Consider the following half‑reactions at $25^{\circ}\text{C}$:\n$\displaystyle \mathrm{Cr_2O_7^{2-}+14H^+ +6e^- \rightarrow 2Cr^{3+}+7H_2O}$ $E^{\circ}=+1.33\ \text{V}$\n$\displaystyle \mathrm{Fe^{3+}+e^- \rightarrow Fe^{2+}}$ $E^{\circ}=+0.77\ \text{V}$\nA solution contains $0.050\ \text{M}$ $\mathrm{Fe^{2+}}$ and $0.020\ \text{M}$ $\mathrm{Cr_2O_7^{2-}}$ in excess $\mathrm{H^+}$ ($[\mathrm{H^+}]=1.0\ \text{M}$).\n(a) Identify which species will be oxidised and which will be reduced when the solutions are mixed, and state whether the overall redox process is spontaneous.\n(b) Calculate the cell electromotive force (EMF) at $25^{\circ}\text{C}$ using the Nernst equation, assuming activities equal the given concentrations.\n(c) If $0.010\ \text{mol}$ of $\mathrm{Fe^{2+}}$ reacts completely, determine the mass of $\mathrm{Fe^{3+}}$ formed.

Question

Consider the following half‑reactions at $25^{\circ}\text{C}$:\n$\displaystyle \mathrm{Cr_2O_7^{2-}+14H^+ +6e^- \rightarrow 2Cr^{3+}+7H_2O}$ $E^{\circ}=+1.33\ \text{V}$\n$\displaystyle \mathrm{Fe^{3+}+e^- \rightarrow Fe^{2+}}$ $E^{\circ}=+0.77\ \text{V}$\nA solution contains $0.050\ \text{M}$ $\mathrm{Fe^{2+}}$ and $0.020\ \text{M}$ $\mathrm{Cr_2O_7^{2-}}$ in excess $\mathrm{H^+}$ ($[\mathrm{H^+}]=1.0\ \text{M}$).\n(a) Identify which species will be oxidised and which will be reduced when the solutions are mixed, and state whether the overall redox process is spontaneous.\n(b) Calculate the cell electromotive force (EMF) at $25^{\circ}\text{C}$ using the Nernst equation, assuming activities equal the given concentrations.\n(c) If $0.010\ \text{mol}$ of $\mathrm{Fe^{2+}}$ reacts completely, determine the mass of $\mathrm{Fe^{3+}}$ formed.

Accepted Answer

Dichromate is reduced and Fe^{2+} is oxidised; the reaction is spontaneous with E ≈ 0.56 V. Complete oxidation of 0.010 mol Fe^{2+} gives ≈0.56 g Fe^{3+}.

Gce Model Questions Vol1 2020 Chemistry Question 27

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