The method and the results are on the chemical equilibrium lab. Do the analysis yourself first, then check the answers below.
Quick answer: At chemical equilibrium the forward and reverse reaction rates are equal, so concentrations stay constant. These answers go with the AP Chemistry lab on chemical equilibrium: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
The four values of K agree within a few per cent, which shows that K depends on temperature, not on starting concentrations. The qualitative tests are all explained by comparing Q with K or by the sign of ΔH. Keep two significant figures because the colorimeter readings have two.
Mean = (268 + 266 + 268 + 262)/4 = 266 M⁻¹. The range is 262–268, within 1.2 % of the mean. K is constant although [Fe³⁺]₀ changed fourfold.
Question 5 Explaining the disturbances
Adding Fe³⁺ raises the denominator of Q, so Q < K and the forward reaction runs: deeper red. NaOH removes Fe³⁺ as Fe(OH)₃, so Q rises above K and the reverse reaction runs: paler.
Question 6 Temperature
Warming makes the solution paler, so the reverse reaction is favoured and K decreases. The reverse is endothermic, so the forward reaction is exothermic (ΔH < 0).
Question 7 Dilution
Both concentrations halve, so Q = (½[FeSCN²⁺])/(½[Fe³⁺] × ½[SCN⁻]) = 2K > K. The system shifts to the left (towards more particles), and the colour is paler than half the original.
