Categories
Chemistry 9th Class Chemistry

Chapter 7 Electrochemistry 9th Class Chemistry Notes

Chapter 7 Electrochemistry 9th Class Chemistry Notes

 

Question#1: Define electrochemistry? Also write a detailed note on oxidation reduction reactions?


ELECTROCHEMISTRY:

Electrochemistry is the branch of Chemistry that deals with the relationship between electricity and chemical reactions.

OXIDATION AND REDUCTION REACTIONS ON THE BASIS OF ADDITION OR REMOVAL OF OXYGEN:

Oxidation: Oxidation is defined as addition of oxygen during a chemical reaction.

Reduction: The removal of oxygen during a chemical reaction is called reduction.

Example: A reaction between zinc oxide and carbon takes place by the removal of oxygen (reduction) from zinc oxide and addition of oxygen (oxidation) to carbon. It is represented as

Reaction of Zinc with C

 

OXIDATION AND REDUCTION REACTIONS ON THE BASIS OF ADDITION OR REMOVAL OF HYDROGEN:

Oxidation: The removal of hydrogen during a chemical reaction is called oxidation.

Reduction: The addition of hydrogen during a chemical reaction is called Reduction.

Example: A reaction between hydrogen sulphide and chlorine takes place by oxidation of hydrogen sulphide and reduction of chlorine. Hydrogen is being removed from H2S and added to chlorine. It is represented as:

Reaction of H2S With Cl

 

Oxidation and Reduction in Terms of Loss or Gain of Electron: Reactions which involve ‘loss or gain of electrons’ are also called oxidation and reduction reactions.

Oxidation: Loss of electrons by an atom or an ion is called Oxidation.

Example:

oxidation in zinc and iron

 

Reduction: Gain of electrons by an atom or ion is called reduction.

Reduction in Hydrogen and Chlorine

 

The overall redox reaction is sum of both processes, written as

Reaction of Zinc with Hydrogen ion

 

Summary of all the Oxidation and Reduction reactions

OxidationReduction
Addition of oxygenRemoval of oxygen
Removal of hydrogenAddition of hydrogen
Loss of electronsGain of electrons

Question#2: Describe the rules for assigning the oxidation state?

Answer: OXIDATION STATE OR OXIDATION NUMBER:

Oxidation state or oxidation number (O.N.) is the apparent charge assigned to an atom of an element in a molecule or in an ion. For example: in HCl, oxidation number of H is +1 and that of CI is -1.

Rules for assigning oxidation numbers (O.N.)

  1. The oxidation number of all elements in the free state is zero.
  2. The oxidation number of an ion consisting of a single element is the same as the charge on the ion.
  3. The oxidation number of different elements in the periodic table is: in Group 1 it is +1, in Group 2 it is +2 and in Group 13 it is +3.
  4. The oxidation number of hydrogen in all its compounds is +1. But in metal hydrides it is -1.
  5. The oxidation number of oxygen in all its compounds is -2. But it is -1 in peroxides and +2 in OF2.
  6. In any substance, the more electronegative atom has the negative oxidation number.
  7. In neutral molecules, the algebraic sum of the oxidation numbers of all the elements is zero.
  8. In ions, the algebraic sum of oxidation number equals the charge on the ion.
REMEMBER

It is important to note that while assigning oxidation numbers the sign precedes the number. It is written as +2. Whereas, the apparent charge on an atom, ion or molecule which is called valency, is written as the sign followed by the number i.e. 2+.

Question#3) What are oxidizing and reducing agents?

Answer:  OXIDIZING AGENTS:

An Oxidizing agent is the specie that oxidise a substance by taking electrons from it is called Oxidizing agent. The substance (atom or ion) which is reduced itself by gaining electrons is also called oxidizing agent. Non-metals are oxidizing agents because they accept electrons being more electronegative elements.

Reducing agent:  Reducing agent is the specie that reduces a substance by donating electron to it. The substance (atom or ion) which is oxidized by losing electrons is also called reducing agent .  Almost all metals are good reducing agents because they have the tendency to lose electrons. An outline of oxidation and reduction processes is given below.

 

DO YOU KNOWOxidation is ‘losing electrons in a chemical reaction’

Reduction is ‘gaining electrons in a chemical reaction’

Reducing agent – is a substance that oxidizes itself and reduces other.

Oxidizing agent – is a substance that reduces itself and oxidizes other.

 

 

Question#4) what do you know about Oxidation Reduction in terms of increasing or decreasing state?

Answer: OXIDATION – REDUCTION REACTIONS IN TERMS OF INCREASING OR DECREASING OXIDATION STATE:

Chemical reactions in which the oxidation state of one or more substances changes are called oxidation-reduction or redox reactions.

Oxidation: The reaction in which oxidation number of an atom or ion increases is called oxidation reaction.

Reduction: The reaction in which oxidation number of an atom or ion decreases is called reduction reaction.

Following are the examples of redox reactions. Each reaction system consists of oxidizing and reducing agents.

Let us discuss a reaction of zinc metal with hydrochloric acid:

Reaction of Zinc with Hydrogen ion

 

 

 

 

Let us find out the atoms that are oxidized or reduced or whether there is a change in their oxidation state, it is indicated as follows:

 

 

Electrochemistry 9th Class

 

Question#5) Define electrolyte? What are different types of electrolytes?

Answer:  Electrolytes:

The substances, which can conduct electricity in their aqueous solutions or molten states, are called electrolytes. For example, solutions of salts, acids or bases are good electrolytes. The electricity cannot pass through solid NaCl but in aqueous solution and in molten state, it does conduct. Electrolytes are classified into two groups depending upon their extent of ionization in solution.

Strong Electrolytes:

The electrolytes which ionize almost completely in their aqueous solutions and produce more ions, are called strong electrolytes. Example of strong electrolytes are aqueous solutions of NaCl, NaOH and H2SO4, etc.

 

 Weak Electrolytes:

The electrolytes which ionize to a small extent when dissolved in water and could not produce more ions are called weak electrolytes. Acetic acid (CH3COOH) and Ca(OH)2when dissolved in water, ionize to a small extent and are good examples of weak electrolytes. Weak electrolytes do not ionize completely, As a result the weak electrolytes are poor conductors of electricity.

.  For example, ionization of acetic acid in water produces less ions.

 

Non-Electrolytes

The substances, which do not ionize in their aqueous solutions and do not allow the current to pass through their solutions, are called non-electrolytes. For example, sugar solution and benzene are non-electrolytes.

Question#6) What is an electrochemical cell?

Answer:  ELECTROCHEMICAL CELLS:

Electrochemical cell is a system in which two electrodes are dipped in the solution of an electrolyte which are connected to the battery. Electrochemical cell is an energy storage device in which either a chemical reaction takes place by using electric current (electrolysis) or chemical reaction produces electric current(electric conductance).

Types of  Electrochemical cells: Electrochemical cells are of two types.

  1. Electrolytic cells
  2. Galvanic cells

 

Question#7) How can a non-spontaneous reaction be carried out in an electrolytic cell? Discuss in detail?

Answer:  Electrolytic cells:

The type of electrochemical cell in which a non-spontaneous (energy is used) chemical reaction takes place when electric current is passed through the solution, is called an electrolytic cell.

Process or Reaction in Electrochemical cell: The process that takes place in an electrolytic cell is called electrolysis.

Electrolysis: The chemical decomposition of a compound into its components by passing current through the solution of the compound or in the molten state of the compound.

Examples of Electrlytic cell: Downs cell, Nelson’s cell, etc.

 

Construction of an Electrolytic Cell:

An electrolytic cell consists of a solution of an electrolyte, two electrodes (anode and cathode)

Electrolytic Cell

that are dipped in the electrolytic solution and connected to the battery. The electrode connected to positive terminal is called anode and electrode connected to the negative terminal is called cathode.

Working of an electrolytic cell: When electric current is applied from battery, the ions in the electrolyte migrate to their respective electrodes. The anions, which are negatively charged, move towards the anode and discharge there by losing their electrons. Thus oxidation takes place at anode. While cations, which are positively charged ions, move towards cathode. Cations gain electrons from the electrode and as a result reduction takes place at cathode.

For example, when fused salt of sodium chloride is electrolysed the following reactions take place during this process:

Sodium Chloride Splitting

Oxidation reaction at anode:

Oxidation of Chloride ion

Reduction reaction at cathode:

Reduction of Sodium Ion

Overall reaction:

Question#8) Discuss the electrolysis of water?

Answer: Electrolysis of Water

Pure water is a very weak electrolyte. It ionizes to a very small extent. The concentrations of hydrogen ions (H+) and hydroxyl ions (OH-) are both at 10-7 mol dm-3 respectively. When a few drops of an acid are added in water, its conductivity improves.

When an electric current is passed through this acidified water, OH- anions move towards positive electrode (anode) and H+ cations move towards negative electrode (cathode) and discharge takes place at these electrodes.They produce oxygen and hydrogen gases respectively at anode and cathode as shown in figure.

Fig. Electrolytic cell showing electrolysis of water

 

The redox reaction taking place in the electrolytic bath can be shown as following:

Oxidation reaction at anode:

Reduction reaction at cathode:

Overall reaction:

 

 

Question#9) Discuss the construction and working of a cell in which electricity is produced? Or write a detailed note on Galvanic cell?

Answer:

Galvanic Cell:

The electrochemical cell in which a spontaneous chemical reaction takes place and generates electric current is called galvanic or voltaic cell. Example of this type of cell is a Daniel cell. Galvanic cell was named after Luigi Galvani and Alessandro volta.

 

Construction of a Daniel Cell : A galvanic cell consists of two cells, each called as half-cell, connected electrically by a salt bridge. In each of the half-cell, an electrode is dipped in 1 M solution of its own salt and connected through a wire to an external circuit.

The left half-cell consists of an electrode of zinc metal dipped in 1 M solution of zinc sulphate. The right half-cell is a copper electrode dipped in I M solution of copper sulphate. Salt bridge is a U shaped glass tube. It consists of saturated solution of strong electrolyte supported in a jelly type material. The ends of the U tube are sealed with a porous material like glass wool. The function of the salt bridge is to keep the solutions of two half-cells neutral by providing a pathway for migration of ions.

Galvanic Cell

 

 

Working of the Cell

The Zn metal has tendency to lose electrons more readily than copper. As a result oxidation takes place at Zn-electrode. The electrons flow from Zn-electrode through the external wire in a circuit to copper electrode. These electrons are gained by the copper ions of the solution and copper atoms deposit at the electrode. The respective oxidation and reduction processes going on at two electrodes are as follows:

Half-cell reaction at anode (oxidation)

Half cell reaction at cathode (reduction)

 

Overall galvanic reaction is the sum of these two half-cell reactions

As a result of redox reaction, electric current is produced. The batteries which are used for starting automobiles, running calculators and toys and to lit the bulbs work on the same principle.

Question#10) Compare electrolytic and galvanic cell?

Answer:

A Comparison of Electrolytic and Galvanic Cells

Electrolytic cellGalvanic cell
It consists of one complete cell, connected to a batteryIt consists of two half-cells connected through a salt bridge.
Anode has positive charge while cathode has negative chargeAnode has negative charge while cathode has positive charge
Electrical energy is converted into chemical energyChemical energy is converted into electrical energy.
Current is used for a non-spontaneous chemical reaction to take placeRedox reaction takes place spontaneously and produce electric current.

Question#11) what do you know about electrochemical industries? Discuss in detail the manufacturing of sodium on industrial scale?

Answer:  ELECTROCHEMICAL INDUSTRIES: The industries in which various chemicals are produced by the use of electrolytic cell or electric current are called electrochemical industries.

Manufacture of Sodium Metal from Fused NaCl:

On the industrial scale, molten sodium metal is obtained by the electrolysis of fused NaCl in the Downs cell.

Construction: The Down cell is an electrolytic cell like a circular furnace. In the center there is a large block of graphite, which acts as an anode while cathode around it is made of iron.

Working of Downs Cell

The fused NaCl produces Na+ and Cl- ions, which migrate to their respective

electrodes on the passage of electric current. The electrodes are separated by steel gauze to prevent the contact between the products. The CI- ions are oxidized to give CI2 gas at the anode. It is collected over the anode within an inverted cone-shaped structure. While Na+are reduced at cathode and molten Na metal floats on the denser molten salt mixture from where it is collected in a side tube.

Basic reactions: Following reactions take place during the electrolysis of the molten sodium chloride:

 

Molten NaCl ionizes as:

2NaCl  >> 2Na+(l) +2Cl(l)

Half-cell reaction at anode (oxidation)

2Cl(l)  → Cl2(g) + 2e

Half-cell reaction at cathode (reduction)

2Na+(l) + 2e → 2Na (l)

Overall galvanic reaction is the sum of these two half-cell reactions

2NaCl(fused) → Cl2 (g) + 2Na(l)

Question#12) How can we prepare NaOH (Sodium hydroxide) on commercial scale? Discuss its chemistry along with the diagram?  OR  What is Nelson cell and for what purpose it is used?

Answer:   Manufacture of NaOH from Brine

On industrial scale caustic soda (sodium hydroxide) NaOH, is produced in Nelson’s cell by the electrolysis of aqueous solution of NaCl called brine.

Raw materials: Concentrated aqueous solution of sodium chloride called brine.

Construction of cell: It consists of a steel tank in which graphite anode is suspended in the center of a U shaped perforated iron cathode. This iron cathode is internally lined with asbestos diaphragm. Electrolyte brine is present inside the iron cathode.

Working of Nelson’s Cell

Aqueous solution of sodium chloride consists of Na+, CI, H+ and OH ions. These ions move towards their respective electrodes and redox reactions take place at these electrodes. When electrolysis takes place Cl ions are discharged at anode and CI2 gas rises into the dome at the top of the cell. The H+ ions are discharged at cathode and H2

gas escapes through a pipe. The sodium hydroxide solution slowly percolates into a catch basin.

 Brine ionizes to produce ions:

2NaCl(aq) → 2Na+(aq) + 2Cl-(aq)

Reaction at anode (oxidation)

2Cl-(aq)  → Cl2(g) + 2e-

Reaction at cathode (reduction)

2H2O(l) + 2e-  → H2(g) + 2OH- (aq) + 4e-

2Na+(aq) + 2OH- (aq)  → 2NaOH (aq)

0verall cell reaction of this process:

 2NaCl(aq) + 2H2O(l)  → H2(g) + Cl2(g) + 2NaOH(aq)

Question#13) Define corrosion and rusting? Discuss the redox reaction taking place in the rusting of iron in detail?

 CORROSION:

Corrosion is slow and continuous eating away of a metal by the surrounding medium. It is a redox chemical reaction that takes place by the action of air and moisture with the metals.

Example: The most common example of corrosion is rusting of iron.

Rusting of Iron

The corrosion of iron is called rusting.

Condition: The important condition for rusting is moist air (air having water vapours in it). There will be no rusting in water vapours free of air or air free of water.

Causes: Stains and dents on the surface of the iron provide the sites for this process to occur. This region is called anodic region(oxidation occur) and following oxidation reaction takes place here:

Oxidation:   2Fe  →  2Fe+2 + 4e-

This loss of electrons damage the object. The free electrons move through iron sheet ,until they reach to a region of relatively high O2 concentration near the surface surrounded by water layer. This region acts as cathode and electrons reduce the oxygen molecule in the presence of H+ ions:

Reduction:   O2(g) + 4H+(aq) + 4e-  → 2H2O(l)

The H+ ions are provided by the carbonic acid, which is formed because of presence of CO2 in water. That is why acidic medium accelerates the process of rusting. The overall redox process is completed without the formation of rust.

Formation of Rust: 2Fe(s) + O2(g) + 4H+(aq)  → 2Fe+2(aq) + 2H2O(l)

The Fe+2 formed spreads through out the surrounding water and react with O2 to form the salt Fe2O3 .nH2O which is called rust. It is also a redox reaction.

2Fe+2 (aq) + 1/2 O2(g) + (2+n)H2O(l)  → Fe2O3.nH2O(s) + 4H+(aq)

In the above formula “n” shows the number of H2O molecules. It is different at different conditions. If “n” is considered as 1, then:

2Fe+2 (aq) + 1/2 O 2(g) + (2+1)H2O(l)  → Fe2O3.1H2O(s) + 4H+(aq)

2Fe+2 (aq) +1/2 O 2(g) + 2H2O(l)  → Fe2O3.H2O(s) + 4H+(aq)

The rust layer of iron is porous and does not prevent further corrosion. Thus rusting continues until the whole piece of iron is eaten away

Rusting

 

Does Aluminium Rust?

Aluminium corrodes but it does not rust. Rust refers only to iron and steel corrosion. A very hard material aluminium oxide protects the aluminium from further corrosion.

In comparison to that when iron corrodes, its color changes and produces large red flakes known as rust. Unlike aluminium oxide, the expanding and flaking of rust exposes new metal surface to further rusting.

 

 

Question#14) How corrosion can be prevented? What is electroplating? Write down procedure of electroplating? What is the principle of electroplating? How electroplating of chromium is carried out?

Answer: Prevention of Corrosion:

Removal of stains:

The regions of stains in an iron rod act as the site for corrosion. If the surface of iron is properly cleaned and stains are removed, it would prevent rusting.

Paints and greasing

Greasing or painting of the surface can prevent the rusting of iron. With development of technologies, modern paints contain a combination of chemicals called stabilizers that provide protection against the corrosion in addition to prevention against the weathering and other atmospheric effects.

Alloying: Alloy is a homogeneous mixture of one metal with one or more other metals or non-metals. Alloying of iron with other metals has proved to be very successful technique against rusting. The best example of alloying is the ‘stainless steel’, which is a good combination of iron, chromium and nickel.

Metallic coating

The best method for protection against the corrosion of metals exposed to acidic conditions is coating the metal with other metal. Corrosion resistant metals like Zn, Sn and Cr are coated on the surface of iron to protect it from corrosion. It is the most widely applied technique in the food industry where food is ‘tin-packed’. The containers of iron are coated with tin to give it a longer life.

Methods of Metallic coating: Metallic coating can take place by physical as well as electrolytic methods.

1- Physical Methods (galvanizing and tin coating)

  1. Zinc coating or Galvanizing

The process of coating a thin layer of zinc on iron is called galvanizing.

Method or process or mechanism: This process is carried out by dipping a clean iron sheet in a zinc chloride bath and then heating it. After this iron sheet is removed, rolled into molten zinc metal bath and finally air-cooled.

Advantages: Advantage of galvanizing is that zinc protects the iron against corrosion even after the coating surface is broken.

  1. Tin Coating:

It involves the dipping of the clean sheet of iron in a bath of molten tin and then passing it through hot rollers. Such sheets are used in the beverage and food cans. The tin protects the iron only as long as its protective layer remains intact. Once it is broken and the iron is exposed to the air and water, a galvanic cell is established and iron rusts rapidly.

2- Electrolytic method (Electroplating)

Electroplating is depositing of one metal over the other by means of electrolysis.

Advantages and uses: This process is used to protect metals against corrosion and to improve their appearance.

Principle of electroplating: Principle of electroplating is to establish an electrolytic cell in which anode is made of the metal to be deposited and cathode of the object on which metal is to deposit. The electrolyte is in aqueous solution of a salt of the respective metal.

Procedure for Electroplating

In this process the object to be electroplated is cleaned with sand, washed with caustic soda solution and finally it is thoroughly washed with water. The anode is made of the metal, which is to be deposited like Cr, Ni. The cathode is made up of the object that is to be electroplated like some sheet made up of iron.

The electrolyte in this system is a salt of the metal being deposited. The electrolytic tank is made of cement, glass or wood in which anode and cathode are suspended. These electrodes are connected with a battery. When the current is passed, the metal from anode dissolves in the solution and metallic ions migrate to the cathode and discharge or deposit on the cathode (object). As a result of this discharge, a thin layer of metal deposits on the object, which is then pulled out and cleaned. Some examples of electroplating are Silver electroplating, chromium electroplating, Tin Plating and Zinc plating.

 

  1. Electroplating of Silver

The electroplating of silver is carried out by establishing an electrolytic cell. The pure piece of silver strip acts as anode that is dipped in silver nitrate solution. The cathode is the metallic object to be coated such as spoon. When the current is passed through the cell, the anode dissolves to produce Ag+ ions, that migrate towards the cathode. At cathode they are discharged and deposited on the object e.g. spoon. The chemical reaction can be represented as:

The above is anodic while below is cathodic reaction.

Figure: Electroplating of an object

Common examples of silver plating are tablewares, cutlery, jewelry and steel objects.

  1. Electroplating of Chromium

The electroplating of chromium is carried out in the same way as that of silver. The object to be electroplated is dipped in aqueous solution of chromium sulphate containing a little sulphuric acid, that acts as an electrolyte. The object to be electroplated acts as cathode while anode is made of antimonial lead. The electrolyte ionizes and provides Cr3+ ions, which reduce and deposit at cathode.

Electrolyte produces the following ions:

Reactions at the electrodes are as follows:

For practical convenience, the steel is usually plated first with nickel or copper and then by chromium because it does not adhere well on the steel surface. Moreover, it allows moisture to pass through it and metal is stripped off. The nickel or copper provides adhesion and then chromium deposited over the adhesive layer of copper lasts longer. This type of electroplating resists corrosion and gives a bright silvery appearance to the object.

  1. Electroplating of zinc

The target metal is cleaned in alkaline detergent solutions and it is treated with acid, in order to remove any rust or surface scales. Then, the zinc is deposited on the metal by immersing it in a chemical bath containing electrolyte zinc sulphate. A current is applied, which results in depositing of zinc on the target metal i.e. cathode.

  1. Electroplating of tin:

Tin is usually electroplated on steel by placing the steel into a container containing a solution of tin salt. The steel is connected to an electrical circuit, acting as cathode. While the other electrode made of tin metal acts as anode. When an electrical current passes through the circuit, tin metal ions present in the solution deposit on steel.

  1. Electrolytic refining of copper:

Impure copper is refined by the electrolytic method in the electrolytic cell. Impure copper acts as anode and a pure copper plate acts as cathode as shown in figure 7.8. Copper sulphate solution in water is used as an electrolyte. Oxidation reaction takes place at the anode. Copper atoms from the impure copper lose electrons to the anode and dissolve in solution as copper ions:

Reduction reaction takes place at the cathode. The copper ions present in the solution are attracted to the cathode. Where they gain electrons from the cathode and become neutral and deposit on the cathode.

In the process impure copper is eaten up and purified copper atoms deposit on the cathode.

Refining of copper in an electrolytic cell

Short Answer Questions.

  1. Define oxidation in terms of electrons. Give an example.

Answer: The loss of electrons by an atom or an ion is called oxidation.

 Example: Zn  →  Zn+2 + 2e-

 

 

  1. Define reduction in terms of loss or gain of oxygen or hydrogen. Give an example.

Answer; Reduction:  See in redox reactions

 

 

  1. What is difference between valency and oxidation state?

Answer: Difference between valency and oxidation state:

               Valency             Oxidation
1) It is the combining capacity of an atom to other atom.1) It is the apparent charge assigned to an atom of an element in a molecule or in an ion.
2)  It is a number only. It has no plus or minus sign with it.2) It may be positive or negative number.
3)  It can not be zero.3) It may be zero.
4)It is always a whole number.4) It may be whole number or fraction.

 

  1. Differentiate between oxidizing and reducing agents

Answer:

         Oxidizing agent      Reducing agent
A species that oxidize a substance by taking electrons from it is called oxidizing agent.A species that reduces a substance by donating electrons to it is called reducing agent.
Non metals being high electronegative take electrons from others and are good oxidizing agent.Metals being high electropositive donate electrons to others and are good reducing agents.

 

  1. Differentiate between strong and weak electrolytes.

Answer:

 

          Strong electrolytes           Weak electrolytes
The electrolytes which ionize almost completely in their aqueous solutions and produce more ions, are called strong electrolytes.The electrolytes which ionize to a small extent when dissolved in water and could not produce more ions are called weak electrolytes.
Example of strong electrolytes are aqueous solutions of NaCl, NaOH and H2SO4, etc.Acetic acid (CH3COOH) and Ca(OH)2when dissolved in water, ionize to a small extent and are good examples of weak electrolytes.

 

  1. How electroplating of tin on steel is carried out?

Answer: Electroplating of tin:

Tin is usually electroplated on steel by placing the steel into a container containing a solution of tin salt. The steel is connected to an electrical circuit, acting as cathode. While the other electrode made of tin metal acts as anode. When an electrical current passes through the circuit, tin metal ions present in the solution deposit on steel.

  1. Why steel is plated with nickel before the electroplating of chromium.

Answer: The steel is usually plated first with nickel or copper and then by chromium because it does not adhere well on the steel surface. Moreover, it allows moisture to pass through it and metal is stripped off. The nickel or copper provides adhesion and then chromium deposited over the adhesive layer of copper lasts longer. This type of electroplating resists corrosion and gives a bright silvery appearance to the object.

  1. How can you explain, that following reaction is oxidation in terms of increase of oxidation number?

Answer: Oxidation is the loss of electrons from a substance. In the given reaction aluminum losses three electrons, so it is an oxidation reaction.

 

  1. How can you prove with an example that conversion of an ion to an atom is an oxidation process?

Answer: The loss of electrons by an atom or an ion is called oxidation. The loss of electrons from a negative ion results in the formation of an atom.

Example 1: The removal of an electron from a hydride ion results in the formation of hydrogen atom.

H- → H + e

Example 2: The removal of an electron from a chloride ion results in the formation of chlorine atom.

Cl → Cl + e

  1. Why does the anode carries negative charge in galvanic cell but positive charge in electrolytic cell? Justify with comments.

Answer: In electrolytic cell anode carries positive charge because it is connected to positive terminal of the battery. In a galvanic cell the anode carries negative charge due to the presence of electrons lost by the Zinc (Zn) metal.

  1. Where do the electrons flow from Zn electrode in Daniel’s cell?

Answer: The electrons flow from Zn electrode through external wire to the copper electrode. These electrons are gained by the copper ions of the solution and copper atoms deposited on the electrode.

  1. Why do electrodes get their names ‘anode’ and cathode in galvanic cell?

Answer: In a galvanic cell, the electrode from which electrons flow out of the cell is called anode. The electrode from which electrons move into the cell is called cathode.

  1. What happens at the cathode in a galvanic cell?

Answer: Reduction reaction takes place at cathode in the galvanic cell.

  1. Which solution is used as an electrolyte in Nelson cell?

Answer: Aqueous concentrated solution of NaCl (Brine) is used as an electrolyte in the Nelson cell.

  1. Name the by-products produced in Nelson’s cell?

Answer: Hydrogroen and chlorine gases are produced as by-products in the galvanic cell.

  1. Why is galvanizing done?

Answer: Galvanizing is the coating of thin layer of zinc on iron. It is done to protect iron from rusting.

  1. Why an iron grill is painted frequently?

Answer: An iron grill is painted frequently to protect it from corrosion (rusting).

  1. Why is O2 necessary for rusting?

Answer: A region of high oxygen concentration sorrounding by water act as cathode and electron reduce the oxygen molecule in the presence of hydrogen ions and water is fomed.

O2 + 4H+ +4e  →  2H2O

  1. In electroplating of chromium, which salt is used as an electrolyte?

Answer: Chromium sulphate , Cr2.(SO4)3 is used in electroplating of chromium,

  1. Write the redox reaction taking place during the electroplating of chromium?

Answer: Following redox reaction taking place during the electroplating of chromium.

 

  1. In electroplating of silver, from where Ag+ ions come and where they deposit?

Answer: When the current is passed through the cell, the anode dissolves to produce Ag+ ions, that migrate towards the cathode. At cathode they are discharged and deposited on the object e.g. spoon. The chemical reaction can be represented as:

 

  1. What is the nature of electrode used in electrolyting of chromium?

Answer: In electroplating of chromium the object to be electroplated act as cathode while anode is made of antimonial lead.

 

Long Answer Questions

  1. Describe the rules for assigning the oxidation state?

Answer: Please see answer of question number 2.

  1. Find out the oxidation numbers of the underlined elements in the following compounds.
    (a) Na2SO4(b) AgNO3(c) KMnO4
    (d) K2Cr2O7 (e) HNO2

Answer:

  • Na2SO4:

Solution:  2[O.N of Na] + [O.N of S] + 4[O.N of O]

2[+1] + [O.N of S] + 4[-2]

2 +  [ O.N of S ] +  [-8 ]

[ O.N of S ] + 2 -8.

Oxidation number of Sulphur = 8-2

Oxidation number of Sulphur = +6.

(b) AgNO3 :

Solution: [O.N of Ag] + [O.N of N] + 3[O.N of O] = 0

[+1] + [0.N of N] + 3[-2] = 0

[0.N of N] + [-6] + [+1] = 0

Oxidation number of nitrogen: 6 – 1

Oxidation number of nitrogen in AgNO3 = +5

(c) KMnO4:

Solution: [ O.N of K] + [O.N of Mn] + 4[O.N of O] = 0

[+1] +  [O.N of Mn] + 4[-2] = 0

[O.N of Mn] + [-8] + [+1] = 0

Oxidation number of Mn = 8-1

Oxidation number of Mn in KMnO4 = +7

(d) K2Cr2O7 :

Solution : 2[O.N of K] + 2[O.N 0f Cr] + 7[O.N of O] = 0

2[+1] + [O.N of Cr] + 7[-2] = 0

[O.N of Cr] + [-14] + [+2] = 0

2[Oxidation number of Chromium] = 14 – 2

2[Oxidation number of Chromium] = 12

Oxidation number of Chromium in K2Cr2O7 = 12/2

Oxidation number of Chromium in K2Cr2O7 = +6

(e) HNO2 :

[O.N of H] + [O.N of N] + 2[O.N of O] = 0

[+1] + [O.N of N] + 2[-2] = 0

[O.N of N] + [-4] + [+1] = 0

Oxidation number of Nitrogen = 4 -1

Oxidation number of Nitrogen in HNO2 = +3

 

  1. How can a non-spontaneous reaction be carried out in an electrolytic cell? Discuss in detail.

Answer: Please see answer of question number 7.

 

  1. Discuss the electrolysis of water?

Answer: Please see answer of question number 8.

 

  1. Discuss the construction and working of a cell in which electricity is produced?

Answer: Please see answer of question number 9.

 

  1. How can we prepare NaOH on commercial scale? Discuss its chemistry along with the diagram?

Answer: Please see answer of question number 12.

 

  1. Discuss the redox reaction taking place in the rusting of iron in detail?

Answer: Please see answer of question number 13.

 

  1. Discuss, why galvanizing is considered better than that of tin plating.

Answer: Zinc protects the iron against corrosion even after the coating surface is broken. While the tin coating once broken and the iron is exposed to the air and water, a galvanic cell is established and iron rusts rapidly, that is why galvanizing is considered better than that of tin plating.

  1. What is electroplating? Write down procedure of electroplating.

Answer: Please see answer of question number 14.

  1. What is the principle of electroplating? How electroplating of chromium is carried out?

Answer: Please see answer of question number 14.

Test yourself 7.1

  1. How can you justify that a reaction between magnesium and oxygen is a redox reaction, while the reaction shows only addition of oxygen (oxidation)

 

Answer: In a reaction between magnesium and Oxygen, each of the Mg atom loss 2 electrons which are accepted by the Oxygen. As in the reaction Oxygen reduces while Magnesium oxidizes, so the reaction is redox reaction.

  1. A reaction between carbon and oxygen involved only addition of oxygen (oxidation), but, it is called a redox reaction. Comment on this.

Answer: Reaction between Carbon and Oxygen is as follows:

C + O2 → CO2

In the reaction carbon loss 4 electrons, its oxidation number increases from 0 to +4, so it oxidizes while each of the Oxygen atom gain two electrons and their oxidation number decreases from 0 to -2, so they reduces and that’s why the reaction is redox reaction as both oxidation and reduction occurs at the same time.

  1. Oxidation and reduction proceed simultaneously. Explain, with an example.

Answer: Oxidation and reduction proceed simultaneously in a reaction.as:

 

In the reaction Magnesium loss electrons and becomes oxidize while Oxygen gain electrons and thus reduces, so both the reactions occurs at a time.

  1. Identify which of the following is oxidation or reduction reaction?

 

Answer: a) K loss electrons, so it is oxidation reaction.

  1. b) In the reaction Br gains electrons, so it is a reduction reaction.
  2. c) In the reaction Cu loss electrons, so the reaction is oxidation.
  3. d) In the reaction Iodide ion loss electron so it is an oxidation reaction.
  4. e)  In the reaction Fe+2 ion loss electron so it is an oxidation reaction.

 

  1. An element M reacts with another element X to form MX2In terms of loss or gain of electrons, identify the element which is oxidized and which is reduced.

Answer: M + X → MX2

According to chemistry rule the electropositive atom is always written before the electronegative. In the given compound M before X shows that M is electropositive. M atom loss two electrons and becomes oxidizes, while each of the X atom gain one electron and reduces.

  1. How can you justify that the following reaction is not only an oxidation reaction but also a complete redox reaction.

 

Answer: In the reaction FeO loss Oxygen so it reduces, while CO oxidizes as it gains Oxygen. So the reaction is complete redox reaction.

  1. Explain the term oxidation on the basis of electronic concept with an example.

Answer: Oxidation: Loss of electrons by an atom or an ion is called Oxidation.

Example:

 

 

 

 

Test Yourself 7.2:

 

  1. Find out the oxidation numbers of the following elements marked in bold in the formulae:
    (A) Ba3(PO4)2.

(B) CaSO4.

(C) Cu(NO3)2.

(D) Al2(SO4)3

Answer:

(A) To calculate the oxidation number of phosphorous (P) in Ba3(PO4)2.

Solution: 3[O.N of Ba] + 2[O.N of P] + 2×4[O.N of O] = 0

3[+2] + 2[O.N of P] + 8[-2] = 0

2[O.N of P] + [-16] + [+6] = 0

2[O.N of P] = 16 – 6

2[O.N of P] = 10

Oxidation number of P = 10/2

Oxidation number of P in Ba3(PO4)2 is = 5

(B) To calculate the oxidation number of Sulphur (S) in CaSO4:

Solution: [O.N of Ca] + [O.N of S] + 4[O.N of O] = 0

[+2] + [O.N of S] + 4[-2] = 0

[O.N of S] + [-8] + [+2] = 0

Oxidation number of sulphur = 8 – 2

Oxidation number of sulphur in CaSO4 = 6

(C) Cu(NO3)2.

Solution: To calculate the Oxidation number of Nitrogen (N) in Cu(NO3)2.

[O.N of Cu] + 2[O.N of N] + 3×2[O.N of O] = 0

[+2] + 2[O.N of N] + 6[-2] = 0

2[O.N of N] + [-12] + [+2] = 0

2[Oxidation number of N] = 12 – 2

2[Oxidation number of N] = 10

[Oxidation number of N] = 10/2

Oxidation number of N in Cu(NO3)2 is = 5

 

(D) To calculate the oxidation number of Sulphur (S) in Al2(SO4)3

Solution: 2[O.N of Al] + 3[O.N of S] + 4×3[O.N of O] = 0

2[+3] + 3[O.N of S] + 12[-2] = 0

3[O.N of S] + [-24] + [6] = 0

3[Oxidation number of Sulphur] = 24 – 6

3[Oxidation number of Sulphur] = 18

Oxidation number of Sulphur = 18/3

Oxidation number of Sulphur in Al2(SO4)3 = 6

  1. In a compound MX3, find out the oxidation number of M and X.

Answer: In the formula MX3, M being high electropositive written before X, while X is high electronegaive . Select a most lowest negative oxidation state for X that is -1.

Applying the formula:

[O.N of M] + 3[O.N of X] = 0

[O.N of M] + 3[-1] = 0

[O.N of M] = +3

So Oxidation number of X is -1 while that of M is +3.

  1. Why the oxidation number of oxygen in OF2is +2

Answer: In OF2 molecule flourine being more electronegative gains electrons from Oxygen. Each of the flourine gains 1 electron and attains -1 oxidation state while Oxygen  being less electronegative loss two electrons and attains oxidation state of +2.

  1. In H2S, SO2 and H2SO4 the sulphur atom has different oxidation number. Find out the oxidation number of sulphur in each compound.

Answer: (a) Calculating Oxidation state of sulfur (S) in H2S.

Solution: 2[O.N of H] + [O.N of S] = 0

2[+1] + [O.N of S] = 0

[+2] + [O.N of S] = 0

Oxidation number of Sulphur in H2S = -2

(b) Calculating Oxidation state of sulfur (S) in SO2.

Solution: [O.N of S] + 2[O.N of O] = 0

[O.N of S] + 2[-2] = 0

[O.N of S] + [-4] = 0

Oxidation number of sulphur in SO2 = +4

(c) Calculating Oxidation state of sulfur (S) in  H2SO4.

Solution: 2[O.N of H] + [O.N of S] + 4[O.N of O] = 0

2[+1] + [O.N of S] + 4[-2] = 0

[O.N of S] + [-8] + [+2] = 0

[O.N of S] = 8 – 2

Oxidation state of Sulphur (S) in H2SO4 is 6.

  1. An element X has oxidation state 0. What will be its oxidation state when it gains three electrons?

Answer: An element X with oxidation state 0, when gains three (3) electrons, its oxidation state increases from 0 to -3. It can be represented as:

X0 + 3e  →  X3

  1. An element in oxidation state +7 gains electrons to be reduced to oxidation state +2. How many electrons did it accept?

Answer: An element with oxidation state +7 when gains 5 electrons it reduces to +2 oxidation state. It can be represented as:

X7+ + 5e  →  X2+

  1. If the oxidation state of an element changes from +5 to -3. Has it been reduced or oxidized? How many electrons are involved in this process?

Answer: If an element with oxidation state +5 gains 8 electrons, it reduces to -3 oxidation state. It can be represented as follows:

X5+ + 8e-  →  X3-

 

 

TEST YOURSELF 7.3

 

  1. In the following reaction, how can you justify that H2S is oxidized and SO2is reduced.

 

Answer:

Chemical equation with oxidation numbers can be written as :

S+4O2-4 + 2H2+2S-2  → 2H2+2O-2 + 3S0

In the reaction SO2 reduces because the oxidation state of sulphur decreases from +4 to 0 due to the gain of 4 electrons. On the other hand H2S oxidizes because Sulphur in it loss 2 electrons and change from -2 oxidation state to 0.

  1. The reaction between MnO2and HCl is a redox reaction written as balance chemical equation.

 

Find out:
a. The substance oxidized.
b. The substance reduced.
c. The substance which acts as an oxidizing agent.
d. The substance which acts as a reducing agent.

Answer: Chemical equation with oxidation number is as follows:

Mn+4O2-4 + 4H+1Cl-1  →  Mn+2Cl2-2 + 2H2+2O-2 + Cl20

  • The reaction shows that HCl is oxidized.
  • MnO2is reduced.
  • MnO2acts as an oxidizing agent.
  • HCl acts as a reducing agent.
  1. The following reactions are redox reactions.
    Find out the element which has been reduced and the element which has been oxidized.

 

Answer: (a) Chemical equation with oxidation number will be as follows:

Zn0 + Cu+2SO4-2  →  Zn+2SO4-2 + Cu0

The reaction shows that:

(i) Zn (Zn0) is oxidized.

(ii) Cu (Cu+2) is reduced.

(b) Cu0 + 2Ag+1N+5O3-6  →  Cu+2(N-5O3-6)2 + 2Ag0

The reaction shows that :

(i) Copper (Cu0) is oxidized.

(ii) Silver(Ag+1) is reduced.

(c) H2+2S-1 + Cl20  →  2H+1Cl-1 + S0

(i) Sulphur (S-1) is oxidized.

(ii) Chlorine (Cl0) is reduced.

  1. Why the following reaction is not a redox reaction. Explain with reasons?

 

 

Answer: Chemical equation with oxidation number is as follows:

Na+1O-2H+1 + H+1Cl-1  →  Na+1Cl-1 + H2+2O-2

In the reaction no change occur in the oxidation state of any element, so it is not a redox reaction.

TEST YOURSELF 7.4

  1. Why are the strong electrolytes termed as good conductors?

Answer: Strong electrolytes are termed as good conductors because they ionize almost completely in aqueous solution and due to production of more ions produce more current.

  1. Does non-electrolytes forms ions in solution?

Answer: No, non electrolytes does not form ions in aqueous solution.

 

  1. What is difference between a strong electrolyte and a weak electrolyte?

Answer: Strong Electrolytes :The electrolytes which ionize almost completely in their aqueous solutions and produce more ions, are called strong electrolytes. Example of strong electrolytes are aqueous solutions of NaCl, NaOH and H2SO4, etc.

 

 Weak Electrolytes

The electrolytes which ionize to a small extent when dissolved in water and could not produce more ions are called weak electrolytes. Acetic acid (CH3COOH) and Ca(OH)2 when dissolved in water, ionize to a small extent and are good examples of weak electrolytes. Weak electrolytes do not ionize completely, As a result the weak electrolytes are poor conductors of electricity.

.  For example, ionization of acetic acid in water produces less ions.

 

 

  1. Identify a strong or weak electrolyte among the following compounds:

CuSO4, H2CO3, Ca(OH)2 , HCl, AgNO3

Answer: Strong electrolytes: CuSO4, HCl, AgNO3.

Weak electrolytes: H2CO3, Ca(OH)2.

  1. Which force drives the non-spontaneous reaction to take place?

Answer: An electric force drives the non spontanous reactions to takes place.

  1. Which type of chemical reaction takes place in electrolytic cell?

Answer: Non spontaneous redox reactions takes place in electrolytic cell.

  1. What type of reaction takes place at anode in electrolytic cell?

Answer: Oxidation reaction takes place at anode in electrolytic cell.

  1. Why the positively charged electrode is called anode in electrolytic cell?

Answer: In electrolytic cell, positively charged electrode is connected to positive terminal of the battery, so it is called anode.

“OR”   Anode is the electrode where oxidation takes place, as in electrolytic cell oxidation occurs at positively charged electrode so it is called anode.

  1. In the electrolysis of water, towards which terminal H+ ions move?

Answer: In the electrolysis of water the H+ ions move towards cathode (negative electrode).

  1. In the electrolysis of water, where is the oxygen produced?

Answer: In the electrolysis of water , oxygen is produced at anode.

  1. Towards which electrode of the electrolytic cell moves the cations and what does they do there ?

Answer: In electrolytic cell cations move towards cathode and reduce there by the gain of electrons.

  1. How the half-cells of a galvanic cell are connected? What is function of salt-bridge?

Answer: The two half cells in a galvanic cell are elictrically connected by a salt bridge. The salt bridge provide a pathway for migration of the ions to keep the solutions of two half cells neutral.

Question#13) Define anode and cathode?

Answer: The electrode where oxidation takes place is called anode while the electrode where reduction takes place is called cathode.  “OR”   The electrode which is connected to positive terminal of the battery is called anode while the electrode which is connected to negative terminal of the battery is called cathode.

The metals and non metals of the anode will oxidize going from an oxidation state of 0 to positive oxidation state and it will becomes an ion. At the cathode the metal ion in the solution will accept one or more electrons and the ions oxidation state will reduce from positive to zero.

 

TEST YOURSELF 7.5.

  1. Anode of Downs cell is made of a non-metal, what is its name? What is the function of this anode?

Answer: Anode of down cell is made of graphite. The function of this anode is to oxidize Cl ion to Cl2 by taking electrons from it.

  1. Where does the sodium metal is collected in Downs cell?

Answer: In down cell, the sodium metal is collected at a side tube in molten form.

  1. What is the name of the by-product produced in the Downs cell?

Answer: Chlorine gas is produced as a by-product in down cell.

  1. Are anodes of Downs cell and Nelson cell made of same element? If yes, what is its name ?

Answer: Yes, anodes of both the Downs cell and Nelson cell are made of same element. The name of the element is graphite.

  1. What is the shape of cathode in Nelson’s cell? Why is it perforated?

Answer: In nelson cell, U shaped iron cathode is used. It is perforated for percolation of sodium hydroxide solution into the catch basin, from where it is collected.

  1. Which ions are discharged at cathode in Nelson’s cell and what is produced at cathode?

Answer: The H+ ions are discharged at cathode and H2 gas escapes through a pipe.

 

TEST YOURSELF 7.6

 

 

  1. What is the difference between corrosion and rusting?

Answer: Corrosion is slow and continuous eating away of a metal by the surrounding medium. Corrosion is a general term while corrosion of iron is called rusting.

  1. What happens to iron in the rusting process?

Answer: In rusting process the piece of iron is slowely eaten away or corroded.

  1. Rusting completes in how many redox reactions?

Answer: The rusting of iron completes in four redox reactions.

  1. Explain the role of O2in rusting?

Answer: The region of relatively high O2 concentration near the surface surrounded by water layer act as cathode , where electrons reduce the oxygen molecule in the presence of H+ ions to produce water.

 

  1. State the best method for protection of metal from corrosion.

Answer: The best method for protection of metal from corrosion is metal coating.

  1. What do you mean by galvanizing ?

Answer: The process of coating a thin layer of zinc on iron is called galvanizing.

  1. What is the advantage of galvanizing?

Answer: The advantage of galvanizing is that zinc protects the iron from corrosion , even after the coating surface is broken down.

  1. Why tin plated iron is rusted rapidly when tin layer is broken?

Answer: The tin protects the iron only as long as its protective layer remains intact. Once it is broken and the iron is exposed to the air and water, a galvanic cell is established and iron rusts rapidly.

 

  1. Name the metal which is used for galvanizing iron?

Answer: Zinc (Zn) metal is used for galvanizing of iron.

 

 

TEST YOURSELF 7.7

 

  1. Define electroplating?

Answer: Electroplating is depositing of one metal over the other by means of electrolysis.

  1. How electroplating of zinc is carried out?

Answer: Electroplating of zinc:

The target metal is cleaned in alkaline detergent solutions and it is treated with acid, in order to remove any rust or surface scales. Then, the zinc is deposited on the metal by immersing it in a chemical bath containing electrolyte zinc sulphate. A current is applied, which results in depositing of zinc on the target metal i.e. cathode.

  1. Which material is used to make cathode in electroplating?

Answer: The catode is made of object to be electroplated.

 Read Also Chapter 5

  1. Why is the anode made up of a metal to be deposited during electrolysis?

Answer: The anode is made of the metal that is to be deposited like, chromium. Silver, etc. When the current is passed, metals from anode dissolves in the solution and the metallic ion migrate to the cathode and discharge or deposit on the cathode.

Leave a Reply

Your email address will not be published. Required fields are marked *