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2.2 Rate of chemical change

Syllabus
First assessment 2025
Topic
2.2
Level
SL

Reaction Rate

rate=changeinconcentration/timerate = change in concentration / time

An instantaneous rate is the gradient of a tangent at the stated point on a concentration–time, volume–time or mass–time graph. Keep the units consistent.

Choose two well-separated points on the tangent, not on the curve, to calculate its gradient. A reactant concentration has a negative gradient, so report its disappearance rate as a positive magnitude unless a signed change is requested.

Requested rate Graph operation Evidence check
mean over an interval secant gradient between interval endpoints quote the interval and units
initial tangent gradient at t = 0 choose well-separated points on the tangent
instantaneous at time t tangent gradient at that time do not use two points on the curved trace

A measured mass, pressure or gas volume is a rate proxy only when its change is tied to reaction progress under the stated conditions. Preserve reactant/product slope sign or report a positive disappearance/formation magnitude as requested.

Finding an Instantaneous Rate

Assessment in practice

Representative question

Question 1

[Maximum number: 3]

Determine the instantaneous rate of reaction to two significant figures when [Br2]=0.0080 moldm−3\left[\mathrm{Br}_{2}\right]=0.0080 \mathrm{~mol} \mathrm{dm}^{-3}.

Collision Theory

A successful collision needs sufficient kinetic energy to overcome activation energy and a suitable orientation of the reacting particles.

Temperature raises average kinetic energy and changes collision frequency and the fraction of particles with energy at least Ea. Collision frequency alone does not guarantee reaction.

At one temperature, only the fraction of collisions above Ea and with a productive orientation can react. Raising temperature increases that fraction, not the energy of every particle by the same amount. Use collision frequency to explain concentration or pressure effects and the energy distribution to explain the stronger temperature effect.

Explaining Rate with Collision Theory

Assessment in practice

Representative question

Question 1

[Maximum number: 3]

Explain, using collision theory, how an increase in temperature increases the reaction rate.

Factors Affecting Rate

Change Main collision consequence
concentration or pressure up more frequent collisions
surface area up more collisions at a solid surface
temperature up more frequent and more energetic collisions
catalyst alternative lower-Ea pathway

Explain a predicted rate change through collision frequency or the fraction of effective collisions, not just by saying particles move faster.

Powdered CaCO₃ reacts faster than equal-mass chips because more surface sites are exposed, not because its particles have higher kinetic energy. For each changed condition, identify exactly what changes—collision frequency, energy distribution or pathway—and hold other variables constant in a fair comparison.

Predicting Rate Changes

Assessment in practice

Representative question

Question 1

[Maximum number: 2]

The student then carried out the experiment at other acid concentrations with all other conditions remaining unchanged.

\begin{tabular}{|c|c|}
\hline[H+]/ mol dm−3\left[\mathbf{H}^{+}\right] / \mathbf{~ m o l ~ d m}^{-\mathbf{3}} & Relative rate of reaction \\
\hline 0.05 & 0.0025 \\
\hline 0.10 & 0.0051 \\
\hline 0.20 & 0.0100 \\
\hline
\end{tabular}

State and explain the relationship between the rate of reaction and the concentration of acid.

Activation Energy and Maxwell–Boltzmann Curves

Ea=minimumkineticenergyforaneffectivecollisionEa = minimum kinetic energy for an effective collision

A Maxwell–Boltzmann curve shows the distribution of particle kinetic energies. At higher temperature the peak is lower and shifts right; the area beyond Ea is larger, so more particles can react.

Two Maxwell–Boltzmann curves for the same number of particles have equal total area. At higher T the curve is broader with a lower peak and a larger area to the right of a fixed Ea line; the peak does not move to Ea and no particle count is lost.

Interpreting Maxwell–Boltzmann Distributions

Assessment in practice

Representative question

Question 1

[Maximum number: 3]

Explain why the reaction rate increases with temperature, adding annotations to the following Maxwell-Boltzmann graph to assist your explanation.

Explanation:

Catalysts and Activation Energy

A catalyst provides an alternative reaction pathway with lower activation energy. It does not change the energy levels of the reactants or products.

Because the catalysed Ea is lower, a larger fraction of the same distribution lies beyond the threshold. The reaction therefore has more effective collisions at the same temperature.

At fixed temperature a catalyst does not change the Maxwell–Boltzmann distribution; it moves the threshold to a lower Ea, increasing the area beyond it. On an energy profile it changes the pathway and peak, not ΔH, reactant/product energies or the equilibrium constant.

Showing a Catalyst on Energy Diagrams

Assessment in practice

Representative question

Question 1

[Maximum number: 2]

Sketch the Maxwell-Boltzmann energy distribution curve for this reaction. Label the activation energy with and without a catalyst on the diagram.

Rate and Mechanisms Summary

Retrieve the route: measure a tangent rate, explain effective collisions, map rate factors, read Ea and energy profiles, evaluate mechanisms, determine molecularity and orders, calculate k, then use Arrhenius gradient and intercept for Ea and A.

Check tangent versus average slope, energy versus orientation, barrier labels, intermediate versus transition state, one-variable trial comparisons, order-dependent units, kelvin temperature and the signs of gradient and Ea.

ConceptIB Chemistry SL