Chapter 24 Electrochemistry Flashcards
Standard Cell Potential, ΔE°cell
- The voltage of an electrochemical cell made up of two half-cells at standard conditions.
- Could be any half-cells and neither have to be a standard hydrogen electrode
- Electromotive force (emf)
- Can be measured by voltmeter
Emf of a cell depends on?
- Nature of the electrodes & ions
- Temperature
- Concentrations
Standard Hydrogen Electrode (SHE)
- The ultimate reference when calculating standard cell potential, ΔE°cell
- The SHE is assigned an E° value of 0.00 V
Diagram of SHE
- Hydrogen gas at 100 kPa
- Platinum electrode
- Solution of 1M H+ (aq), e.g. 1M HCl or 0.5M H2SO4
- 298 K or 25 °C
Standard electrode potential
Potential of a half cell compared to the standard hydrogen electrode (SHE)
E°cell formula
E°cell = E°reduction - E°oxidation
- Be careful with sign for E°oxidation
The 4 types of half-cells
- Metal & Ion
- Gases & Ion
- Systems involving oxidation or reduction (ion / ion): solutions of ions in 2 different oxidation states & solutions of oxidising agents in acid solution
Describe the metal / ion half-cell. Use copper (Cu) as an example.
- Reaction
- Electrode
- Solution
- Potential
- R: Cu2+ + 2e- <> Cu (s)
- E: Copper
- S: 1M Cu2+ (aq) / copper sulphate solution
- P: + 0.34 V
Describe the gas / ion half-cell. Use H2 & Cl2 as an example.
- Reaction
- Electrode
- Solution
- Gas
- Potential
H2
- R: 2H+ (aq) + 2e- <> H2 (g)
- E: platinum
- S: H+ (aq) (1M) / 1M HCl / 0.5M H2SO4
- G: H2 at 100 kPa (1 atm)
- P: 0 V
Cl2
- R: Cl2 (g) + 2e- <> 2Cl- (aq)
- E: platinum
- S: Cl- (aq) (1M) / 1M NaCl
- G: Cl2 (g) at 1 atm
- P: +1.36 V
Describe the ion / ion half cell, specifically the solutions of ions in 2 different states. Use Fe3+ & Fe2+ as an example.
- Reaction
- Electrode
- Solution
- Potential
- R: Fe3+ (aq) + e- <> Fe2+ (aq)
- E: Platinum
- S: Fe3+ (aq) (1M) & Fe2+ (aq) (1M)
- P: +0.77 V
Describe the ion / ion half cell, specifically the solutions of oxidising agents in acid solution. Use MnO4- as an example.
- Reaction
- Electrode
- Solution
- Potential
- R: MnO4- (aq) + 8H+ (aq) + 5e- <> Mn2+ (aq) + 4H2O (l)
- E: Platinum
- S: MnO4- (aq) (1M) & Mn2+ (aq) (1M) & H+ (aq)
- P: +1.52 V
Formula for standard cell potential.
Ecellꝋ = Ereductionꝋ – Eoxidationꝋ
- Ecellꝋ > 0 - spontaneous reaction
- Ecellꝋ < 0 - non-spontaneous reaction
Effect of Concentration on E° values.
- Result from changes in concentrations & can be predicted with the Le Chatelier principle.
- Concentration only applies to ions!
What happens when [Cu2+] increases?
- Cu2+ + 2e- <> Cu
- Equilibrium position shifts to the right
- Cu electrode becomes more positively charged
- E° becomes more positive
What happens when [Cu2+] decreases?
- Cu2+ + 2e- <> Cu
- Equilibrium position shifts to the left
- Cu electrode becomes more negatively charged
- E° becomes more negative
