Predicting feasibility and limitationsEdexcel A-Level Chemistry: Flashcards

What these 13 flashcards ask

When is a redox reaction thermodynamically feasible?
Does a positive E°cell mean the reaction will be fast?
State the equation linking E°cell and ΔS(total).
State the equation linking E°cell and K.
Calculate ΔS(total) for E°cell = +1.10 V, n = 2, T = 298 K.
What does a very large K show?
Give the two limitations of predictions from E° values.
Why does acidified MnO₄⁻ not oxidise water in practice, although E°cell = +0.28 V?
What is the electrochemical series?
Which species is the strongest reducing agent in the series?
Define disproportionation.
Write the disproportionation of copper(I).
Why does Fe²⁺ not disproportionate?

Exam questions on Predicting feasibility and limitations

Acidified potassium manganate(VII) solution is added to a solution of iron(II) sulfate at 298 K. Standard electrode potentials: MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ ⇌ Mn²⁺(aq) + 4H₂O(l), E° = +1.51 V; Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V.
State the relationship between E°cell and ΔS(total), and explain why a positive E°cell shows that a reaction is feasible.2 marks
Zinc powder is added to aqueous copper(II) sulfate at 298 K. Standard electrode potentials: Zn²⁺(aq) + 2e⁻ ⇌ Zn(s), E° = −0.76 V; Cu²⁺(aq) + 2e⁻ ⇌ Cu(s), E° = +0.34 V. Use ΔS(total) = nFE°cell / T and ln K = nFE°cell / RT, with F = 96 500 C mol⁻¹ and R = 8.31 J K⁻¹ mol⁻¹.
Explain what the value of K shows about the extent of this reaction.2 marks
A technician stores acidified potassium manganate(VII) solution for use as a titrant, and finds that it stays purple for weeks. Standard electrode potentials: MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ ⇌ Mn²⁺(aq) + 4H₂O(l), E° = +1.51 V; O₂(g) + 4H⁺(aq) + 4e⁻ ⇌ 2H₂O(l), E° = +1.23 V.
Use the data to predict whether acidified manganate(VII) ions should oxidise water to oxygen.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).