topic 14

Question 1

standard electrode potential

Answer 1

• voltage measured under standard conditions when the half cell is connected to a standard hydrogen electrode

Question 2

what are standard conditions

Answer 2

• 298K
• 100 kPa pressure
• 1 mol dm-3 concentration

Question 3

features of standard hydrogen electrode

Answer 3

• hydrogen gas at 100 kPa
• solution with hydrogen ion at 1 mol dm-3
• temp at 298 K

Question 4

why is a reference electrode needed

Answer 4

• impossible to measure the absolute potential of a half electrode on its own
• it must be connected to another half-cell of known potential
• reference electrode is given a potential of 0V
• the p.d. can then be measured between the two

Question 5

setting up an electrochemical cell

Answer 5

1. obtain metals being investigated and clean with sandpaper to remove impurities
2. wash surface of metals with propanone and wear gloves
3. place each metal into solution containing the ion of the same metal
4. make salt bridge from filter paper soaked in saturated KNO3 or KCl
5. connect electrodes with wires, crocodile clips & voltmeter

Question 6

why is the surface washed with propanone

Answer 6

to remove oils

Question 7

why do we clean with sandpaper

Answer 7

to remove any impurities

Question 8

why is a salt bridge used

Answer 8

• to connect the circuit
• free moving ions conduct the charge

Question 9

why is a wire not used to connect the circuit

Answer 9

• metal wire would set up its own electrode system
• wires do not allow the flow of ions

Question 10

what happens if a current is allowed to flow

Answer 10

• reactions will occur separately at each electrode
• as V drops to 0, reactants are used up

Question 11

how to set up a zinc copper electrochemical cell

Answer 11

• zinc is the cathode so loses electrons more easily than copper so it’s oxidised
• copper is the anode so gains the electrons

Question 12

what happens at the anode

Answer 12

oxidation

Question 13

what happens at the cathode

Answer 13

reduction

Question 14

how is an electrochemical cell set up for different metals

Answer 14

• place the metals in solutions of their own ions that are connected with a wire to an external circuit to allow electron flow , form a salt bridge

Question 15

how to set up an electrochemical cell that forms an ion

Answer 15

• use a platinum electrode - as it needs to conduct electricity but be inert so it won’t react with anything else in the half cell
• use a solution containing 2 aqueous ions of the same element

Question 16

how to calculate standard emf, Ecell

Answer 16

Ereduction - Eoxidation

Question 17

cell diagram representation

Answer 17

R O O R

Question 18

hydrogen electrode equilibrium

Answer 18

H2 (g) ⇌ 2H+ + 2e-

Question 19

why are standard conditions needed

Answer 19

• the position of the redox equilibrium will change with conditions

Question 20

e.g. of eqb

Answer 20

Mx+ (aq) + x e- ⇌ M (s)

if Mx+ concentration increases, eqb will move to the right hence the potential is more positive

Question 21

what does the more positive E cell undergo?

Answer 21

• it acts as an oxidising agent - accepts electrons
• so undergoes reduction

Question 22

what does the more negative E cell undergo?

Answer 22

• it acts as a reducing agent - donates electrons
• so undergoes oxidation

Question 23

how to work out the Ecell from two electrode potentials

Answer 23

E red - E ox

Question 24

what determines whether a reaction will be feasible

Answer 24

• if the E cell value is positive, the reaction is likely to occur

Question 25

predicting reaction feasibility

Answer 25

- identify which cell is being oxidised - take oxidised equation and reverse it - combine the two equations - calculate the E cell value to confirm

Question 26

entropy, cell potential & eqb constant

Answer 26

- the larger the cell potential, the larger the total entropy change E ∝ ∆S total - entropy & eqb constant (K) are linked E ∝lnK

Question 27

effect of current on E cell

Answer 27

- if current is allowed to flow, the cell reaction will occur - the E cell will fall to zero as the reaction proceeds - the reactant concentration drops

Question 28

effect of concentration on E cell

Answer 28

- increasing concentration of reactants will increase E cell - decreasing will cause E cell to decrease

Question 29

effect of temperature on E cell

Answer 29

- most cells are exothermic - applying Le Chatelier to temp rise would result in a decrease in E cell because the eqb reactions would shift backwards

Question 30

disproportionation reactions - using electrode potentials

Answer 30

- electrode potential values can be used to calculate EMF - if EMF is positive, the disproportionation will occur - only possible under standard conditions

Question 31

fuel cell

Answer 31

- fuel cells use energy from the reaction of a fuel with oxygen to create a voltage

Question 32

how do fuel cells maintain a constant concentration of reactants

Answer 32

- they maintain a constant voltage overtime as they are fed continuously with fresh O2 and H2

Question 33

advantages of fuel cells

Answer 33

- less pollution and less CO2 - pure hydrogen emits only water whilst hydrogen rich fuels produce only small amounts of air pollutants and CO2 - greater efficiency

Question 34

limitations of hydrogen fuel cells

Answer 34

- expensive - storing & transporting hydrogen (in terms of safety, feasibility & limited life cycle of a solid adsorber or absorber) - limited lifetime - high production cost - use of toxic chemicals in their production