Chemical equilibrium lab answers for A Level chemistry (free PDF download)

Question 1 Complex concentration

[complex] = 2.00 × 10⁻⁴ × A / 0.380. Moreover, tube 1: 2.474 × 10⁻⁴ mol/dm³; tube 2: 2.237 × 10⁻⁴; tube 3: 1.926 × 10⁻⁴; tube 4: 1.505 × 10⁻⁴.

Question 2 Initial concentrations

[Fe³⁺]₀ = 0.0200 × V / 10.0: 1.00 × 10⁻², 8.00 × 10⁻³, 6.00 × 10⁻³ and 4.00 × 10⁻³ mol/dm³. [SCN⁻]₀ = 0.00200 × 2.0 / 10.0 = 4.00 × 10⁻⁴ mol/dm³ in every tube.

Question 3 Equilibrium concentrations

Each mole of complex formed uses one mole of Fe³⁺ and one of SCN⁻. tube 1: [Fe³⁺] = 1.00 × 10⁻² − 2.474 × 10⁻⁴ = 9.75 × 10⁻³; [SCN⁻] = 4.00 × 10⁻⁴ − 2.474 × 10⁻⁴ = 1.53 × 10⁻⁴. Also, tube 2: 7.78 × 10⁻³ and 1.76 × 10⁻⁴. Also, tube 3: 5.81 × 10⁻³ and 2.07 × 10⁻⁴. Moreover, tube 4: 3.85 × 10⁻³ and 2.49 × 10⁻⁴ mol/dm³. In addition, these equilibrium values are used in Kc.

Question 4 Calculating Kc

Kc = [Fe(SCN)²⁺] / ([Fe³⁺][SCN⁻]). Also, tube 1: 166 dm³ mol⁻¹; tube 2: 163; tube 3: 160; tube 4: 157.

Question 5 Mean and consistency

Mean = (166 + 163 + 160 + 157) / 4 = 1.6 × 10² dm³ mol⁻¹ (161.5). The values agree within about ±3 %, as expected because Kc is constant at a fixed temperature, even though the concentrations changed. A small downward trend suggests a systematic effect.

Question 6 Assumption in the standard

The true complex concentration in the standard is about 1.87 × 10⁻⁴ mol/dm³, lower than the 2.00 × 10⁻⁴ assumed. Every calculated complex concentration is therefore too high by about 7 %, so the calculated Kc is too high (about 1.6 × 10² against a true value near 1.4 × 10²).

Question 7 Prediction with silver ions

SCN⁻ is removed, so Q becomes greater than K and the equilibrium shifts to the left, making the solution paler. Kc is unchanged because the temperature is the same.

Question 8 Improving the method

Use a calibration series of several standards and a calibration graph of absorbance against concentration, and keep all tubes in a thermostatic water bath at one temperature. Then, repeat each mixture and average.