Part 2: Measuring rates and required practical 7

Plan initial-rate and continuous-monitoring measurements, interpret tangents and recognise what a clock time actually measures.

Choose a measurable signal

A continuous method records a signal repeatedly during one run: gas volume, mass loss or absorbance are common examples. The signal must relate to the species whose change you want to follow. Absorbance can track a coloured species after calibration; gas volume requires controlled temperature and pressure if it is to represent moles.

The rate of disappearance of a reactant is the positive magnitude of its downward concentration–time gradient. Product formation has a positive gradient. For an equation with unequal coefficients, species rates differ: if 2A → B, disappearance of A is twice formation of B. Check which rate a question defines.

Instantaneous and initial rate

Draw a tangent at the requested time and use two well-separated points on that tangent, not two arbitrary points on the curve. Initial rate uses the tangent at time zero. A chord gives an average rate over an interval.

For an illustrative reactant tangent passing through (20 s, 0.090 mol dm⁻³) and (80 s, 0.030 mol dm⁻³), the gradient is −0.060/60 = −1.00 × 10⁻³ mol dm⁻³ s⁻¹. The disappearance rate is +1.00 × 10⁻³ mol dm⁻³ s⁻¹.

A straight decreasing concentration–time graph is consistent with zero-order disappearance while the conditions and model remain valid: [A] = [A]₀ − kt, so k is the negative gradient. Curvature alone does not distinguish first from second order. First-order decay has constant successive half-lives; second-order single-reactant decay has increasing half-lives. These patterns assume other rate-law factors are controlled.

An iodine clock measures a fixed small change

One possible clock produces iodine from iodide and hydrogen peroxide in acid. A small, fixed amount of thiosulfate removes iodine rapidly. Once the thiosulfate is exhausted, iodine accumulates and gives a blue-black colour with starch. The clock time is therefore the time to form a specified small amount of iodine, not the time to finish the whole reaction.

Control the experiment, then change one variable

For an initial-rate investigation, prepare several mixtures differing in the concentration of one reactant. Replace its reduced solution volume with water to keep total volume fixed; keep the other starting concentrations, starch and thiosulfate threshold constant. Calculate each concentration after mixing using cstock × Vstock/Vtotal. Equilibrate solutions in a water bath, add the final reactant consistently, start timing immediately and use a consistent mixing and endpoint method. Repeat trials and investigate anomalous results.

For continuous gas monitoring, connect a suitable gas syringe without leaks, choose reactant amounts that fit its capacity, and record time and volume at short intervals. Minimise delay between mixing and sealing, or use a setup that mixes after sealing. Control exposed solid surface area if a solid is involved. For mass loss, avoid losing droplets while allowing gas to escape; use the appropriate flask and a loose plug where suitable.

Use eye protection and the teacher-approved concentrations and disposal method. Acids and peroxide require concentration-specific handling; hydrogen-generating experiments need ignition sources excluded. Short reaction times increase relative timing error. Gas leaks and loss during sealing systematically underestimate collected volume; repeat measurements do not remove those biases.

Excess reactant is not proof of zero order

Holding one reactant in large excess can make its concentration nearly constant during a run. Its rate-law factor is then folded into an observed constant, giving a pseudo-order description. That does not demonstrate its true order is zero. To find its order, vary its starting concentration across controlled runs.

A plateau in a gas-volume curve shows no further measurable net gas production; use the chemistry to decide whether a limiting reagent was exhausted or equilibrium was reached. It does not automatically mean every reactant has disappeared. A steeper initial curve can still reach the same final volume if the limiting amount is unchanged.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.

Sources