Question

A student performs a redox titration between 10.0 mL of hypochlorous acid, HClO(aq), and chromium (III) nitrate, Cr(NO₃)₃(aq). The half-reaction are given below. 7 H₂O + 2 Cr³ ⟶ Cr₂O₇²(aq) + 14 H(aq) + 6 e- 2 HClO(aq) + 2 H(aq) + 2 e- ⟶Cl₂(g) + H₂O Give the overall oxidation-reduction reaction. What is the oxidation number of chromium in Cr₂O₇²- ?

Answer 1

The overall redox reaction is formed by combining the balanced half-reactions for hypochlorous acid and chromium, resulting in 6 HClO(aq) + 6 H(aq) + Cr₂O⁷²⁻ (aq) → 3 Cl₂(g) + 2 Cr³⁺ (aq) + 8 H₂O(l). Chromium in Cr₂O⁷²⁻ has an oxidation number of +6.

Step-by-step explanation:

To determine the overall oxidation-reduction reaction for the redox titration between hypochlorous acid (HClO) and chromium(III) nitrate (Cr(NO₃)₃), the provided half-reactions must be combined. First, balance the number of electrons between the two half-reactions by finding a common multiple. In this case, the hypochlorous acid half-reaction generates 2 electrons and the chromium half-reaction consumes 6 electrons. Multiply the hypochlorous acid half-reaction by 3 and the chromium reaction by 1 to make the electrons transfer equal:

6 HClO(aq) + 6 H(aq) + 6 e- → 3 Cl₂(g) + 3 H₂O(l) (Multiplied by 3)

6e- + 14H+ (aq) + Cr₂O⁷²⁻ (aq) → 2Cr³⁺ (aq) + 7H₂O(l) (Original)

Add the two balanced half-reactions to get the overall redox reaction:

6 HClO(aq) + 6 H(aq) + Cr₂O⁷²⁻ (aq) → 3 Cl₂(g) + 2 Cr³⁺ (aq) + 8 H₂O(l)

For the oxidation number of chromium in Cr₂O⁷²⁻, chromium is in the +6 oxidation state. This can be found by setting up an equation where the sum of oxidation numbers equals the charge of the compound. With seven oxygen atoms each with an oxidation number of -2, and the overall charge of the ion being 2-, we get:

2(Cr oxidation number) + 7(-2) = -2

2(Cr oxidation number) - 14 = -2

2(Cr oxidation number) = 12

Cr oxidation number = +6

Answer 2

The oxidation number of chromium in Cr2O72- is +6. To find the overall redox reaction, the two half-reactions for chromium reduction and hypochlorous acid oxidation need to be combined and balanced for charges and atoms.

Step-by-step explanation:

In a redox titration, the overall oxidation-reduction (redox) reaction is obtained by combining the two given half-reactions and balancing the charges and atoms. The half-reaction for chromium is the reduction part where chromium is reduced from +6 oxidation state in dichromate ion (Cr2O72-) to +3 in chromium(III) ion (Cr3+). To find the overall redox reaction, we need to balance the number of electrons exchanged between the two half-reactions. The hypochlorous acid half-reaction involves an oxidation where electrons are produced, while the chromium half-reaction involves a reduction where electrons are consumed.

The oxidation number of chromium in Cr2O72- can be determined by setting up an equation considering the overall charge of the ion. Each oxygen atom has an oxidation number of -2, and the total charge of the ion is -2. We let x be the oxidation number of chromium:

2x + 7(-2) = -2

2x - 14 = -2

2x = 12

x = +6

Therefore, the oxidation number of chromium in the dichromate ion Cr2O72- is +6.