SSCE VCE Chemistry Redox Reactions: In solution, pale yellow-coloure

Question

In solution, pale yellow-coloured Fe³⁺(aq) and colourless SCN⁻(aq) form an equilibrium with Fe(SCN)²⁺(aq). Fe(SCN)²⁺(aq) is red in colour.

Fe³⁺(aq) + SCN⁻(aq) ⇌ Fe(SCN)²⁺(aq)  ΔH negative

**a.** A student investigates this reaction using separate samples of an equilibrium mixture in which significant quantities of Fe³⁺, SCN⁻ and Fe(SCN)²⁺ are present. In each case changes are made and indicated in the table below.

Complete the table by placing ticks in the appropriate boxes to indicate the effect of each change on:
1. the intensity of the red colour of the solution; and 
2. the concentration of Fe³⁺ once the new equilibrium has been established.

| Change to the equilibrium                                       | I. Colour at new equilibrium compared with initial equilibrium | II. [Fe³⁺] at new equilibrium compared with initial equilibrium |
|---------------------------------------------------------------|---------------------------------------------------------------|-----------------------------------------------------------------|
| Sample 1: 1 drop of a concentrated solution of Ag⁺(aq) is added, which forms a AgSCN precipitate  |                                                               |                                                                 |
| Sample 2: 1 drop of a concentrated solution of Fe³⁺(aq) is added                               |                                                               |                                                                 |
| Sample 3: 1 drop of a concentrated solution of HPO₄²⁻(aq) is added, which forms colourless FeHPO₄(aq) |                                                               |                                                                 |
| Sample 4: Addition of a large volume of water                |                                                               |                                                                 |

**b.** The reaction is exothermic. The graph below represents the initial concentrations of the ions at equilibrium. Sketch the changes that would be expected to occur to these concentrations if the temperature of the equilibrium mixture was increased to a new, constant value.

| concentration | 
|---------------| 
|               | 
| Fe(SCN)²⁺    | 
|               | 
| SCN⁻         | 
|               | 
| Fe³⁺        | 
|               | 
| initial equilibrium  | final equilibrium | 
| time  |

In solution, pale yellow-coloured Fe³⁺(aq) and colourless SCN⁻(aq) form an equilibrium with Fe(SCN)²⁺(aq) - VCE - SSCE Chemistry - Question 6 - 2006 - Paper 1

Worked Solution & Example Answer:In solution, pale yellow-coloured Fe³⁺(aq) and colourless SCN⁻(aq) form an equilibrium with Fe(SCN)²⁺(aq) - VCE - SSCE Chemistry - Question 6 - 2006 - Paper 1

Sample 1: 1 drop of a concentrated solution of Ag⁺(aq) is added, which forms a AgSCN precipitate

Sample 2: 1 drop of a concentrated solution of Fe³⁺(aq) is added

Sample 3: 1 drop of a concentrated solution of HPO₄²⁻(aq) is added, which forms colourless FeHPO₄(aq)

Sample 4: Addition of a large volume of water

Sketch the changes that would be expected to occur

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