9701 A Level Chemistry/ Topic 26 Reaction kinetics (A Level)/ 6 interactive simulations/ drag to rotate 3D scenes

Rate equations and reaction kinetics, made interactive

Six simulations covering Topic 26: an initial-rates experiment on a hidden reaction where you choose the concentrations, read the gradients and deduce the orders and the rate constant with its units; the concentration-time and rate-concentration graphs for zero, first and second order with half-life markers; SN1 and SN2 hydrolysis of a halogenoalkane stepped through in 3D to show which rate equation each mechanism predicts; the Boltzmann distribution with a temperature slider and an activation energy line; the Arrhenius plot of ln k against 1/T with the gradient turned into Ea; and energy profiles for homogeneous and heterogeneous catalysts with the iron surface in the Haber process animated.

1. The initial-rates method 2D · 26.1

A hidden reaction A + B → products obeys rate = k[A]^m[B]ⁿ. You cannot see m, n or k. Choose starting concentrations, run the experiment, and read the gradient of the concentration-time curve at t = 0. Change one concentration at a time and the orders fall out of the table.

Hidden reaction

[A]₀0.10

[B]₀0.10

Exp	[A] / mol dm⁻³	[B] / mol dm⁻³	initial rate / mol dm⁻³ s⁻¹

Your deduction

order in Aorder in B

RATE EQUATION AND k

run some experiments first

Method: keep [B] fixed and vary [A], then keep [A] fixed and vary [B]. Doubling a concentration and seeing the rate stay the same means order 0, double means order 1, four times means order 2. Only the first moments of each run are used, because the concentrations are known exactly at t = 0.

2. Graph shapes and half-life 2D · 26.1

The order of a reaction decides the shape of both graphs. Left, concentration against time with the successive half-lives marked. Right, rate against concentration. The moving dot follows the reaction as it runs, and the slider changes the starting concentration so you can see which half-lives depend on it.

Order with respect to A

[A]₀1.00

RATE EQUATION

INITIAL RATE / mol dm⁻³ s⁻¹

NOW: [A] AND RATE

half-life	1st	2nd	3rd
t_½ / s

Exam point: a constant half-life, however far the reaction has gone, shows first order, and then t_½ = ln 2 / k = 0.693 / k. A straight rate-concentration line through the origin also shows first order, and its gradient is k.

3. Mechanism and the rate-determining step 3D · 26.1

Hydroxide ions hydrolyse a halogenoalkane in two different ways. A primary one does it in a single collision (S_N2). A tertiary one first loses its bromide to make a carbocation, and only then meets the hydroxide (S_N1). The slowest step decides the rate, so the two mechanisms predict different rate equations. Step through each and test the prediction.

drag to rotate · wheel to zoom

Mechanism

STEP

PREDICTED RATE EQUATION

Test the prediction

[RX]0.10

[OH⁻]0.10

RATE RELATIVE TO THE FIRST ROW OF THE TABLE

[RX] / mol dm⁻³	[OH⁻] / mol dm⁻³	rate / mol dm⁻³ s⁻¹

Conditions: warm aqueous or ethanolic sodium hydroxide, heated under reflux. S_N2 is typical of primary halogenoalkanes and S_N1 of tertiary ones. Species in the rate equation are those in the rate-determining step or in steps before it. Rate constants here are illustrative.

4. The Boltzmann distribution and temperature 2D · 26.2

Molecules have a spread of energies. Only those with at least the activation energy can react. Raising the temperature flattens and widens the distribution, and because the high-energy tail is so thin, a small shift moves a large proportion of it past E_a. The gold dashed copy of the tail is magnified so that you can see it.

Conditions

temperature300 K

E_a / kJ mol⁻¹50

FRACTION WITH E ≥ E_a AT THIS T

FRACTION AT 300 K (REFERENCE)

THIS T COMPARED WITH 300 K

FACTOR FOR A 10 K RISE FROM HERE

Rule of thumb: for E_a near 50 kJ mol⁻¹ and room temperature, a 10 K rise roughly doubles the fraction of molecules above E_a, and so roughly doubles the rate. The area under the whole curve stays the same, because the number of molecules does not change. Energies are per mole.

5. The Arrhenius plot 2D · 26.2

The rate constant follows k = A e^−E_a/RT. Taking natural logarithms gives ln k = ln A − (E_a/R) × (1/T), a straight line when ln k is plotted against 1/T. Measure k at six temperatures, and the gradient of the line is −E_a/R. Change E_a and watch the table, the curve and the gradient respond.

The reaction

E_a / kJ mol⁻¹60

T / K	k / s⁻¹	1/T / 10⁻³ K⁻¹	ln k

GRADIENT OF LINE / K

E_a = −GRADIENT × 8.314

INTERCEPT = ln A

k AT 340 K / k AT 290 K

Method: here A = 1.0 × 10¹¹ s⁻¹ and R = 8.314 J K⁻¹ mol⁻¹. Convert the gradient result from J to kJ at the end. Take the triangle from points on the line, not from the plotted points, and make it large so that the gradient is accurate.

6. Catalysts and energy profiles 2D · 26.3

A catalyst gives the reaction a different route with a lower activation energy and is regenerated at the end. A homogeneous catalyst is in the same phase as the reactants and works by changing oxidation state. A heterogeneous catalyst is a solid surface where the reactants adsorb, react and desorb. Pick a route, then press play or choose a stage.

Route

highest E_a with catalyst60

STAGE

E_a UNCATALYSED (ILLUSTRATIVE)

100 kJ mol⁻¹

RATE BOOST AT 298 K

The catalyst does not change ΔH, the position of equilibrium or the products. It only lets more molecules react by lowering E_a. Energy values are illustrative; the boost is e^ΔE_a/RT with the pre-exponential factor unchanged.

TheLucidSTEM · Cambridge International A Level Chemistry 9701 · Topic 26 Reaction kinetics (A Level). Every scene runs in the browser; drag to rotate 3D scenes, wheel to zoom. Values are standard textbook data rounded for teaching.

About this lab: Rate equations and reaction kinetics

Six interactive simulations for Cambridge A Level Chemistry 9701 topic 26 Reaction kinetics: an initial-rates experiment on a hidden reaction with a table of rates and a check of the orders, rate constant and units, concentration-time and rate-concentration graphs for orders zero, one and two with half-life markers, SN1 and SN2 hydrolysis of halogenoalkanes in 3D with the rate equation each predicts, the Maxwell-Boltzmann distribution with temperature and activation energy sliders and the fraction of molecules above Ea, the Arrhenius plot of ln k against 1/T with the gradient giving Ea, and enthalpy profiles for homogeneous and heterogeneous catalysts including the three stages on the iron surface in the Haber process.

Rate equations and reaction kinetics is a free, browser-based simulation for Cambridge International AS & A Level Chemistry 9701 (topic 26 Reaction kinetics). The course is assessed in Paper 1 (Multiple Choice (AS)), Paper 2 (AS Structured), Paper 3 (Advanced Practical), Paper 4 (A Level Structured), Paper 5 (Planning, Analysis, Evaluation). Syllabus for examination in 2025-2027. There are 6 tabs, each a self-contained experiment with its own controls and live readings, and nothing to install: it runs on a laptop, tablet or classroom projector.

Syllabus coverage

26.1 Simple rate equations, orders of reaction and rate constants. Lessons: Orders of reaction and rate equations, Rate-determining step and mechanisms.
26.2 Effect of temperature on rate constants and the concept of activation energy. Lessons: Temperature, rate constants and the Arrhenius equation.
26.3 Homogeneous and heterogeneous catalysts. Lessons: Homogeneous and heterogeneous catalysts.

Full syllabus map: A Level Chemistry 9701 topic 26.

How to use this simulation

1. The initial-rates method. A hidden reaction A + B → products obeys rate = k[A]m[B]n.
2. Graph shapes and half-life. The order of a reaction decides the shape of both graphs.
3. Mechanism and the rate-determining step. Hydroxide ions hydrolyse a halogenoalkane in two different ways.
4. The Boltzmann distribution and temperature. Molecules have a spread of energies.
5. The Arrhenius plot. The rate constant follows k = A e−Ea/RT.
6. Catalysts and energy profiles. A catalyst gives the reaction a different route with a lower activation energy and is regenerated at the end.
The buttons at the end of the tab strip give the scene the whole window (Full screen, Hide controls) and, on a dark stage, a projector-friendly palette (Light scene). Many scenes also have drag handles: drag them to change the set-up directly.

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