Aqueous silver ions, Ag+(aq), react slowly with aqueous iron(II) ions, Fe2+(aq). An equilibrium is established. Ag+(aq) + Fe2+(aq) ⇋ Ag(s) + Fe3+(aq) The concentration of Ag+(aq) at equilibrium can be determined by titration with a standard solution of aqueous potassium thiocyanate, KSCN(aq). During the titration, the remaining Ag+(aq) ions react with SCN-(aq) ions to form a precipitate of AgSCN(s). Ag+(aq) + SCN¯(aq) → AgSCN(s) When all Ag+(aq) ions have been removed from solution, excess SCN-(aq) ions react with Fe3+(aq) to form a complex ion, FeSCN2+(aq), which has a red colour. Fe3+(aq) + SCN¯(aq) → FeSCN2+(aq) The appearance of the red colour indicates the end-point. A student carries out an experiment to determine the equilibrium constant, Kc. Kc = [Fe3+(aq)]eqm / [Fe2+(aq)]eqm [Ag+(aq)]eqm The student makes 250.0 cm³ of 0.0200 mol dm⁻³ KSCN(aq) to use in the titration.

1. (a)Calculate the mass of solid potassium thiocyanate, KSCN(s), needed to make 250.0 cm³ of 0.0200 mol dm⁻³ KSCN(aq).[1]
2. (b)Describe how the student should make 250.0 cm³ of 0.0200 mol dm⁻³ KSCN(aq) starting from the mass of KSCN(s) calculated in (a) in a 50 cm³ beaker. Give the name and size of any key apparatus used. Write your answer using a series of numbered steps.[3]
3. (c)(i)Suggest why flask A is sealed with a bung in step 3.[1]
4. (c)(ii)Suggest why flask A is left to stand for twelve hours in step 4.[1]
5. (c)(iii)Identify the precipitate in flask A in step 5.[1]
6. (d)(i)State if concordant titres have been achieved. Explain your answer.[1]
7. (d)(ii)Calculate the percentage error in the titre volume in titration 2. Show your working. percentage error =[1]
8. (d)(iii)The student repeats the experiment using KSCN(aq) at a higher concentration. The student obtains smaller titres. Suggest one reason why a larger titre is better than a smaller titre.[1]
9. (e)(i)Calculate [Ag+(aq)] in the equilibrium mixture in flask A. [Ag+(aq)]eqm =[1]
10. (e)(ii)Calculate [Fe3+(aq)] in the equilibrium mixture in flask A. [Fe3+(aq)]eqm =[1]
11. (e)(iii)The formula for the equilibrium constant, Kc, is shown. Kc= [Fe3+(aq)]eqm / [Fe2+(aq)]eqm [Ag+(aq)]eqm Determine the value of Kc. Give the units of Kc. Kc = units[2]
12. (f)(i)One student suggests that Kc is directly proportional to temperature. State and explain if the results displayed in Fig. 1.1 support this suggestion.[1]
13. (f)(ii)Another student suggests that the data represented in the graph in Fig. 1.1 is reliable. Explain how the graph supports this suggestion.[1]
14. (f)(iii)Use the data displayed in Fig. 1.1 to state if the forward reaction is exothermic or endothermic. Ag+(aq) + Fe2+(aq) ⇋ Ag(s) + Fe3+(aq) Explain your answer. forward reaction explanation[1]

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