1.11 Electrode potentials

Question 1

How do electrochemical cells work?

Answer 1

• Use redox reaction between metals and the flow of electrons.
• Transfer of electrons produces a potential difference, which is measured.

Question 2

What is a half cell?

Answer 2

• A rod of metal dipped into a solution of its own ions.

Question 3

How do you make an electrochemical cell?

Answer 3

Two half cells are connected together by a salt bridge.

Question 4

What does the salt bridge do?

Answer 4

Transfers free ions to opposite electrodes (for red/ox)

Question 5

What are the conditions of a salt bridge?

Answer 5

Needs to be made of a salt that doesn’t react with any ions in the half cells.

Question 6

Which electrode is positive?

Answer 6

Cathode

Question 7

Which electrode is negative?

Answer 7

Anode

Question 8

At which electrode does reduction occur?

Answer 8

Cathode

Question 9

At which electrode does oxidation occur?

Answer 9

Anode

Question 10

What is the standard hydrogen electrode (SHE) ?

Answer 10

A half cell that is used to compare other metals, to see their tendency to release electrons.

Question 11

How does a standard hydrogen electrode work?

Answer 11

1. H₂ is bubbled through concentrated HCl.
2. H₂ is unreactive, so has no conduction, so a platinum wire is used as the electrode.
3. This half cell is connected to another metal half cell with a salt bridge.

Question 12

What are the conditions of the SHE?

Answer 12

• 298K
• 1 atm
• [H⁺] in HCl = 1 moldm¯³

Question 13

What is the potential of the SHE?

Answer 13

0

Question 14

What is the EMF?

Answer 14

The potential difference between the SHE and a metal.

Question 15

What is the EMF of a better reducing agent than the other half cell?

Answer 15

It has a more negative EMF.

Question 16

What are the rules to follow when representing half cells?

Answer 16

SOLID LINE: a phase boundary, between different states.

DOUBLE LINE: represents the salt bridge.

The highest oxidation state goes next to the salt bridge.

If there is no phase change, use a comma.

SHE is always on the left of the salt bridge.

(the most -ve EMF is on the left of the salt bridge)

Question 17

How do you find the E-cell of an electrochemical cell?

Answer 17

Most positive EMF - Most negative EMF (E,red - E,ox)

Question 18

How do you predict the redox reaction equation?

Answer 18

1. Write the most -ve reduction equation on top.
2. Write the less -ve reduction equation on top.
3. The top equation goes from RHS - LHS.
4. The bottom equation goes from LHS - RHS.
5. DO this to see which way the equation is, then recombine and balance.

Question 19

What must the E,cell be for the reaction to be feasible?

Answer 19

Positive

Question 20

When does the E,cell increase?

Answer 20

When the forward reaction is favoured.