AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The redox potential for a transition metal ion changing from a higher to a…
Full specification wording
Variable oxidation states (A-level only): The redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by pH and by the ligand.
A standard electrode potential applies only under standard conditions, and two things routinely shift it.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
Study notes
From the checked teaching notes for this specification point.
The idea
A standard electrode potential applies only under standard conditions, and two things routinely shift it. The first is pH: many reduction half-equations consume H⁺, as in MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. Raising the acid concentration shifts that equilibrium to the right, making the potential more positive and manganate(VII) a stronger oxidising agent; in alkaline conditions it is far weaker and is reduced only as far as MnO₂. The second is the ligand: replacing water changes how readily the metal gives up an electron. [Co(H₂O)₆]²⁺ is barely oxidised by air, but add ammonia and the resulting cobalt(II) ammine complex is oxidised to cobalt(III) readily, because ammonia stabilises the higher oxidation state.
What earns the marks
- Quoting a half-equation that contains H⁺ to show the pH dependence.
- Applying Le Chatelier's principle to the effect of changing the H⁺ concentration on that equilibrium.
- Stating the direction in which the electrode potential moves and what that does to oxidising power.
- A ligand example, such as ammonia making cobalt(II) far easier to oxidise than water does.
Common mistakes and how to put them right
Assuming standard electrode potentials apply whatever the pH or the ligand.
Claiming acid always makes a species a better reducing agent, when for an oxoanion it does the opposite.
Saying a reaction cannot happen because standard potentials forbid it, while ignoring the non-standard conditions given.
Confusing the effect of a ligand on colour with its effect on redox potential.
Exam tips
- Look for H⁺ in the half-equation, because if it is there then pH will change the potential.
- Use the phrase the equilibrium shifts to the right when arguing the effect of added acid.
- Say explicitly whether the species becomes a stronger or a weaker oxidising agent.
Try the lesson
The start of the Variable oxidation states (A-level only): The redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by pH and by the ligand. lesson, made from checked teaching notes for this specification point.
Have a go first
The redox potential for the reduction of VO₂⁺ to VO²⁺ in acidic solution is affected by pH. Which statement explains why the potential becomes less positive as pH increases?
Listen
A standard electrode potential applies only under standard conditions, and two things routinely shift it. The first is pH: many reduction half-equations consume H⁺, as in MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. Raising the acid concentration shifts that equilibrium to the right, making the potential more positive and manganate(VII) a stronger oxidising agent; in alkaline conditions it is far weaker and is reduced only as far as MnO₂. The second is the ligand: replacing water changes how readily the metal gives up an electron. Co(H₂O)₆]²⁺ is barely oxidised by air, but add ammonia and the resulting cobalt(II) ammine complex is oxidised to cobalt(III) readily, because ammonia stabilises the higher oxidation state. A strong answer earns marks for these points. quoting a half-equation that contains H⁺ to show the pH dependence. applying Le Chatelier's principle to the effect of changing the H⁺ concentration on that equilibrium. stating the direction in which the electrode potential moves and what that does to oxidising power. a ligand example, such as ammonia making cobalt(II) far easier to oxidise than water does. Exam tip: Look for H⁺ in the half-equation, because if it is there then pH will change the potential.
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