3.1 - Redox & SEP

Question 1

What is oxidation?

Answer 1

Oxidation is the loss of electrons

Question 2

What is reduction?

Answer 2

Reduction is the gain of electrons

Question 3

What is a redox reaction?

Answer 3

When both oxidation and reduction occur in the same equation simultaneously

Question 4

What happens to the species in a redox reaction?

Answer 4

The species being oxidised loses the electrons which are then donated and gained by the other species which is being reduced

Question 5

How do electrochemical cells use redox? (+ potential difference)

Answer 5

The electron transfer between products creates a flow of electrons, this flow of charged particles is an electrical current which flows between electrons in the cell.
Potential difference is produced between 2 electrodes which can be measured.

Question 6

What are the rules for oxidation numbers?

Answer 6

• elements = 0
• neutral compounds = 0
• charged compounds = total of the charge
• hydrogen = +1 (ex. metalhydrides where its -1)
• oxygen = -2 (ex. peroxides and F2O where it is -1)
• all halogens = -1
• G1 metals = +1

Question 7

What makes up an electrochemical cell?

Answer 7

2 solutions with metal electrodes, a voltmeter and a salt bridge

Question 8

What does a salt bridge do in an electrochemical cell?

Answer 8

A salt bridge is a tube of unreactive ions that can move between the solutions to carry the flow of the charge but will not interfere with the reaction
Usually made out of KNO3 or KCl

Question 9

Why are half equations used?

Answer 9

Used to show the separate oxidation and reduction reactions that occur in a redox reaction

Question 10

How do you balance half equations?

Answer 10

• balance all species except oxygen and hydrogen
• balance oxygen using H2O
• balance hydrogen using H+ ions
• balance charges using e- (electrons)

Question 11

How are standard cells represented?

Answer 11

• the half cell with the most negative potential goes on the left
• the most oxidised species from each half cell goes next to the salt bridge
• a salt bridge is shown using a double line
• always include state symbols

Question 12

What is a cell potential?

Answer 12

If measured under standard conditions, cell potentials are measured compared to the standard hydrogen electrode (SHE) to give a numerical value for the half cell potential

Question 13

What does a more positive cell potential mean?

Answer 13

The substances are more easily reduced and will gain electrons

Question 14

What does a more negative cell potential mean?

Answer 14

The substances are more easily oxidised and will lose electrons

Question 15

What is a standard hydrogen electrode and its conditions?

Answer 15

cell potential of 0.00V, measured under standard conditions
- solutions of 1 moldm-3 concentration
- temp of 298K
- 100kPa pressure

Question 16

What does a standard hydrogen electrode consist of?

Answer 16

Hydrochloric acid, hydrogen gas and platinum electrodes

Question 17

Why are platinum electrodes ideal for SHE?

Answer 17

Very useful as they are metallic,so they will conduct electricity but not interfere with the reaction

Question 18

Whats the equation for cell emf?

Answer 18

emf (cell) = E* (right) - E* (left)
- most postive potential minus the most negative potential
- reduced species potential minus oxidised species potential

Question 19

What does it mean if a cell emf if positive?

Answer 19

If the overall cell potential is a positive value, the reaction taking place is spontaneous and favorable
The more positive the cell potential, the more favourable the reaction

Question 20

What is the anticlockwise rule?

Answer 20

• write the most negative emf out of the pair on top
• draw anticlockwise arrows around the reactions
• balance the electrons on both sides of the reaction
• write out the cell reaction

Question 21

What are fuel cells?

Answer 21

Used to generate an electrical current wihout needing to be recharged

Question 22

What is the most common type of fuel cell?

Answer 22

The most common type of fuel cell is hydrogen which uses a continuous supply of hydrogen and oxygen in the air to generate a continuous current

Question 23

What is an advantage of using hydrogen fuel cells?

Answer 23

The reaction that takes place produces only water as a waste product, so relatively environmentally friendly

Question 24

What is a disadvantage of using hydrogen fuel cells?

Answer 24

• energy is required to produce a supply of oxygen and hydrogen, for example electrolysis of water which requires electricity, indicating that hydrogen fuel cells are not carbon neutral
• hydrogen is highly flammable so requires careful storage