RP7: Measuring Reaction Rates | Chemistry Made Easy

1. Continuous Monitoring vs Initial Rate Methods

In OxfordAQA A2 Chemistry (CH03 and Paper 5), reaction rates are evaluated using two distinct experimental methodologies:

Method A: Continuous Monitoring

Tracks how a physical property changes continuously over the entire course of a single reaction run:

Gas Syringe Collection: Measuring volume of gas evolved (e.g. H2 from Mg + HCl, or CO2 from CaCO3 + HCl) at regular time intervals (every 10 or 15 s).
Colorimetry: Tracking absorbance of a colored ion or halogen over time.
Continuous Mass Loss: Placing a reaction flask on an analytical balance to measure mass drop as gas escapes.

Method B: Initial Rate (Clock Reactions)

Measures the time taken for an observable chemical event to occur at the very beginning of the reaction across multiple separate runs with varied concentrations:

Iodine Clock Reaction: Persulfate oxidising iodide in the presence of fixed thiosulfate and starch, timing the sudden switch from colourless to blue-black.
Disappearing Cross: Timing sulfur turbidity (RP3).
Initial Rate: Evaluated at t = 0 before reactant concentrations change appreciably.

2. Continuous Gas Volume Monitoring Protocol

To investigate the reaction between magnesium ribbon and hydrochloric acid:

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Clamp a clean, dry 100 cm3 glass gas syringe horizontally. Push the plunger fully in to read exactly 0 cm3. Check that the plunger moves freely with zero friction.
Measure 25.0 cm3 of 1.0 mol dm^-3 hydrochloric acid into a conical flask using a volumetric pipette.
Clean a 5 cm strip of magnesium ribbon with emery paper to remove unreactive magnesium oxide (MgO) surface coating.
Add the magnesium ribbon to the acid, immediately replace the rubber bung tightly, and start the stopwatch simultaneously.
Swirl continuously to prevent hydrogen bubbles from adhering to the metal surface.
Record the gas syringe volume reading every 15 seconds until the volume reaches a constant plateau (reaction complete).

3. Tangent Construction for Initial Rate (t = 0)

Plotting gas volume against time produces a curve with steep initial rise that gradually flattens as reactants are consumed:

Constructing an Initial Rate Tangent 1. Align a transparent plastic ruler so that it forms a tangent to the curve at the origin (t = 0 s, V = 0 cm3).
2. Draw a long, thin, straight line extending across at least 50% of the graph grid.
3. Construct a large gradient triangle: Gradient = delta y / delta x = delta V / delta t.
4. Units of Initial Rate: cm3 s^-1 (or converted to mol dm^-3 s^-1).

Examiner Warning: Gas Loss upon Stopper Insertion

A major source of systematic error in gas syringe experiments is gas escaping into the room during the 1 to 2 seconds between dropping in the magnesium and inserting the rubber bung.
Examiner-Approved Solution: Suspend the magnesium ribbon inside the flask above the acid on a thread held by the bung, or place the magnesium in a small test tube inside the flask, stopper the system, and tip the flask to begin the reaction without opening the system to air.

4. Iodine Clock Reaction (Peroxodisulfate & Iodide)

The standard clock reaction in OxfordAQA Chemistry investigates the oxidation of iodide ions by peroxodisulfate(VI) ions (S2O8^2-):

Main Kinetics Reaction S2O8^2-(aq) + 2I-(aq) → 2SO4^2-(aq) + I2(aq)

Scavenger Reaction with Thiosulfate 2S2O3^2-(aq) + I2(aq) → S4O6^2-(aq) + 2I-(aq)

How the Clock Mechanism Functions:

A known, constant volume of sodium thiosulfate (Na2S2O3) and starch indicator is included in the reaction mixture.
As iodine (I2) is generated by the main reaction, it is immediately reduced back to iodide by the thiosulfate scavenger reaction. The solution remains completely colourless.
The exact moment all thiosulfate is consumed, the very next drop of I2 produced is free to complex with starch, triggering an instantaneous colour switch to blue-black.
The time taken (t) for the blue-black flash to appear is inversely proportional to the initial rate: Initial Rate ∝ 1 / t.

5. Deducing Reaction Orders & The Rate Equation

To determine the rate equation: Rate = k * [S2O8^2-]^m * [I-]^n, a series of experiments is performed varying one concentration while keeping the other constant, with total volume maintained constant by adding deionised water:

Experiment	Vol of KI / cm3	Vol of K2S2O8 / cm3	Vol of Na2S2O3 / cm3	Vol of Water / cm3	Time to Blue-Black (t) / s	Relative Rate (1/t) / s^-1
1	10.0	10.0	5.0	25.0	50.0	0.0200
2	20.0 (x2)	10.0	5.0	15.0	25.0	0.0400 (x2)
3	10.0	20.0 (x2)	5.0	15.0	25.0	0.0400 (x2)

Analysis of Data:

Comparing Exp 1 and Exp 2: [I-] doubles while [S2O8^2-] is constant → Rate doubles (0.0200 → 0.0400). Therefore, the reaction is first order with respect to I- (n = 1).
Comparing Exp 1 and Exp 3: [S2O8^2-] doubles while [I-] is constant → Rate doubles (0.0200 → 0.0400). Therefore, the reaction is first order with respect to S2O8^2- (m = 1).
Overall Rate Equation: Rate = k * [S2O8^2-] * [I-] (overall second order).

6. Worked Kinetics Rate Constant Problem

Worked Example: Calculating Rate Constant k with Units

Problem: In an initial rate experiment, when [S2O8^2-] = 0.040 mol dm^-3 and [I-] = 0.060 mol dm^-3, the initial rate of reaction is measured as 1.44 * 10^-4 mol dm^-3 s^-1. The reaction follows the rate equation Rate = k * [S2O8^2-] * [I-].

Calculate the value of the rate constant k and state its units.

Step 1: Rearrange the rate equation for k

k = Rate / ([S2O8^2-] * [I-])
k = (1.44 * 10^-4) / (0.040 * 0.060)
k = (1.44 * 10^-4) / (2.40 * 10^-3) = 0.060

Step 2: Determine units of k

Units of k = (mol dm^-3 s^-1) / ((mol dm^-3) * (mol dm^-3))
Units of k = (mol dm^-3 s^-1) / (mol^2 dm^-6) = mol^-1 dm^3 s^-1

Final Answer: k = 0.060 mol^-1 dm^3 s^-1

7. Practice Exam Questions

Question 1: In the iodine clock reaction, why must the amount of sodium thiosulfate added be kept small and constant in all experiments?

A. To ensure that the blue-black color appears while the reaction is still in its initial rate phase (<10% reactant consumption). B. Excess thiosulfate acts as a catalyst for peroxodisulfate decomposition. C. Starch decomposes irreversibly if exposed to thiosulfate for more than 60 seconds. D. To keep the temperature strictly at 0 deg C.

Show Answer & Explanation

Correct Answer: A

The initial rate assumption (Rate ∝ 1/t) is only valid if the timed event occurs before reactant concentrations change appreciably (typically within the first 1-5% of reaction progress). Adding too much thiosulfate delays the endpoint beyond the initial rate region.

Question 2: Why must a gas syringe be clamped completely horizontally during continuous rate experiments?

A. To prevent the glass barrel from cracking due to thermal expansion. B. Clamping vertically would introduce gravitational resistance on the plunger (if pointed up) or cause the plunger to drop under its own weight (if pointed down), introducing systematic volume error. C. To allow air bubbles to escape into the room. D. To make the volume markings readable without glasses.

Show Answer & Explanation

Correct Answer: B

Horizontal alignment ensures that the only force moving the plunger is the pressure of the generated gas, eliminating gravity-induced friction and pressure bias.