Redox

Question 1

Define galvanic cell.

Answer 1

Electrochemical cells in which spontaneous redox reactions occur. They convert chemical energy to electrical energy. Also known as voltaic cell or Daniel cell

Question 2

Define electrode potential.

Answer 2

When there is a separation of charges and thus a potential difference between a metal electron and ion in solution arises.

Question 3

Define standard electrode potential.

Answer 3

Potential of reduction half-equation under standard conditions of 298K, 1 bar and 1 mol dm^-3 measured relative to S.H.E.

Question 4

Oxidation is when…

Answer 4

O.N of any element is increased, gain of oxygen or loss of electrons.

Question 5

Reduction is when…

Answer 5

O.N of any element is decreased, loss of oxygen or gain of electron.

Question 6

When is hydrogen’s charge not +1?

Answer 6

In metal hydrides, hydrogen has a -1 charge.

Question 7

When is oxygen’s charge not -2?

Answer 7

In peroxides, oxygen has a -1 charge.

Question 8

In a galvanic cell, where does oxidation occur ?

Answer 8

At the anode.

Question 9

What charge do anode and cathode have in galvanic cells ?

Answer 9

Anode is the negative electrode, cathode is the positive electrode.

Question 10

Why are the reactants separated in galvanic cells ?

Answer 10

If they are in direct contact, electrons will be directly transferred from one reactant to the other and can’t be used to provide current. Separating them creates a current.

Question 11

What is the purpose of salt bridges ?

Answer 11

They maintain the charge balance in each half-cell. Ion mobility for cation and anion of salt bridge must be similar.

Question 12

How are half-cells formed ?

Answer 12

Different chemical systems (ex: Cu and Zn), same solutions at different concentrations (ex: Cu at 1M, Cu at 2M) or the same gas at different pressure (hydrogen at 1 bar, hydrogen at 2 bar).

Question 13

What are the standard conditions ?

Answer 13

298K/25 degrees Celsius, 1 bar, concentration of solution is 1M and gas is bubbled at 1 bar (if there is gas).

Question 14

What is a standard hydrogen electrode made of ?

Answer 14

A platinum electrode coated with platinum black, solution containing 1M hydrogen ions, hydrogen gas at 100kPa pressure continuously bubbled through the solution over Pt electrodes.

Question 15

Why use platinum black coating ?

Answer 15

It helps to increase surface area of electrode.

Question 16

Define standard cell potential.

Answer 16

The potential difference between the cathode (reduction half-cell) and the anode (oxidation half-cell) of a cell.

Question 17

Define electrochemical series.

Answer 17

A series in which elements (metals and non-metals) are arranged in order of their standard electrode potential.

Question 18

Define redox series.

Answer 18

A series of all possible electrode potentials arranged in increasing values down the series (values get more positive down the series). Electrochemical series is part of the redox series.

Question 19

Meaning of standard cell potential.

Answer 19

More positive standard electrode potential, the stronger oxidising agent it is. More negative standard electrode potential, the stronger reducing agent it is.

Question 20

Define electrolytic cell.

Answer 20

Electrochemical cells in which non-spontaneous chemical reactions are made to occur by the forced input of electrical energy.

Question 21

Define electrolysis.

Answer 21

The process of using an electric current to bring about chemical change.

Question 22

What charge do anode and cathode have in electrolytic cells ?

Answer 22

Anode is the positive electrode. Cathode is the negative electrode.

Question 23

In electrolytic cells, where do oxidation and reduction occur ?

Answer 23

Oxidation is at the anode and reduction is at the cathode.

Question 24

Why is the voltage applied across the electrodes in an electrolytic cell higher than the standard cell potential ?

Answer 24

Because of electrical resistance and problem of overvoltage.

Question 25

Define overvoltage.

Answer 25

Slow rate of attainment of equilibrium at the electrodes.

Question 26

What is the formula of cryolite and why is it used in the electrolysis of aluminum oxide ?

Answer 26

Na3AlF6 is used to lower the high melting point of aluminum oxide.

Question 27

State Faraday’s first law.

Answer 27

The mass of an element liberated at an electrode during electrolysis is directly proportional to the quantity of electricity that passes through the electrolyte solution.

Question 28

State Faraday’s second law.

Answer 28

When the same quantity of electricity is passed through different electrolytes connected in series, the masses of different substances deposited or liberated are proportional to their chemical equivalent masses.

Question 29

Define BOD.

Answer 29

It’s the amount of oxygen needed to oxidize the organic components of a water sample over 5 days at a specific temperature.