SC18 · Rates of reaction · Topic 7 — Rates of reaction and energy changes | GCSE Edexcel Separate Sciences

Revise the key ideas

Reaction rate describes how quickly reactants are used up or products are formed. Average rate = amount of change ÷ time taken, with units appropriate to the measurement.
Choose a method that fits the reaction: measure gas volume with a gas syringe, mass loss on a balance if gas escapes, or a change in colour or cloudiness (turbidity).
For 40 cm³ gas in 20 s, average rate is 40 ÷ 20 = 2 cm³ s⁻¹. A mass loss of 0.6 g in 30 s gives 0.02 g s⁻¹.
On a product-volume versus time graph, gradient gives rate. A steeper curve means faster production; the plateau means gas production has stopped, usually because a reactant is used up.
Same reactant amounts and completion; speed differs but final yield need not.
The rate at one particular moment (instantaneous rate) is the gradient of a tangent to the curve at that time. Average rate over an interval is change in measured quantity ÷ change in time: the gradient of a straight line joining the two ends of that interval (a secant).
Choose a sufficiently large triangle; the tangent estimates rate at one instant. Tangent geometry is calculated at a point on this schematic curve.
On a reactant-mass graph the gradient can be negative because mass falls; rate of consumption is usually reported as a positive magnitude. Check what the vertical axis measures.

Reacting particles must collide with sufficient energy, at least the activation energy, for a successful reaction. Not every collision produces reaction.
Higher concentration means more reactant particles per volume and more frequent collisions. Keep other variables controlled when explaining the effect.
Higher pressure for reacting gases puts more particles into a given volume, increasing collision frequency. This does not apply directly to merely pressing a solid reactant.
Smaller solid pieces have a larger surface area for the same total mass. More exposed particles are available for collisions, so powder usually reacts faster than large lumps.
Schematic faces represent cubes; cutting increases exposed area at fixed total volume.
Increasing temperature makes particles move faster and collide more frequently. More importantly, a greater fraction of collisions have enough energy to overcome activation energy.
Temperature affects both frequency and the energy distribution.
As reactants are used up, concentration or available solid decreases, so the rate often slows. A reaction graph commonly becomes less steep with time.
Smaller solid pieces or a catalyst can make a reaction finish sooner. They do not change the final amount of product if the same amounts of reactants react completely in the same reaction.

Calcium carbonate marble reacts with hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Carbon dioxide volume can be recorded against time.
Use a conical flask connected through a stopper and delivery tube to a gas syringe. Check connections for leaks and ensure the plunger moves freely; do not create a sealed pressure vessel with no expansion route.
Check for leaks; gas must be able to expand into the syringe.
Measure acid volume and concentration, add measured marble chips, connect promptly and start the timer consistently. Record gas volume at regular intervals.
To compare surface area, use the same mass of marble as different-sized chips, with the same acid volume/concentration and temperature. If acid is sufficient for complete reaction, final gas volume should be the same.
To compare acid concentration, control acid volume, chip mass/size and temperature. Make sure the chosen limiting reactant is understood: changing concentration can also change final yield if acid limits reaction.
An alternative is recording mass loss as CO₂ escapes from an unsealed flask on a balance. A loose cotton-wool plug can reduce spray without trapping the gas; cotton wool is not a gas-tight stopper.
Use eye protection, appropriate dilute acid and teacher supervision. Repeat trials, compare results and investigate anomalies rather than discarding inconvenient readings without reason.

Sodium thiosulfate reacts with hydrochloric acid to form a cloudy sulfur precipitate. Place a flask over a drawn cross and time until the cross can no longer be seen.
Use the same depth, cross and viewing conditions; endpoint judgement has uncertainty.
The method uses the same visual endpoint to compare relative rate: for the same endpoint, rate is proportional to 1/time. Reciprocal time has units s⁻¹, not a measured gas-volume rate.
To vary thiosulfate concentration, dilute measured portions with water while keeping total mixture volume the same. Keep acid volume/concentration, temperature, flask and viewing conditions controlled.
Record the time from consistent mixing to disappearance of the cross. A shorter time indicates faster reaction; repeat and use a suitable mean, noting uncertainty in judging the endpoint.
The reaction releases sulfur dioxide, so use suitable ventilation, small volumes and the specified safe school procedure. Stop and follow the teacher's guidance rather than breathing fumes.
The cross disappearing is a visual endpoint, not necessarily the end of the reaction. How cloudy the liquid is, the lighting and the observer’s judgement all affect when the cross seems to disappear.

A catalyst speeds up a reaction and is not used up overall. It is chemically unchanged and has the same mass at the end. It may react in individual steps, but is made again (regenerated) by the end.
A catalyst provides an alternative reaction pathway with lower activation energy, so more collisions succeed at the same temperature. It does not make every collision successful.
The catalyst is regenerated overall; final products and equilibrium position are unchanged.
For a complete reaction with the same limiting-reactant amount, a catalyst does not increase theoretical final product amount. It also does not change equilibrium yield under fixed conditions.
Catalysts can allow useful industrial rates at lower temperatures, reducing energy costs. A suitable catalyst depends on the reaction; there is no universal catalyst.
Vehicle catalytic converters convert pollutants such as carbon monoxide and nitrogen oxides to less harmful gases. Carbon dioxide produced still contributes to greenhouse effects, so “all products harmless” is too broad.
Enzymes are biological catalysts. Yeast enzymes catalyse fermentation of sugars in alcoholic-drink production; temperature and conditions affect enzyme activity.
For a rate experiment with and without a catalyst, keep reactant amounts and other conditions the same, then compare rates rather than just noting a different total time without controls.