Chemical Change & Rate Of Reaction

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Chemical Change & Rate Of Reaction (Chemistry, CIE) — use them in your lesson, or run the topic as a live class game.

Notes

Physical & Chemical Changes

**Physical changes** do not produce new substances; they are often easy to reverse (e.g., melting, dissolving).
**Chemical changes** form new substances with different properties; signs include colour change, temperature change, effervescence.
Example of colour change: copper (orange-brown) + silver nitrate → silver solid + blue solution.
Temperature changes: **exothermic** reactions release heat (e.g., CaO + water); **endothermic** reactions absorb heat (e.g., dissolving ammonium chloride).
Effervescence (fizzing) indicates gas production, e.g., alkali metals + water produce hydrogen.

Rates of Reaction Factors

Rate of reaction is affected by: **concentration** (or pressure for gases), **surface area** of solids, **temperature**, and **catalysts**.
Higher concentration/pressure → more particles per volume → steeper initial gradient, same final product amount.
Higher surface area (e.g., powder vs lumps) → more exposed particles → steeper initial gradient, same final product.
Higher temperature → particles have more kinetic energy → steeper initial gradient, same final product.
**Catalysts** speed up reaction without being consumed; they provide an alternative pathway with lower activation energy.

Collision Theory (Extended)

For a reaction to occur, particles must **collide** with sufficient energy (≥ **activation energy**) and correct orientation.
**Successful collisions** lead to product formation; **unsuccessful collisions** result in particles bouncing off unchanged.
Rate depends on: number of particles per unit volume, collision frequency, kinetic energy of particles, and activation energy.
Increasing any factor that raises collision frequency or energy increases the number of successful collisions per second.

Explaining Rates Using Collision Theory (Extended)

**Concentration**: more particles per volume → more collisions per second → higher rate.
**Pressure** (gases): same particles in smaller volume → more collisions per second → higher rate.
**Surface area**: more exposed particles → more collisions per second → higher rate.
**Temperature**: particles gain kinetic energy → more collisions and a greater proportion exceed activation energy → rate increases sharply (≈ doubles per 10 °C).
**Catalysts**: lower activation energy → more collisions have sufficient energy → higher rate.

Investigating the Rate of a Reaction

Common methods: **mass loss** on a balance, **gas volume** (downward displacement or gas syringe), **disappearing cross** (precipitate formation).
Disappearing cross: measure time for a cross to become obscured by sulfur precipitate from sodium thiosulfate + HCl.
Gas collection: e.g., magnesium + acid → hydrogen; measure volume over time.
Catalyst investigation: e.g., hydrogen peroxide decomposition with MnO₂ catalyst; compare gas volumes.
Advantages/disadvantages: mass loss is simple but unsuitable for low‑mass gases; gas syringes are accurate but fragile.

Interpreting Data

Rate is fastest at the start (steepest gradient) because reactant concentration is highest.
As reaction proceeds, gradient decreases; when one reactant is used up, the line becomes horizontal (rate = 0).
To find rate at a specific time, draw a **tangent** to the curve and calculate gradient = change in y / change in x.
Comparing experiments: higher concentration, temperature, surface area, or catalyst gives a steeper initial curve and same final product amount (if limiting reactant unchanged).
If the amount of limiting reactant is increased, the final product amount increases.

Particle arrangement in solids, liquids, and gases. Solids have fixed, closely packed particles; liquids have slightly more spacing; gases have particles far apart and moving rapidly.

Graph comparing reaction rates at low and high concentration: higher concentration gives a steeper initial gradient and reaches the same final volume sooner.

Graph showing that a higher temperature (20 °C) gives a steeper initial gradient and faster reaction than a lower temperature (10 °C), reaching the same final volume sooner.

Practice questions

Free preview — 8 of 40 questions. Sign up to see them all.

1.Which of the following processes represents a physical change?
Easy
- AAcid base neutralisation
- BMetal displacement reactions
- CBoiling ethanol
- DCombustion of magnesium
2.Physical and chemical changes are different processes. Which of the following rows correctly identifies characteristics of each one?
Easy
- Achemical change - no new substance formed; physical change - change of state
- Bchemical change - no new substance formed; physical change - involves electron transfer
- Cchemical change - new substance formed; physical change - involves electron transfer
- Dchemical change - new substance formed; physical change - change of state
3.Which process is a physical change?
Easy
- Acracking of an alkane
- Bhydration of ethene
- Cfractional distillation of petroleum
- Dthermal decomposition of calcium carbonate
4.The rate of a reaction was monitored by recording the volume of gas produced every 5 seconds. What is the mean rate of reaction in the first 10 seconds of the reaction, in cm³/s? (Data: at 0 s, 0 cm³; at 5 s, 15 cm³; at 10 s, 35 cm³)
Easy
- A2.9
- B3.0
- C3.5
- D2.5
5.A student was investigating the rate of reaction between iron and a solution of sulfuric acid. Two experiments were performed, P and Q, in which the volume of hydrogen was recorded at regular intervals. Which change could explain the difference in results between P and Q? (Graph shows P has steeper initial slope and reaches higher final volume than Q)
Medium
- AThe student added a catalyst in P.
- BLarger pieces of iron were used in P.
- CLess concentrated acid was used in P.
- DLarger quantities of both reactants are used in P.
6.A student investigates the rate of reaction between aqueous sodium thiosulphate and aqueous hydrochloric acid by measuring the time taken for a precipitate to obscure a cross underneath the beaker. Which statement is correct for the reaction?
Medium
- AThe precipitate will be formed faster if ice is placed in contact with the beaker.
- BThe precipitate will be formed faster if 10.0 cm³ of water are added to the aqueous sodium thiosulphate.
- CThe precipitate will be formed slower if a catalyst is added to the reaction mixture.
- DThe precipitate will be formed faster if drops of concentrated hydrochloric acid are added to the reaction mixture.
7.A student investigates the effect of temperature on the rate of a reaction. In Experiment 1, the reaction is performed at 10 °C. In Experiment 2, the reaction is repeated at 20 °C. All other conditions stay the same. Which graph shows the results for the volume of gas produced against time? (Options: four graphs showing curves; correct one has steeper initial slope and reaches same final volume sooner for 20°C)
Medium
- AGraph A: both curves identical
- BGraph B: 20°C curve has steeper slope and higher final volume
- CGraph C: 20°C curve has steeper slope and same final volume, levels off earlier
- DGraph D: 20°C curve has less steep slope and lower final volume
8.Magnesium metal reacts with an excess of hydrochloric acid solution to form magnesium chloride and hydrogen: Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g). Which of the following will not increase the rate of this reaction?
Medium
- AIncrease the temperature of the acid
- BIncrease the surface area to volume ratio of the pieces of magnesium
- CIncrease the concentration of the hydrochloric acid
- DIncrease the volume of hydrochloric acid solution used

Slides

Physical & Chemical Changes

Physical changes do not produce new substances and are often easy to reverse. Examples: melting, dissolving.
Chemical changes form new substances with different properties. Signs include colour change, temperature change, effervescence.
Example: copper + silver nitrate → silver + copper nitrate (colour change from orange-brown to silver and colourless to blue).

Which of the following is a physical change?

Acid-base neutralisation
Metal displacement reaction
Boiling ethanol
Combustion of magnesium

Factors Affecting Rate of Reaction

Rate of reaction is affected by: concentration (or pressure for gases), surface area of solids, temperature, and presence of a catalyst.
Higher concentration, pressure, surface area, temperature, or using a catalyst increases the rate.
Graphs show steeper initial gradient and same final product amount when rate is increased.

Which change would increase the rate of reaction between magnesium and hydrochloric acid?

Using larger pieces of magnesium
Decreasing the temperature
Increasing the concentration of acid
Adding water to dilute the acid

Collision Theory (Extended)

For a reaction to occur, particles must collide with sufficient energy (≥ activation energy).
Successful collisions lead to product formation; unsuccessful collisions do not.
Rate depends on frequency of collisions and proportion with energy > activation energy.

What is activation energy?

The energy released when particles collide
The minimum energy required for particles to react when they collide
The maximum energy required for particles to react
The total energy of the reactants

Explaining Rate Factors Using Collision Theory (Extended)

Increasing concentration: more particles per volume → more collisions per second → higher rate.
Increasing pressure: same particles in smaller volume → more collisions per second.
Increasing surface area: more exposed particles → more collisions per second.
Increasing temperature: particles gain kinetic energy → more collisions and higher proportion with energy > activation energy.
Catalyst: provides alternative pathway with lower activation energy → more successful collisions.

Which of these are true about catalysts?

Catalysts are used up during the reaction.
Catalysts provide an alternative reaction pathway with lower activation energy.
Catalysts increase the amount of product formed.
Enzymes are biological catalysts.

Investigating Rate of Reaction

Common methods: measuring mass loss, gas volume (downward displacement or gas syringe), or time for a colour change (disappearing cross).
For the disappearing cross experiment: sodium thiosulfate + HCl → sulfur precipitate; time to obscure a cross is measured.
Gas collection methods: inverted measuring cylinder (simple but can lose gas) or gas syringe (accurate but fragile).

In the disappearing cross experiment, what is the effect of adding more concentrated hydrochloric acid?

The cross disappears faster
The cross disappears slower
No change
The cross becomes clearer

Interpreting Data from Rate Experiments

Rate is fastest at the start (steepest gradient) and decreases as reactants are used up.
When the line becomes horizontal, the reaction has stopped (one reactant used up).
To find rate at a specific time, draw a tangent and calculate gradient = change in y / change in x.

The graph shows volume of gas vs time for a reaction. Which statement is correct?

The rate is constant throughout.
The rate is highest at the start.
The rate is highest at the end.
The rate is zero at the start.

A catalyst speeds up a reaction by providing an alternative pathway with a lower ____ energy.

Comparing Physical and Chemical Changes

Physical change: no new substance, reversible, e.g., change of state, dissolving.
Chemical change: new substance formed, often irreversible, signs include colour change, temperature change, gas produced.
Examples: boiling water (physical), burning magnesium (chemical).

Which row correctly identifies physical and chemical changes?

Chemical: new substance formed; Physical: change of state
Chemical: no new substance; Physical: involves electron transfer
Chemical: new substance; Physical: involves electron transfer
Chemical: no new substance; Physical: change of state

Effect of Temperature on Rate

Increasing temperature increases rate: particles have more kinetic energy, move faster, collide more often and with greater energy.
Rough rule: for every 10°C rise, rate approximately doubles for aqueous/gaseous systems.
Temperature is the only factor that directly increases the energy of collisions.

Why does increasing temperature increase reaction rate?

Particles have less kinetic energy
Particles have more kinetic energy, leading to more successful collisions
Particles move slower, increasing collision time
Activation energy decreases

Effect of Surface Area

Smaller particles have larger surface area to volume ratio, exposing more reactant to collisions.
Powdered solids react faster than lumps (same mass).
Graph: steeper initial gradient, same final amount.

Which would increase the rate of reaction between marble chips and acid?

Using larger marble chips
Using powdered marble
Decreasing temperature
Using less acid

Summary of Rate Factors

Concentration/pressure: more particles per volume → more collisions.
Surface area: more exposed area → more collisions.
Temperature: more kinetic energy → more collisions and higher energy collisions.
Catalyst: lowers activation energy → more successful collisions.

Unlock all 40 questions, slides & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

🗂️ Coming soon

Notes

Physical & Chemical Changes

Rates of Reaction Factors

Collision Theory (Extended)

Explaining Rates Using Collision Theory (Extended)

Investigating the Rate of a Reaction

Interpreting Data

Particle arrangement in solids, liquids, and gases. Solids have fixed, closely packed particles; liquids have slightly more spacing; gases have particles far apart and moving rapidly.

Graph comparing reaction rates at low and high concentration: higher concentration gives a steeper initial gradient and reaches the same final volume sooner.

Graph showing that a higher temperature (20 °C) gives a steeper initial gradient and faster reaction than a lower temperature (10 °C), reaching the same final volume sooner.

Practice questions

Unlock all 40 questions, slides & more

Past papers

Graph showing that a higher temperature (20 °C) gives a steeper initial gradient and faster reaction than a lower temperature (10 °C), reaching the same final volume sooner.