physical chem; electrode potentials & electrochemical cells

Question 1

What is a half cell?

Answer 1

Its one half of an electrochemical cell

Question 2

In what ways can half cells be constructed?

Answer 2

A metal is dipped into a solution containing its ions, or a plantinum electrode with 2 aqueous ions

Question 3

What else is used in an electrochemical reaction with 2 aqueous ions?

Answer 3

An inert but electrically conductive electrode — e.g platinum

Question 4

What are electrochemical cells are made?

Answer 4

2 half cells joined by a wire, voltmeter & a salt bridge

Question 5

What happens when 2 half cells are connected?

Answer 5

On one side a reduction reaction occurs and the other side an oxidation reaction — redox reaction

Question 6

What does an electrode potential value tell us?

Answer 6

How easily the half cell gives up e- (oxidised) — measured in volts

Question 7

How are electrochemical cells written (equation form)?

Answer 7

In a reduced form — reduction in forward direction

Question 8

NO PRoblem

Answer 8

The most Negative half cell will undergo Oxidation
The most Positive half cell with undergo Reduction

Question 9

What is the standard hydrogen electrode (SHE) used for?

Answer 9

Used as a reference to measure standard electrode potentials

Question 10

SHE value

Answer 10

0.00V

Question 11

SHE standard conditions

Answer 11

• temperature 298K
• pressure 100kPa
• conc of ions at 1 moldm-3

Question 12

Electrochemical series

Answer 12

It’s a list of half cell reactions & their standard electrode potentials

Question 13

How is the standard cell potential calculated?

Answer 13

E cell = E reduced - E oxidised

Question 14

Cell Notation

Answer 14

Standard way of representing cells:
Reduced form|Oxidised Form||Oxidised form|Reduced form

Question 15

What are standard electrode potentials used to predict?

Answer 15

If a stated reaction is likely to proceed under standard conditions

Question 16

Batteries

Answer 16

Are electrochemical cells that come in 2 main forms — rechargeable & non-rechargeable

Question 17

Example of rechargeable battery

Answer 17

Lithium ion batteries

Question 18

Difference between rechargeable & non-rechargeable

Answer 18

Non-rechargeable are cheaper
Rechargeable are reversible & can last longer — chaper in the long term

Question 19

How is a negative electrode identified?

Answer 19

By finding which electrode is producing the e- (oxidation)

Question 20

How is electricity generated?

Answer 20

By a continuous external supply of chemicals

Question 21

Example of a fuel cell

Answer 21

An alkaline hydrogen-oxygen fuel cell

Question 22

Describe how an alkaline hydrogen-oxygen fuel cell works

Answer 22

1. Hydrogen feed — H reacts with OH- ions in solution
2. Flow of e- — e- produced in reaction travel through platinum electrode
3. Component — the flow of e- is used to power something
4. Oxygen feed — O reacts with water, & 4 e- forming OH- ions
5. Negative electrode (cathode) — e- flow to the -ve electrode
6. Electrolyte — made from KOH solution, carries the OH- ions from the cathode to the anode
7. Positive electrode (anode) — e- flow from +ve electrode
8. Water emitted
9. Movement of OH- ions — ions carried towards anode via electrolyte

Question 23

Ion exchange of membranes

Answer 23

Lines electrode (e.g. platinum) & allows ions (e.g. OH-) to pass through but not gas (e.g. H & O)

Question 24

Advantage of fuel cells

Answer 24

• fuel cells are more efficient as more energy is converted into KE, engines waste a lot of thermal energy
• fuel cells do not need to be recharged whereas batteries do — they need supply of H & O
• only waste product is water, no CO2 emitted

Question 25

Disadvantage of fuel cells

Answer 25

- H is highly flammable & must be stored & transported correctly - Expensive to transport & store H, stored in pressurised containers - Energy is required to make the H & O, fossil fuels are used to pass water through electrolysis process — use of fossil fuels contributes to CO2 emissions