short question answers chapter 10

 Chapter # 10            chemistry first year        F.Sc                     

Electro chemistry

Q1. What is Electrochemistry?

Ans. Electrochemistry is the branch of chemistry which deals with the conversion of electrical energy into chemical energy in the electrolytic cells and vice versa.

Q.2. what are electrolytic cells and voltaic cells?

Ans.  An Electrochemical cell (in which electric current is used to derive a non spontaneous reaction) is called Electrolytic cell.  During this nonspontaneous reaction the substance is deposited at respective electrodes and electrolyte is decomposed. E.g., Down cell, Nelson’s cell,etc.

Voltaic or Galvanic cell:  an electrochemical cell in which a spontaneous redox reaction generates electric current is called Voltaic or Galvanic cell.  A Galvanic cell consist of two half cells. Each half cell is a portion of cell where half reactions takes places e.g., Daniel cell, Fuel cell.

Q.3 what is the difference between ionization and electrolysis?

Ans.  Ionization is a process in which molten (fused) or solid ionic compound is dissolved in water splits into charged ions(positive and negative)   or particles.

                    NaCl                                                   Na⁺ (aq)    +    Cl^-                (aq)

Electrolysis is a process in which nonspontaneous reaction takes place at the expense of electrical energy. Substances (molten compounds or ions in electrolyte) are deposited at electrodes and electrolyte is decomposed.

Q.4 How the conduction through metals occurs?

Ans. In metals electric current passes by the flow of electrons. No chemical change takes place in metals.

Conduction is decreased by increase of temperature.

Q.5. How the conduction through molten electrolyte occurs?

Ans.  Electric current is carried by movement of charged ions in molten electrolyte.  Chemical change occurs in molten electrolyte. Conductivity is increased with the increase of temperature.

Q.6. what is electrolytic conduction?

Ans.  Electrolytic conduction is the passage of electric current through electrolyte in molten state or in the solution of ionic compounds.

Q.7. what is salt bridge? Describe its function in Galvanic cell?

Ans. A salt bridge is U-shaped tube filled electrolyte like KCl or KNO₃ in gelatin. It serves three purposes.

1. It allows electrical contact between two electrolytic solutions.
2. It prevents mixing of two electrolytic solutions.
3. It maintains electrical neutrality in each half cell.

Q.8. Salt bridge maintain electrical neutrality in galvanic cell explain.

Ans.  Galvanic cell consist of two half cells that are electrically connected by external circuit and are connected by a salt bridge internally. The electrons flow in external circuit (from anode to cathode.)

As oxidation and reduction proceeds at electrodes ions from salt bridge migrates to neutralize charge in separate half cells. Cations flow in the solution opposite to the direction of electrons. Thus a salt bridge maintains the electrical neutrality in Galvanic cell.

Q.9. what is a battery?

Ans. Battery is portable self contained electrochemical power source that consists of one or more voltaic cells. Greater voltages can be achieved by using multiple voltaic cells in a single battery. A 12volt battery contain 6 voltaic cell connected in series.

Q.10. What is cell?

Ans. Cell is a system consisting of electrodes that dip into an electrolyte  and chemical reaction either  uses or generate electric current. Cell is not Self contained system e.g fuel cell. Nelson’s cell.

Q.11. what is difference between a primary and secondary battery?

Ans. Primary battery is not rechargeable, e.g. alkaline battery,

         Secondary Battery is rechargeable e.g.  Lead acid accumulator, nickel cadmium battery.

Q.12. What is lead accumulator (or lead acid Battery)?

Ans. Lead accumulator is used as a car battery. It is a secondary battery. A 12V car battery consists of voltaic cells in series, each can produce 2V. The cathode of each cell consists of lead dioxide, PbO₂ , packed on a metal grid. The anode of each cell is composed of lead. Both electrodes are immersed in 30% sulphuric acid.

Q.13. write-down the electrode reactions that occur during discharge in lead storage battery.

Ans.  Lead acts as anode and PbO₂ acts as cathode in lead storage battery.

At Cathode:

PbO_{2 (s)}    +     SO₄^-2 _(aq) + 4H⁺  _(aq)  + 2e^-                                                           PbSO_{4 (s)} + 2H₂O            (reduction)

At Anode:

Pb _(s)    +     SO₄^-2 _(aq)                                                                                                  PbSO_{4 (s)} + 2e^-       (oxidation)

Overall reaction: PbO_{2 (s)}    +    Pb _(s)   + 2SO₄^-2 _(aq) + 4H⁺  _(aq)                               2 PbSO_{4 (s)}   +  2H₂O

At anode the lead atom release two electrons which pass round an external circuit as an electric current. At the cathode these electrons are accepted by  PbO₂ and H⁺.

Q.14.Lead acid accumulator is a chargeable battery. Explain it.

Ans. Lead acid accumulator is a Secondary battery. During the process of recharging the anode and cathode of the external source are connected to the anode and cathode of external source are connected to anode and cathode of all cell respectively. The redox reactions are reversed on respective electrodes regenerating  PbO_{2 (s)} and   Pb _(s).

Overall reaction:

2PbSO_{4 (s)}   +  2H₂O                                           PbO_{2 (s)}    +    Pb _(s)   + 2SO₄^-2 _(aq) + 4H⁺  _(aq)

Q.15. A salt bridge is not required in  lead acid battery. Why?

Ans.  The reactants Pb and PbO2 between which electron transfers occur serves as the electrodes, are solid and they cannot come into direct physical contact. That’s why salt bridge is not required to separate them into anode and cathode compartment in lead storage battery.

Q.16. what is alkaline battery or dry alkaline cell?

Ans. In dry alkaline cell, zinc rod acts as anode and manganese dioxide as the cathode. The electrolyte is KOH. The reactions are..

At anode: Zn (s)  +    2OH^- (aq)                                                    Zn(OH)₂   + 2e^-   (oxidation)

At cathode:  2MnO₂   +  H₂O   + 2e^-                                                                   Mn₂O_3(s)   + 2OH^-  (oxidation)

Overall reaction: Zn (s) +   2MnO₂   + H₂O _(l)                                                                 Zn (OH)₂  +  Mn₂O_3(s)

The voltage of cell is 1.5V.

Q.17. what is nickel cadmium cell (NiCad battery).

Ans. It is voltaic cell consisting of anode of cadmium and a cathode of nickel oxide, NiO₂ .The electrolyte is KOH. The half cell reactions during discharge are.

At Anode:  Cd _(s)   + 2OH^- (aq)                                                       Cd (OH)_{2 (s)}    +  2e^-   (Oxidation)

At Cathode:

NiO_{2 (s) +} 2 H₂O   + 2e^-                                                                          Ni (OH)_{2 (s)}  +2OH^-  (reduction)

Overall reaction:

Cd _(s)   +  NiO_{2 (s) +} 2 H₂O                                                   Cd (OH)_{2 (s)}     +   Ni (OH)_{2 (s)}

Rechargeable cells are used in portable calculators, cordless razors and photoflash units. Etc.

Q.18. what is silver oxide battery?

Ans. It is a voltaic cell consisting of an anode zinc metal and a cathode of silver oxide Ag₂O. The electrolyte used is basic in nature.

At Anode:  Zn _(s)   + 2OH^- (aq)                                                       Zn (OH)_{2 (s)}    +  2e^-   (Oxidation)

At Cathode:

Ag₂O _{(s) +} 2 H₂O   + 2e^-                                                                         2Ag _(s)  +2OH^-  (reduction)

Overall reaction:

Zn _(s)   + Ag₂O _{(s) +} 2 H₂O                                                    Zn(OH)_{2 (s)}     +   2Ag_(s)

It is used as power source in electric watches auto exposure cameras and electronic calculators etc.

Q.19. what is fuel cell?

Ans. A Galvanic cell in which reactants are continuously fed into the cell as the cell produces electrical energy is called fuel cell.

Q.20. How fuel cell can be used as drinking source for an astronaut?

Ans. The fuel cell H₂ is operated at high temperature so that the water formed as a product of cell reaction evaporates and may be condensed and used as drinking source for an astronaut.

Q.21. How power is generated by using fuel cell?

Ans. Fuel cells are voltaic cells that utilizes redox reactions using  H₂ as fuel. The fuel H₂ is oxidized to water, releases electrons and Oxygen is reduced to OH^- ions by accepting electrons. This flow of electrons develops the electrical current thus power is generated.

At Anode:  [H_{2 (g)}   + 2OH^- (aq)                                                                     2H₂O _(s)    +  2e^- ]x2   (Oxidation)

At Cathode:

O _{2(g) +} 2 H₂O   + 4e^-                                                                                   4OH^-  (reduction)

Overall reaction:

2H_{2 (g)}   +  O_2(g)                                                                                     2H₂O (aq)

The electrical power production requires the burning of a fuel to vapourize water, which can be used to run turbines connected to electrical generators.

Q.22. what is standard electrode potential?

Ans. The potential setup when an electrode is in contact with one molar solution of its own ions at 298K. Measured relative to standard Hydrogen electrode is called standard electrode potential or standard reduction potential of the element. It is represented as E.    Standard electrode potential of hydrogen has  arbitrarily been chosen to zero while for other elements can be determined by comparing them with standard hydrogen electrode.

Q.23. what is standard Hydrogen Electrode (S.H.E)?

Ans. SHE stands for standard hydrogen electrodes.  It is based on following reaction.

2H⁺ _(aq)       +2e^-                                                                    H_{2 (g)}

It consist of electrode with finely divided Pt in contact with H₂  at 1atm pressure and 1M HCl solution at 25c.

Q.24. How the Daniel cell is represented?

Ans. Galvanic cell (Daniel cell)  are represented by a shorthand notation called a cell diagram. Notations for Daniel cell are :

Zn _(s) | Zn⁺² _(aq) 1M || Cu⁺² _(aq) | Cu_(s)                 E    = 1.1V

Vertical lines represented the phase boundaries and a double line || represented a salt bridge. Anode or oxidation half cell is always written on left the cathode or reduction half cell is written on right side.

Q.25. What are redox reactions?

Ans. Chemical  reactions in which electrons are transferred between two (or more) species or those reactions in which oxidation state of one or more substance changes are called Redox-reactions or

Oxidation reduction reactions. Oxidation refers  to loss of electrons while reduction Refers to gain of electrons.

          Oxidation:  Zn _(s)                                                                     Zn⁺²  _(aq)    

         Reduction:     2H⁺     + 2e^-                                                      H_{2   (g)}

Electrons are transferred from Zn to hydrogen ions.

Q.26. Define Anode and Cathode.

Ans. In the electrochemical cell electrode at which Oxidation occurs called anode.

   Electrodes at which Reduction  occurs is called Cathode.

Q.27. Na, K can displace hydrogen from acid but Pt, Pd, and Cu cannot why?

Ans. Greater  value of standard reduction potential value of a metal it has less tendency to lose electrons to form metal ions as well as it become weaker to displace hydrogen from acids. Pt, Pd, Cu have high reduction potential  so they cannot displace hydrogen while Na, K have low reduction potential.

Q.28. what is electromotive force (emf)?

Ans. Maximum potential difference between electrodes of a voltaic cell is called electromotive force (emf) or  E_cell . Because E_cell is measured in volts so it is also called cell voltage.

Q.29. what is Electroplating?

Ans. The process which involves using electrolysis to deposit a thin layer of one metal on another metal in order to improve shining or resistance to corrosion is called electroplating.

Q.30. How copper can be purified Electrolytically?

Ans. Electrolytic cell  is used for purification of copper. Impure Copper is made the anode and a thin sheet of pure copper sheet is made the cathode. These electrodes are dipped in CuSO₄ solution. The atoms of Cu from impure Copper converted to Cu⁺² ions and migrate to cathode which is made of pure Cu. By this process Cu is purified.

Q.31. How Al anodized in an Electrolytic cell?

Ans. Anodized aluminum is prepared by making Al metal an anode in an electrolytic cell containing H2SO4 or H2CrO4 which coat a thin layer of oxide on it. This layer of aluminum oxide is anodized aluminum.

Q.32. Define the Oxidizing agent and Reducing agent.

Ans. Oxidizing agent  is a substance which oxidizes other substance and itself reduces in a chemical reaction. Reducing agent  is a substance which reduces other substances and itself oxidized.

                                               

                                                                                                                                                                Prepared by:

                                                                                                                                                RanaMuhammad Asif

    Lecturer Chemistry