Study Portal 7 – Chapter 7

Electrochemistry

 

 

Q1.      Define Electrochemistry.

Ans.    Electrochemistry is a branch of chemistry in which the conversion of chemical energy into electrical energy and electric change produced by an electric current are studied. Electrochemistry deals with situations where oxidation and reduction reactions take place.

Q2.   Explain the terms oxidation and reduction with examples.

Ans.    Oxidation

Oxidation is the process which involves the addition of oxygen or the removal of hydrogen. When carbon burns in the presence of oxygen carbon dioxide is produced.

C + O2    CO2 or

Hydrogen sulphides are oxidized by bromine water to sulphur.

H2S + Br2           2HBr + S

Reduction

The process which involves the addition of hydrogen or the removal of oxygen is called reduction e.g.

Hydrogen has been added to chlorine thus Cl2 has been reduced

H2S + Cl 2HCl + S2

And copper oxide is reduced to copper metal due to the removal of oxygen by hydrogen

CuO + H2           Cu + H2O

The modern view of explaining the oxidation and reduction process is in term of electron transfer the oxidation is loss of electron, while reduction is the gain of electron. The reaction in which oxidation and reduction go side by side is called redox reaction.

Q3.      What is meant by oxidation number / state?

Ans.    The apparent charge positive, negative or Zero on an atom of an element in a molecule or ion is called oxidation number or oxidation state.

Rules for assigning oxidation state:

  • The oxidation state of a free element is always zero.
  • The sum of the oxidation states of all the atoms or ions in a natural compound is zero.
  • The sum of the oxidation states of all the atoms in an ion is equal to charge on the ion.
  • The more electronegative element in a substance is given a negative oxidation state. The less electronegative one is given a positive oxidation state.

Q4.      Explain oxidizing and reducing agent with examples.

Ans.    Oxidizing agent

An oxidizing agent is the substance that provides oxygen or removes hydrogen from any other substance. Hydrogen peroxide (H2O2), potassium per manganate (KMnO4) and potassium dichromate (K2Cr2O7) are some important oxidizing agents.

S + O2   SO2

Oxygen is the oxidizing agent because it has oxidized sulphur to sulphur dioxide.

Reducing agent

A reducing agent is the substance that provides hydrogen or removes oxygen from any other substance carbon (C) hydrogen bromide (HBr) and hydrogen sulphides are some important reducing agents.

ZnO + C           Zn + Co

Carbon is the reducing agent because it has reduced ZnO by removing oxygen from ZnO.

Q5.   Define oxidation and reduction in terms of loss or gain electrons.

Ans.    Oxidation

Oxidation is a chemical process in which there is a loss of electron. The species which is oxidized becomes more positive or less negative

Na    Na+ + e (Na0 is oxidized to Na+)

Reduction

Reduction is a chemical process in which there is a gain of electron. The species which is reduced become more negative or less positive e.g.

+ 2 e          2Cl (is reduced to Cl)

Q6.      What is an electrochemical cell?

Ans.    An arrangement which consists of electrodes dipped into an electrolyte in which a chemical reaction uses or generates electric currents is called electrochemical cell. They have two types.

  • Electrolytic cell
  • Voltaic or galvanic cell
  • The Electrochemical Cell in which a reaction occurs with the help of electric current is called electrolytic cell.
  • The Electrochemical Cell in which a reaction generates electric current is called voltaic or galvanic cell.

Q7.      Define concept of electrolyte.

Ans.    The substance which form conducting solutions when dissolved in a polar solvent are called electrolytes and those substance forming non-conducting solutions are called non-electrolytes. The electrolytes which give highly conducting solutions when dissolved in water are called strong electrolytes. The substances which give poorly conducting solutions when dissolved in water are called weak electrolytes.

Q8.      Describe briefly the working of an electrolytic cell.

Ans.    The movement of charged ions through the electrolytic solution is known as electrolytic conduction. This electrolytic conduction is carried in a cell called electrolytic cell.

A source of direct current is connected to the electrodes of the cell dipped into cations and anions of the electrolyte. The direct current, source serves as an electron pump. It pulls electrons away from one electrode and pushes them through the external wiring on to the other electrode. The electrode from which the electrons are removed becomes positively charged. The other electrode becomes negatively charged. When an electric current is passed. Positively charged ions move towards cathode and negatively charged ions towards anode.

Q9.      What is galvanic or voltaic cell?

Ans.    The electrochemical cell in which a reaction generates electric current   is   called   voltaic   cell   (Daniel   cell).  In  this cell Zinc (Zn)

undergoes oxidation and copper (Cu) undergoes reduction. A galvanic cell consist of two half cells. They are connected together electricity. Each of the half cells is a part of the total cell in which half reaction occurs.

Q10. What do you know about the electrochemical industries?

Ans.    Those industrial processes in which electrochemical cells manufacture certain materials on large scale are included in the category of electrochemical industries. Following are some of important chemical industries based on electrolytic process.

  • Extraction of magnesium, calcium and aluminum
  • Purification of copper
  • Preparation of anodized aluminum.

Q11. Describe the manufacture of caustic soda from brine.

Ans.    The caustic soda is a very important chemical and is used in countless industries. It is commonly known as caustic soda and is industrially prepared from the electrolysis of brine in Nelson’s cell which is a specialized electrolytic cell.

 

The process differs from the Down’s method in the sense that in brine some water is also present which is reduced during the process and form OH ions. Na+ and OH combine and gives NaOH. During the electrolysis Cl2 is given out at the anode, H2 at the cathode and NaOH is collected from the base of the cell.

Q12. What is meant by corrosion? How corrosion can be prevented?

Ans.    The eating way of metals by environmental gases and water is called corrosion. Corrosion is a natural redox process that oxidizes metal to their oxides ands sulphides under atmospheric conditions

Prevention: In order to prevent corrosion, metals must be protected from air. Air contains oxygen and water.

The following methods are generally used to prevent corrosion.

  • Barrier protection
  • Electroplating
  • Sacrificial protection.

Q13. What is Electroplating?

Ans.    Electroplating is the art of depositing one metal over another with the help of the electric current; the electroplating is done for two purposes.

(i) Preservation               (ii) Decoration

For electroplating an object we must connect it with cathode. The anode is made of metal which are want to electroplate.

There are several types of electroplating processes classified on the basis of type of compound which is used for electroplating coating. Some of these are.

  • Electroplating of tin
  • Zinc plating
  • Silver electroplating
  • Chromium electroplating.
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