6.2Chemical Reactions
Rate of reaction Core — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 11 parts Lesson 2 of 7 1 quiz
Lesson summary
6.2 Rate of reaction Core — Notes — IGCSE Chemistry notes: Explain how concentration, pressure, surface area, temperature and catalysts affect the rate of reaction using collision theory, and describe how rate is measured.Rate of reaction: the change in the amount (or concentrati…
- Activation energy
- Rate of reaction: the change in the amount (or concentration) of a reactant or product per unit time
- It is measured by the change in mass, the formation of a gas or a colour change
- A higher rate means faster production and a more efficient, sustainable process
- A faster reaction has a steeper gradient at the start, becomes horizontal sooner and forms the same amount of product
- Concentration of solutions: a higher concentration increases the rate
Section 1 of 11Rate of reaction
1
Rate of reaction: the change in the amount (or concentration) of a reactant or product per unit time.
2
It is measured by the change in mass, the formation of a gas or a colour change.
3
A higher rate means faster production and a more efficient, sustainable process.
4
A faster reaction has a steeper gradient at the start, becomes horizontal sooner and forms the same amount of product.
Section 2 of 11Factors that change the rate
1
Concentration of solutions: a higher concentration increases the rate.
2
Pressure of gases: a higher pressure increases the rate.
3
Surface area of solids: powdering a solid increases the rate.
- Cutting a solid into smaller pieces increases its total surface area.
- Fine powders such as flour and coal dust can explode because of the very large surface area and very fast combustion.
4
Temperature: a higher temperature increases the rate, and the rate roughly doubles for every 10 °C rise.
5
Catalyst: adding a catalyst increases the rate and removing it decreases the rate.
Section 3 of 11Catalysts and enzymes
1
Catalyst: speeds up a reaction without being used up, so its mass is the same at the start and end and it is not in the equation.
2
A catalyst decreases the activation energy by providing an alternative pathway.
3
Examples: MnO2 catalyses 2H2O2 → 2H2O + O2, and iron catalyses the Haber process.
4
Enzymes are biological catalysts that speed up reactions in living things.
- They work best at an optimum temperature and pH.
- They are denatured at high temperature.
5
A catalyst changes only the time taken, not the amount of product.
6
On an energy profile the peak is lower but the energies of the reactants and products (and ΔH) are unchanged.
Section 4 of 11Collision theory
1
Collision theory: particles react only when they collide with at least the activation energy and in the correct orientation.
2
Activation energy Ea: the minimum energy colliding particles need to react.
3
Successful collision: the particles have enough energy and the correct orientation and rearrange into products.
4
Unsuccessful collision: the particles bounce off unchanged because of too little energy or the wrong orientation.
5
The number of successful collisions depends on the particles per unit volume, the collision frequency, the kinetic energy of the particles and the size of the activation energy.
Section 5 of 11Explaining the factors
| Factor | Change | Effect on rate | Explanation |
|---|---|---|---|
| Concentration | Increase | Faster | More particles per unit volume, so more frequent collisions |
| Pressure (gases) | Increase | Faster | Same particles in a smaller volume, so more frequent collisions |
| Surface area | Powder | Faster | More particles exposed, so more frequent collisions |
| Temperature | Increase | Faster | Particles move faster and collide more often, and a greater proportion of collisions have energy at least Ea |
| Catalyst | Add | Faster | Alternative pathway with lower Ea, so a greater proportion of collisions are successful |
Explaining the factors
Only temperature and catalysts change the proportion of collisions with energy at least Ea.
Concentration, pressure and surface area change only collision frequency, so they do not change the energy of the particles.
For temperature the increase in energy is the more important effect.
Section 6 of 11Measuring the rate
1
Follow how quickly reactants are used up or products formed, using a property proportional to concentration (colour, mass or volume).
2
Marble chips with hydrochloric acid
- Place a measured volume of acid in a conical flask, keeping the volume and concentration fixed.
- Add a known mass of marble chips, start the stop-clock at once and fit a cotton-wool plug on a balance or connect the bung to a gas syringe.
- Record the mass or gas volume every 30 s until readings stop changing.
- Repeat, changing only one variable (smaller chips, a different concentration or a different temperature).
- Plot mass lost or volume against time for each run on the same axes and compare the gradients.
3
Mass loss: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g), where the mass decreases as CO2 escapes.
4
Mass loss is unsuitable for hydrogen and other low-Mr gases because the change in mass is too small to measure.
5
Rate at a chosen time is the gradient of a tangent drawn on the graph.
Section 7 of 11Other rate experiments
1
Disappearing cross
- Place 50 cm3 of sodium thiosulfate solution over a cross, add 5 cm3 of dilute HCl and start the clock.
- A cloudy sulfur precipitate forms and the clock is stopped when the cross is no longer visible.
- Diluting the thiosulfate with water gives other concentrations, and relative rate = 1 ÷ time.
2
Surface area: react magnesium ribbon in different piece sizes (same total mass) with acid and collect the gas over water in a fixed time.
3
Temperature: heat the acid in a water bath and measure the time for a magnesium strip to disappear.
4
Catalyst: compare hydrogen peroxide with and without MnO2, collecting oxygen in an inverted measuring cylinder.
5
Fair test: change only one independent variable and control volume, concentration, mass, temperature and particle size.
Section 8 of 11Evaluating methods
| Method | Advantage | Disadvantage |
|---|---|---|
| Disappearing cross | Simple, no specialist equipment | Subjective, because people judge the cross to disappear at different cloudiness |
| Gas syringe | Works for any gas and collects all of it | Fragile, expensive, can stick and has a limited capacity |
| Inverted measuring cylinder | Cheap, common equipment | Tube can pop out, scale is hard to read and gas is lost while fitting the bung |
| Mass loss on a balance | Easy to set up | Unsuitable for gases of low Mr such as hydrogen |
Section 9 of 11More from the specification
1Monitoring Reaction Rates by Mass Loss
- Reactions producing carbon dioxide or hydrogen can be monitored by placing the reaction flask on an analytical balance, measuring the continuous decrease in mass over time.
2Gas Syringe Method for Gaseous Products
- Collecting evolving gas in a graduated gas syringe at regular time intervals produces a volume-time curve whose initial tangent slope quantifies the initial rate of reaction.
3Disappearing Cross Method for Precipitates
- In the sodium thiosulfate and hydrochloric acid reaction, timing how long sulfur precipitate takes to obscure a black cross beneath the conical flask provides an inverse measure of reaction rate.
4Thermal energy
- Total kinetic energy of the particles in a substance.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Rate of reaction | the change in amount of a reactant or product per unit time. |
| Collision theory | particles react only if they collide with at least the activation energy and the correct orientation. |
| Activation energy | the minimum energy colliding particles need to react. |
| Catalyst | a substance that speeds up a reaction and is not used up. |
| Enzyme | a biological catalyst. |
| Surface area | the total area of a solid exposed to react. |
Section 11 of 11Exam technique
1
Saying particles collide morethey collide more frequently, with more collisions per second.
2
Saying temperature only increases the number of collisions: it also increases the proportion with energy at least Ea.
3
Saying a catalyst is used up or gives more product: it is unchanged at the end and gives the same amount, only faster.
4
Saying concentration changes the energy of the particles: it changes only how often they collide.
5
Saying a larger surface area means bigger pieces: powder has the larger surface area.
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