QUESTION 126 (5 marks)
A red solution contains the equilibrium . A small amount of Fe(NO₃)₃ is added. Its volume is negligible and the temperature is unchanged.
(a) Use collision theory to explain the change in the forward rate immediately after the addition. [2 marks]
(b) Explain how the forward and reverse rates change as the solution reaches its new equilibrium, and predict its colour change. [3 marks]
Practice marking scheme
Answer
The forward rate initially increases because Fe³⁺–SCN⁻ collisions are more frequent. Net formation of FeSCN²⁺ deepens the red colour. As reactants are consumed the forward rate falls from its initial raised value, while the reverse rate rises until the rates are equal.
Working
The reactant concentration changes instantly, increasing effective collisions between reactants. The complex concentration does not change instantly, so the reverse rate is initially unchanged. More complex then forms: reactant concentrations fall and the reverse reaction has more complex available. A new equality of rates is reached with a greater red-complex concentration.
Marking criteria
- Identifies the increase in Fe³⁺ concentration. [1 mark]
- Links it to more frequent effective reactant collisions and a higher forward rate. [1 mark]
- States that the reverse rate initially remains unchanged and then rises as complex forms. [1 mark]
- Explains that the forward rate falls from its initial raised value until both rates are equal. [1 mark]
- Predicts a deeper red colour due to increased FeSCN²⁺ concentration. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.