# Consider the unbalanced redox reaction: Cr_2O_2^-7(aq)+Cu(s)-->Cr^3+(aq)+Cu^2+(aq) PART A: ...

## Question:

Consider the unbalanced redox reaction:

Cr_2O_2^-7(aq)+Cu(s)-->Cr^3+(aq)+Cu^2+(aq)

PART A:

Balance the equation.

PART B:

Determine the volume of a 0.575 M K_2Cr_2O_7 solution required to completely react with 5.64 g of Cu. Express your answer using three significant figures.

## Redox Reaction:

A redox reaction involves electron transfer between two species. The species that loses/gains electrons is oxidized/reduced respectively. Each of these processes can be written as a balanced half-reaction showing the electron loss or gain. When summed together the net redox reaction must be balanced in mass and charge with no electrons present.

Question a)

• Copper is oxidized here from zero in the metal to +2 in the aqueous ion.
• Oxidation half-reaction: {eq}3Cu (s) \rightarrow 3Cu^{2+} (aq) + 6e^- {/eq}
• Chromium is reduced here from +6 in dichromate anion to +3 in the aqueous ion.
• Reduction half-reaction: {eq}14H^+ (aq) + Cr_2O_7^{2-} (aq) + 6e^- \rightarrow 2Cr^{3+} (aq) + 7H_2O (l) {/eq}
• Net reaction: {eq}3Cu (s) + 14H^+ (aq) + Cr_2O_7^{2-} (aq) \rightarrow 3Cu^{2+} (aq) + 2Cr^{3+} (aq) + 7H_2O (l) {/eq}

Question b)

• moles of copper = {eq}5.64 g \times \dfrac{1mol}{63.546 g} = 0.0888 mol {/eq}

For equivalence this requires (1/3)(0.0888) = 0.0296 mol of potassium dichromate solution.

• volume of potassium dichromate solution required = {eq}\dfrac{n}{C} = \dfrac{0.0296 mol}{0.575 M} = 0.0515 L = 51.5 mL {/eq}