Chemistry A2 - REDOX and eletrode potentials (23)

Question 1

What are the steps for balancing half equations?

Answer 1

1) Write down the species before and after a reaction
2) Balance any atoms apart from oxygen and hydrogen
3) Balance any oxygens with water
4) Balance any hydrogens with H+
5) Balance charges with electrons

Question 2

What is the half equation which shows that (MnO4)- has been reduced?

Answer 2

(MnO4)- + 8H+ + 5e- —> Mn2+ + 4H2O

Question 3

Why add excess dilute sulfuric acid into the conical flask containing the reducing agent when carrying out a redox titration?

Answer 3

To ensure there are sufficient H+ ions to allow the reduction of the oxidising agent

Question 4

What is a half cell?

Answer 4

Half of an electrochemical cell

Question 5

When do we use platinum in a half cell?

Answer 5

When we have a half cell with 2 aqueous ions

Question 6

Why do we use platinum in half cells?

Answer 6

It is inert and so doesn’t react with any of the chemicals or ions in the beaker

Question 7

How is an electrochemical cell formed?

Answer 7

By joining 2 different half cells together

Question 8

What are the two half cells in an electrochemical cell joined by?

Answer 8

A wire, voltmeter, and a salt bridge

Question 9

Where do the electrons flow in an electrochemical cell?

Answer 9

From the more reactive metal to the less reactive one

Question 10

What is the salt bridge made from and what is its purpose?

Answer 10

It is filter paper with saturated KNO3 solution. Ions flow through it which balances the charges

Question 11

What is the electrode potential measured in and what does it tell us?

Answer 11

It is measured in volts and tells us how easily the half cell gives up electrons (oxidised)

Question 12

How are the equations in electrochemical cells always written?

Answer 12

In the reduced form, meaning we always show the equations with reduction in the forward direction

Question 13

How do we know which element is undergoing oxidation/reduction by using the electrode potentials?

Answer 13

Using the NO PRoblem rule:
The most Negative half cell will undergo Oxidation
The most Positive half cell will undergo Reduction

Question 14

How to we measure electrode potentials?

Answer 14

By using the standard hydrogen electrode. We compare them to this which has an electrode potential value equal to 0.00V

Question 15

What are the conditions for standard electrode potentials?

Answer 15

1. Temperature at 298K
2. Pressure at 100kPa
3. Concentrations of ions are [1]

Question 16

How do we calculate the standard cell potential?

Answer 16

Reduced - oxidised

Question 17

What are the four steps for carrying out a manganate(VII) titration?

Answer 17

1. A standard solution of potassium manganate(VII) is added to the burette
2. Using a pipette, add a measured volume of the solution being analysed into the conical flask. An excess of dilute sulfuric acid is also added to provide the H+ for the reduction of (MnO4)-
3. During the titration, the manganate(VII) solution reacts and is decolourised as it is added. The end point is judged by the first permanent pink colour, indicating an excess of (MnO4)- ions
4. Repeat until you obtain two concordant titres

Question 18

Why do we read the meniscus from the top when the solution is potassium manganate(VII)?

Answer 18

It is a very deep purple colour and so it is difficult to see the bottom

Question 19

What is the equation for the oxidation of thiosulfate ions?

Answer 19

(2S2O3)2- –> (S4O6)2- + 2e-

Question 20

What is the equation for the reduction of iodine?

Answer 20

I2 + 2e- –> 2I-

Question 21

What colour is CuI (s)?

Answer 21

White

Question 22

What are primary cells?

Answer 22

They are non-rechargeable and designed to be used once only

Question 23

Why are primary cells only used once?

Answer 23

When in use, electrical energy is produced by oxidation and reduction at electrodes. However, the reactions cannot be reversed. Eventually the chemicals will get used up, voltage will fall, the battery will go flat, and the cell will be discarded or recycled

Question 24

What are secondary cells?

Answer 24

They are rechargeable

Question 25

Why are secondary cells rechargeable?

Answer 25

The cell reaction producing electrical energy can be reversed during recharging

Question 26

What are 3 common examples of secondary cells?

Answer 26

• lead-acid batteries used in car batteries
• nickel-cadmium cells and nickel-metal hydride used in radios and torches
• lithium-ion and lithium-ion polymer cells used in modern appliances such as laptops, tablets, and phones

Question 27

What is a fuel cell?

Answer 27

A cell which uses the energy from the reaction of a fuel with oxygen to create a voltage

Question 28

What are 3 properties of fuel cells?

Answer 28

• The fuel and oxygen flow into the fuel cell and the products flow out. The electrolyte remains in the cell
• They can operate continuously provided fuel and oxygen are supplied into the cell
• They do not have to be charged

Question 29

Why is hydrogen the most common fuel for a fuel cell?

Answer 29

They produce no carbon dioxide during combustion as water is the only product