1.11 Electrode Potentials and Cells

Question 1

What happens when a rod of metal is dipped into a solution of its own ions ?

Answer 1

An equilibria is set up between solid metal and aqueous metal ions

Question 2

Write a half equation for Zn (s) to Zn (II)

Answer 2

Zn (s) -> Zn 2+ + 2e-

Question 3

Write a half equation for Cu (II) to Cu (III)

Answer 3

Cu 2+ -> Cu 3+ + e-

Question 4

What is the simplest salt bridge made of ?

Answer 4

Filter paper soaked in KNO3

Question 5

Why are salt bridges necessary ?

Answer 5

•They complete the circuit, but do not react
•Ion movement balances the charge.

Question 6

What symbol is used to represent a salt bridge in standard notation ?

Answer 6

I I

Question 7

What does I indicate in standard notation ?

Answer 7

A phase boundary ( Solid I Liquid I gas).

Question 8

How would an aluminium/copper cell be represented in standard notation ?

Answer 8

Al (s) I Al 3+ II Cu 2+ I Cu (s)

Question 9

What happens at the left hand electrode in a chemical cell (if drawn correctly)

Answer 9

•Left hand electrode is where oxidation occurs
•Left hand electrode has the most negative E value.

Question 10

What happens at the right hand electrode in an electrochemical cell (if drawn correctly)

Answer 10

•Right hand electrode is where reduction occurs
•Right hand electrode is the half cell with the more positive E cell value.

Question 11

What side of the cell has the most negative Eo value ? What occurs there ?

Answer 11

Left hand electrode - oxidation

Question 12

What are the conditions for the standard hydrogen electrode used ?

Answer 12

•298K
•100kPa
•[H+] = 1dm-3.

Question 13

What is the standard hydrogen electrode used for ?

Answer 13

•Comparing other cells against Eo
•Eo of hydrogen is stated to be 0, so all other values are compared against it.

Question 14

Why might you use other standard electrodes occasionally?

Answer 14

They would be cheaper and easier to set up, platinum is expensive

Question 15

If the Eo of a reaction is more negative, what does this mean about the oxidising/reducing power ?

Answer 15

It means it’s a better reducing agent (easier to oxidise)

Question 16

If the Eo value is more positive, what does this mean about the oxidising/reducing power ?

Answer 16

Better oxidising agent (easier to reduce)

Question 17

What factors change Eo values

Answer 17

•Temperature
•Concentration of ions.

Question 18

What happens if you reduce the concentration of ions in the left hand side of an electrochemical cell ?

Answer 18

•Equilibrium shifts to oppose the change of removing the ions, this releases more e-
•The left Eo becomes more negative, making the EMF more positive.

Question 19

How do you calculate Ecell from Eo values ?

Answer 19

Ecell = right Eo - left Eo

Question 20

When would you use a platinum electrode ?

Answer 20

When both the oxidised and reduced versions of a species are in solution

Question 21

Why is platinum used as an electrode ?

Answer 21

•It’s inert so doesn’t take part in reactions
•Good conductor so completes the circuit.

Question 22

How would you predict if a reaction would occur in chemical cells ?

Answer 22

•Take both half equations
•Find the species that is being reduced
•Find it’s Ecell value and subtract the oxidation value from it
•If 0<, reaction will occur.

Question 23

What are disposable cells made from ?

Answer 23

Zinc/copper (II)

Question 24

What are the two reactions that take place in zinc/carbon cells ?

Answer 24

•Zn is oxidised into Zn2+
•NH4+ is reduced to NH3 at carbon electrode

Question 25

What reactions occur in a lead/acid battery ?

Question 26

How are cells recharged (if applicable)

Answer 26

Reactions are reversible and are reversed by providing a higher voltage through the cell than its Ecell

Question 27

Where are lithium ion cells used ?

Answer 27

Phones and laptop batteries.

Question 28

What reactions occur in lithium ion cells ?

Answer 28

•Li+ + e- + CoO2 -> Li+[CoO2]
•Li -> Li+ + e-

Question 29

What is a fuel cell ?

Answer 29

A cell that is used to make an electrical current, does not require recharging.