A2/A-level Chemistry: Standard Electrode Potential

Let's go over some specific terms about electrochemistry mentioned in A-level Chemistry!

• 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)

• 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 = E^o_reduction – E^o_oxidation = 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)

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 E_cell is on LHS of the equation.

• Stronger reducing agent with the more negative E_cell is on RHS of the equation. 
• **Strong oxidizing agent = weak reducing agent, vice versa

Remarks:

• Eº_cell > +0.6V ---> reaction should go to completion
• 0V < Eº_cell < +0.6V ---> products predominate
• -0.6V < Eº_cell < 0V ---> reactants predominate
• Eº_cell < -0.6V ---> NO reaction

• A reaction with a positive Eº_cell  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~

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