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Let's go over some specific terms about electrochemistry mentioned in A-level Chemistry!
Definition
- Electromotive force (E.m.f.): the ‘push’ that a cell is able to provide to a current flowing through it (volts)
- Standard electrode potentials (SEP): the potential difference of a half-cell relative to the standard hydrogen electrode (SHE)
Electrochemical cell
- e.g. zinc-copper cell
- Component: zinc and copper electrode, metal ion solution, salt bridge, high-resistance voltmeter
- Oxidation: Zn electrode loses electron to form zn2+ ions
- Reduction: Cu2+ ions in solution gain electrons to form Cu metal
- Zinc electrode decreases in mass while copper electrode increases

- Half-cell potential for copper = +0.34V
- Half-cell potential for zinc = -0.76V
- Overall reaction potential = Eoreduction – Eooxidation = 1.10V
- E.m.f. calculated by reversing the least positive value then add up two half-cell potentials (if not familiar with the minus sign)
- For a feasible reaction, cell potential MUST BE POSITIVE (usually over +0.6V)
Measurement of standard potential with SHE
Standard hydrogen electrode (SHE)

- Reference half-cell (with standard potential 0.00V)
- Used to measure other electrode potential
- Condition: Hydrogen gas at 1 atm pressure, 298K, inert platinum electrode in 1.00 mol dm-3 H+(aq) ions

- Anode: Metal electrode
- Cathode: Standard Hydrogen Electrode (SHE)
- other components include metal electrolyte, sald bridge, voltmeter
- Oxidation half-cell: Metal(s) ---> Metal ion(aq) + electrons
- Reduction half-cell: H+(aq) + electron ---> 1/2 H2(g)
- Overall reaction: Metal(s) + H+(aq) ---> Metal ion(aq) + H2(g)

- In A-level Chemistry, the condition of standard potential is always at 1 atm pressure and 298K
Stronger oxidising agent with the more positive Ecell is on LHS of the equation.
- Stronger reducing agent with the more negative Ecell is on RHS of the equation.
- **Strong oxidizing agent = weak reducing agent, vice versa
Remarks:
- Eºcell > +0.6V ---> reaction should go to completion
- 0V < Eºcell < +0.6V ---> products predominate
- -0.6V < Eºcell < 0V ---> reactants predominate
- Eºcell < -0.6V ---> NO reaction
Limitations
- A reaction with a positive Eºcell may not actually take place – gives no indication of reaction rate
- The reaction conditions may be very different from standard conditions
- SEPs ONLY apply to aqueous equilibria, still there are many reactions that take place which are not in aqueous solutions
That's all~
