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It is proposed to indirectly determine the concentration of Fe<sup>3+</sup> ions in a solution by using UV-visible spectroscopy to measure the concentration of red-coloured FeSCN<sup>2+</sup> ions generated by the equilibrium reaction Fe<sup>3+</sup>(aq) + SCN<sup>−</sup>(aq) ⇌ FeSCN<sup>2+</sup>(aq) ΔH = negative colourless red This procedure would provide the most accurate estimate of the concentration of Fe<sup>3+</sup> 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 Fe<sup>3+</sup> ions in a solution by using UV-visible spectroscopy to measure the concentration of red-c... show full transcript
Worked Solution & Example Answer:It is proposed to indirectly determine the concentration of Fe<sup>3+</sup> ions in a solution by using UV-visible spectroscopy to measure the concentration of red-coloured FeSCN<sup>2+</sup> ions generated by the equilibrium reaction Fe<sup>3+</sup>(aq) + SCN<sup>−</sup>(aq) ⇌ FeSCN<sup>2+</sup>(aq) ΔH = negative colourless red This procedure would provide the most accurate estimate of the concentration of Fe<sup>3+</sup> 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<sup>−</sup> is used, and the analysis is carried out at a low temperature.
Answer
To accurately determine the concentration of Fe<sup>3+</sup> ions, using an excess of SCN<sup>−</sup> is crucial when the equilibrium constant is large. A large equilibrium constant indicates that the formation of the product (FeSCN<sup>2+</sup>) is favored. By ensuring that SCN<sup>−</sup> is in excess, we shift the equilibrium to increase the product's concentration, making it easier to measure the absorbance of the red-colored complex.
Furthermore, performing the analysis at a low temperature helps to minimize the reverse reaction, thereby favoring product formation. This combination of conditions provides the most reliable estimate of the original concentration of Fe<sup>3+</sup> ions.