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It is proposed to indirectly determine the concentration of Fe³⁺ ions in a solution by using UV-visible spectroscopy to measure the concentration of red-coloured FeSCN²⁺ ions generated by the equilibrium reaction Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) ΔH = negative colourless red This procedure would provide the most accurate estimate of the concentration of Fe³⁺ ions in the original solution if A - VCE - SSCE Chemistry - Question 5 - 2011 - Paper 1
Question 5
It is proposed to indirectly determine the concentration of Fe³⁺ ions in a solution by using UV-visible spectroscopy to measure the concentration of red-coloured FeS... show full transcript
Worked Solution & Example Answer:It is proposed to indirectly determine the concentration of Fe³⁺ ions in a solution by using UV-visible spectroscopy to measure the concentration of red-coloured FeSCN²⁺ ions generated by the equilibrium reaction Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) ΔH = negative colourless red This procedure would provide the most accurate estimate of the concentration of Fe³⁺ ions in the original solution if A - VCE - SSCE Chemistry - Question 5 - 2011 - Paper 1
Step 1
the value of the equilibrium constant is large, an excess of SCN⁻ is used, and the analysis is carried out at a low temperature.
Answer
In this scenario, using an excess of SCN⁻ shifts the equilibrium to the right, favoring the formation of FeSCN²⁺ ions. This ensures that a more significant product concentration is available for measurement, leading to a more accurate estimation of the original Fe³⁺ concentration. A large equilibrium constant indicates that the forward reaction is favored, which, coupled with the excess SCN⁻, maximizes the reaction's progress. Additionally, conducting the analysis at a low temperature may help prevent decomposition of sensitive species and enhance the stability of the measured product.