Br2(l) + 2e– ⇌ 2Br–(aq) Eꝋ = +1.09 V
2H+(aq) + 2e– ⇌ H2(g) Eꝋ = 0.00 V
Na+ (aq) + e– ⇌ Na(s) Eꝋ = -2.71 V
2H+ (aq) + 2e– ⇌ H2(g) Eꝋ = 0.00 V
2H+ (aq) + 2e- ⇌ H2 (g)
The standard electrode potential of a half-cell can be determined by connecting it to a standard hydrogen electrode
Example of a metal / metal ion half-cell connected to a standard hydrogen electrode
Ag+ (aq) + e- ⇌ Ag (s) Eꝋ = + 0.80 V
2H+ (aq) + 2e- ⇌ H2 (g) Eꝋ = 0.00 V
Br2 (aq) + 2e- ⇌ 2Br- (aq) Eꝋ = +1.09 V
2H+ (aq) + 2e- ⇌ H2 (g) Eꝋ = 0.00 V
Example of a non-metal / non-metal ion half-cell connected to a standard hydrogen electrode
MnO4- (aq) + 8H+ (aq) + 5e- ⇌ Mn2+ (aq) + 4H2O (l) Eꝋ = +1.52 V
2H+ (aq) + 2e- ⇌ H2 (g) Eꝋ = 0.00 V
Ions in solution half cell
Ecellꝋ = Erightꝋ - Eleftꝋ
Ecellꝋ = Ereductionꝋ - Eoxidation
Calculating the standard cell potential
Calculate the standard cell potential for the electrochemical cell below and explain why the Cu2+ / Cu half-cell is the positive pole. The half-equations are as follows:
Cu2+(aq) + 2e- ⇌ Cu(s) Eꝋ = +0.34 V
Zn2+(aq) + 2e- ⇌ Zn(s) Eꝋ = −0.76 V
Answer
Step 1: Calculate the standard cell potential. The copper is more positive so must be the right hand side.
Ecellꝋ = Erightꝋ - Eleftꝋ
Ecellꝋ = (+0.34) - (-0.76)
= +1.10 V
The voltmeter will therefore give a value of +1.10 V
Step 2: Determine the positive and negative poles
The Cu2+ / Cu half-cell is the positive pole as its Eꝋ is more positive than the Eꝋ value of the Zn2+ / Zn half-cell
A helpful mnemonic for remembering redox in cells
Lio the lion goes Roor!
Lio stands for 'Left Is Oxidation' and he is saying ROOR because that is the order of species in the cell:
Reduced/Oxidised (salt bridge) Oxidised/Reduced
转载自savemyexams
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