This question is about an experiment to investigate the effect of temperature on the equilibrium constant, $$\(K_{1}\)$$, of the reaction shown. $$\[ \mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{SCN}^{-}(\mathrm{aq}) \rightleftharpoons \mathrm{FeSCN}^{2+}(\mathrm{aq}) \]$$ The data collected is used to determine the value of the enthalpy change of the reaction. To set up the equilibrium, aqueous iron(III) nitrate, $$\(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}(\mathrm{aq})\)$$, is mixed with aqueous potassium thiocyanate, $$\(\mathrm{KSCN}(\mathrm{aq})\)$$. Aqueous iron thiocyanate ions, $$\(\operatorname{FeSCN}^{2+}(\mathrm{aq})\)$$, have a red colour. A colorimeter is used to measure the absorbance of the reaction mixture. A calibration graph can then be used to determine the concentration of $$\(\mathrm{FeSCN}^{2+}(\mathrm{aq})\)$$ in the reaction mixture. Table 2.1 shows the solutions for the experiments. Another student does the same experiment for seven different temperatures, plots a graph and draws the line of best fit, as shown in Fig. 2.2. Theory predicts that the relationship between $$\(K_{1}\)$$ and $$\(T\)$$ is given by equation 2. $$\[ \text { equation } 2 \quad \log K_{1}=\frac{-\Delta H}{2.303 R T}+\text { constant } \]$$ $$\(\Delta H\)$$ is the enthalpy change of reaction and $$\(T\)$$ is the temperature in Kelvin. (i) Explain why the graph supports the relationship between $$\(K_{1}\)$$ and $$\(T\)$$ given in equation 2. ................................................................................................................................. (ii) Circle the point on the graph in Fig. 2.2 that you consider to be most anomalous. There were no errors in the measurements in the experiment. A student correctly suggests that the anomaly was caused because the absorbance was lower than expected by the line of best fit. Suggest why the absorbance was lower than expected. ....................................................................................................................................... . ................................................................................................................................. (iii) Determine the gradient of the line of best fit in Fig. 2.2. State the coordinates of both points you use in your calculation. These must be selected from the line of best fit. Give the gradient to three significant figures. coordinates 1 ......................................... coordinates 2 ......................................... gradient $$\(=\)$$ .......................... K (iv) Use the gradient calculated in (e)(iii) and equation 2 to calculate a value for the enthalpy change of reaction, $$\(\Delta H\)$$. $$\[ \text { equation } 2 \quad \log K_{1}=\frac{-\Delta H}{2.303 R T}+\text { constant } \]$$ (If you were unable to obtain an answer to (e)(iii), then use the value 635 K . This is not the correct answer.) $$\(\Delta H=\)$$ .......................... $$\(\mathrm{kJ} \mathrm{mol}^{-1}\)$$

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