Redox II Flashcards
What does an electrochemical cell do?
Converts chemical energy into electrical energy using a redox reaction
Draw an electrochemical cell set up (zinc sulphate solution and copper sulphate solution):
What is each metal that comes in contact with a solution of its ions called?
Half-cell
What happens at the negative electrode?
- Oxidation
- Zn (s) –> Zn2+ (aq) + 2e-
- So Zn electrode gets lighter over time
What happens at the positive electrode?
- Reduction
- Cu2+ (aq) + 2e- –> Cu(s)
- So Cu electrode gets heavier over time
What does the overall equation look like?
Zn(s) + Cu2+ (aq) ⇌ Zn2+ (aq) + Cu(s)
Describe what is happening in the digram:
- The electrons flow in a clockwise direction, from the zinc rod to the copper
- The salt bridge, completes the circuit by allowing the passage of ions from the copper sulphate solution to the zinc sulphate solution. This salt bridge is usually a strip of filter paper soaked in saturated potassium nitrate
What does the standard hydrogen electrode measure?
Standard for half-cell potentials. It has a cell potential of 0.00V, measured under standard conditions
What are the standard conditions?
- Solutions of 1.0 mol dm-3 concentration
- 298K
- 100KPa (1 atm)
What is SHE used for?
For reference on all half-cell potentials as it has a standard electrode potential of zero
What do positive potentials mean?
The substances are more easily reduced and will gain electrons
What do negative potentials mean?
The substances are more easily oxidised and will lose electrons to be come more stable
Draw an example of a standard hydrogen electrode that consists of HCl, H2 and platinum electrodes and give the equation of the reaction taking place:
Why are platinum electrodes used?
They are metallic, so will conduct electricity, but inert, so will not interfere with the reaction
How are conventional cells represented?
- 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
- State symbols are always included
How do you calculate cell Emf?
EMF∅ = E∅ cathode (RHS) - E∅ anode (LHS)
What does E∅ being negative mean?
- Less likely to be reduced, more likely to be oxidised
- Position of equilibrium is to the left
What does E∅ being positive mean?
- More likely to be reduced, less likely to be oxidised
- Position of equilibrium is to the right
How do you calculate E∅ cell?
E∅cell = reduced - oxidised
How do you know whether a reaction is feasible or not using electrode potentials?
A reaction is feasible if the E∅cell value is positive overall
What are the limitations of predictions using E∅cells?
- They have been worked out using a standard hydrogen electrode and done under standard conditions. However, this may not necessarily be the case when done under lab conditions
- Sometimes the reaction is feasible but is prevented from happening because the reaction has a very high activation energy, which means that at the temperature being used the reaction will be extremely slow. “kinetically unfeasible”
- Barrier eg/ Al2O3 layer on Al surface so Al + acid - really slow
What does a hydrogen fuel cell look like?
Give the half equations happening at the anode and the cathode of the hydrogen fuel cells?
Anode: H2 –> 2H+ + 2e-
Cathode: 4H+ + O2 + 4e- –> 2H2O
Give the overall equation happening at the anode and the cathode of the hydrogen fuel cells?
2H2 + O2 –> 2H2O
Why can this process be describe as green?
Only product is H2O, no CO2/CO/NOx produced
- However electricity to recharge the fuel cell may not be from ‘green’ sources
Give some uses for a hydrogen fuel cell
Boats and submarines
Plane, trains, buses
Name a suitable metal for making the electrodes from and explain your answer
Platinum - it’s inert so won’t react with H2O/H+
Would a hydrogen fuel cell work indefinitely or would it eventually stop working?
No - in theory it should work indefinitely but actually the electrode can be ‘poisoned’ by binding to heavy metals or problems with leaking gases/cracks
Are there any safety issues with this system?
- H2 is flammable + explosive if mixed with O2
- Very thick walled cylinders and pipes are needed to store hydrogen which has economic impacts
- The production of hydrogen is a by-product of the crude oil industry, which means it relies on a non-renewable, finite resource
