This question is about an experiment to investigate the effect of temperature on the equilibrium constant, K₁, of the reaction shown. Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(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, Fe(NO3)3(aq), is mixed with aqueous potassium thiocyanate, KSCN(aq). Aqueous iron thiocyanate ions, FeSCN²⁺(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 FeSCN²⁺(aq) in the reaction mixture. [Table 2.1] shows the solutions for the experiments. Table 2.1 | solution | ion | concentration/mol dm⁻³ | |---|---|---| | A | SCN⁻(aq) | 0.00920 | | B | SCN⁻(aq) | 0.00200 | | C | Fe³⁺(aq) | 0.00200 | | D | Fe³⁺(aq) | 0.500 |

1. (a)Describe how you would prepare 100.0 cm³ of solution B from solution A. Include a calculation of the volume of solution A required for the preparation of solution B. Give the name and capacity of any key apparatus that should be used. Write your answer as a series of numbered steps.[3]
2. (b)Before starting the experiment, solutions B and D are used to produce a calibration graph. Known volumes of each solution are added together and the absorbance for each mixture is recorded. The calibration graph is shown in [Figure 2.1]. The concentration of solution D is much greater than the concentration of solution B in order that solution D is in excess. Suggest a reason why solution D is in excess.[1]
3. (c)(i)Identify the dependent variable.[1]
4. (c)(ii)Describe how you would adjust the temperature of the water in the large beaker to obtain a temperature of 10°C.[1]
5. (d)(i)Use the calibration graph in [Figure 2.1] to complete column 3 in [Table 2.2]. Record values to one decimal place.[1]
6. (d)(ii)Use equation 1 to complete column 4 in [Table 2.2]. Record values to the nearest whole number.[1]
7. (e)(i)Explain why the graph supports the relationship between K₁ and T given in equation 2.[1]
8. (e)(ii)Circle the point on the graph in [Figure 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.[2]
9. (e)(iii)Determine the gradient of the line of best fit in [Figure 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 [2][2]
10. (e)(iv)Use the gradient calculated in (e)(iii) and equation 2 to calculate a value for the enthalpy change of reaction, ∆H. (If you were unable to obtain an answer to (e)(iii), then use the value 635 K. This is not the correct answer.) ∆H = .............................................. kJ mol⁻¹ [2][2]

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