Chemical Change And Rate Of Reaction

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Physical & Chemical Changes

Physical changes do not produce new substances; they are easy to reverse (e.g., melting, dissolving).
Chemical changes form new substances with different properties; they are difficult to reverse.
Signs of chemical change include: colour change, temperature change, effervescence (fizzing), and formation of a precipitate.
Example: Copper displaces silver from silver nitrate – solid changes from orange-brown to silver, solution from colourless to blue.
Exothermic reactions release heat (e.g., calcium oxide + water); endothermic reactions absorb heat (e.g., dissolving ammonium chloride in water).

The metal displacement reaction of silver and copper

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 → more collisions per second → faster rate.
Higher surface area (e.g., powder vs. lumps) → more exposed particles → more collisions per second → faster rate.
Higher temperature → particles have more kinetic energy → more frequent and energetic collisions → faster rate.
Catalyst speeds up reaction without being consumed; it provides an alternative pathway with lower activation energy.

Graph showing the effect of concentration on rate of reaction

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 apart.
Rate depends on: number of particles per volume, collision frequency, kinetic energy, and activation energy.
Increasing concentration, pressure, surface area, or temperature increases the number of successful collisions per second.
Catalysts lower activation energy, so a greater proportion of collisions are successful.

A successful collision

Explaining Rates Using Collision Theory (Extended)

Concentration: More particles per unit volume → more collisions per second → faster rate.
Pressure: Same number of particles in smaller volume → more collisions per second → faster rate.
Surface area: More exposed area → more collisions per second → faster rate.
Temperature: Particles gain kinetic energy → more collisions and a higher proportion with energy ≥ activation energy → rate increases significantly (≈doubles per 10 °C rise).
Catalyst: Lowers activation energy → more particles have sufficient energy → more successful collisions per second.

Diagram showing the effect of increasing concentration

Investigating the Rate of a Reaction

Rate can be measured by how quickly reactants are used up or products are formed (e.g., gas volume, mass loss, colour change).
Sodium thiosulfate + acid: measure time for a cross to disappear; higher concentration → shorter time.
Magnesium + acid: collect gas by downwards displacement; smaller Mg pieces (larger surface area) → faster rate.
Temperature effect: heat acid in water bath, add Mg, time until Mg disappears; higher temperature → faster rate.
Catalyst effect: hydrogen peroxide decomposition with MnO₂ catalyst; measure oxygen volume; catalyst increases rate.

Diagram showing the apparatus needed to investigate the effect of concentration on the rate of reaction

Interpreting Data

Rate is fastest at the start (steepest gradient) because reactant concentration is highest.
As reaction proceeds, rate decreases (gradient becomes less steep) until reactants are used up (line becomes horizontal).
The limiting reactant determines the final amount of product; excess reactant does not increase product yield.
To find rate at a specific time, draw a tangent to the curve and calculate its gradient: rate = Δy/Δx.
Graphs of volume vs. time or mass vs. time show a curve that levels off when reaction stops.

Calculating the rate of reaction from a tangent

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Practice questions

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1.Which of the following is a sign of a chemical change?
Easy
- AMelting of ice
- BDissolving sugar in water
- CEffervescence (fizzing)
- DBoiling water
2.A catalyst is consumed during a chemical reaction.
Easy
True or false?
3.State two factors that affect the rate of a chemical reaction.
Easy
4.The rate of a reaction approximately doubles for every 10 °C rise in temperature. If the rate at 20 °C is 2.0 cm³/s, what is the approximate rate at 40 °C?
Medium
- A8
- B10
- C7
- D9
5.Complete the sentence using the correct word.
Easy
A catalyst provides an alternative reaction pathway with a ____ activation energy.
6.Match each factor to its effect on the rate of reaction.
Medium
- Increasing temperature
- Increasing concentration
- Adding a catalyst
- Increases frequency and energy of collisions
- Increases number of particles per unit volume
- Lowers activation energy
7.Arrange the following steps in the correct order to investigate the effect of concentration on the rate of reaction using sodium thiosulfate and hydrochloric acid.
Medium
- Add hydrochloric acid to the flask and start the stopwatch.
- Draw a cross on paper and place it under the flask.
- Measure 50 cm³ of sodium thiosulfate solution into a flask.
- Stop the stopwatch when the cross is no longer visible.
- Repeat with different concentrations of sodium thiosulfate.
8.In the reaction between magnesium and hydrochloric acid, which change would increase the rate of reaction the most?
Medium
- AUsing magnesium ribbon instead of powder
- BDecreasing the temperature from 30 °C to 20 °C
- CUsing 1.0 mol/dm³ acid instead of 0.5 mol/dm³ acid
- DAdding a catalyst

More questions

9.Explain why increasing the temperature increases the rate of reaction in terms of collision theory.
Medium
10.In an experiment, 0.2 g of manganese(IV) oxide was added to 25 cm³ of 0.1 mol/dm³ hydrogen peroxide. The volume of oxygen produced over time was recorded. The maximum volume of oxygen produced was 40 cm³. If the experiment is repeated with 25 cm³ of 0.2 mol/dm³ hydrogen peroxide, what maximum volume of oxygen (in cm³) would be expected?
Hard
- A96
- B88
- C72
- D80
11.Increasing the pressure of a gas reaction increases the rate because it increases the number of particles per unit volume.
Easy
True or false?
12.Which statement about a catalyst is correct?
Easy
- AIt is used up in the reaction.
- BIt increases the activation energy of the reaction.
- CIt provides an alternative reaction pathway with a lower activation energy.
- DIt increases the yield of the product.
13.Which of the following is a physical change?
Easy
- ABurning wood
- BRusting iron
- CMelting ice
- DDigesting food
14.A catalyst increases the rate of a chemical reaction by being consumed in the reaction.
Easy
True or false?
15.Which statement about collision theory is correct?
Medium
- AAll collisions between reactant particles result in a reaction.
- BIncreasing temperature increases the number of particles per unit volume.
- CA successful collision requires particles to have energy greater than or equal to the activation energy.
- DA catalyst increases the activation energy of a reaction.
16.In an experiment to investigate the effect of surface area on reaction rate, magnesium ribbon and magnesium powder of the same mass are reacted separately with excess hydrochloric acid. Which statement is correct?
Medium
- AThe magnesium powder produces more hydrogen gas.
- BThe magnesium ribbon produces hydrogen gas faster.
- CThe magnesium powder produces hydrogen gas faster.
- DBoth forms produce hydrogen gas at the same rate.
17.Which of the following are signs that a chemical change has occurred? (Select all that apply)
Medium
- AColour change
- BChange of state (e.g. melting)
- CEffervescence (fizzing)
- DTemperature change
- EDissolving sugar in water
18.Which of the following is a chemical change?
Easy
- AMelting ice
- BDissolving sugar in water
- CBurning wood
- DBoiling water
19.Match each factor to its effect on the number of successful collisions per second.
Medium
- Increasing concentration
- Increasing temperature
- Adding a catalyst
- Increasing surface area
- Lowers activation energy
- Increases number of particles per unit volume
- Increases kinetic energy and collision frequency
- Exposes more particles to collisions
20.Arrange the following steps in the correct order to investigate the effect of temperature on the rate of reaction between magnesium and hydrochloric acid.
Medium
- Add a strip of magnesium to the acid
- Heat the acid to a set temperature using a water bath
- Stop the stopwatch when the magnesium has fully reacted
- Start the stopwatch immediately after adding the magnesium
21.The graph shows the volume of gas produced over time for a reaction. At which point is the rate of reaction the greatest?
Medium
- AAt the start (0 s)
- BHalfway through the reaction
- CWhen the line becomes horizontal
- DThe rate is constant throughout
22.A student investigates the rate of reaction between magnesium and hydrochloric acid. They use the same mass of magnesium but in two forms: magnesium ribbon and magnesium powder. Which graph shows the expected results? (Volume of hydrogen gas produced over time.)
Medium
- AGraph A: steeper initial gradient for ribbon, same final volume
- BGraph B: steeper initial gradient for powder, same final volume
- CGraph C: steeper initial gradient for powder, lower final volume
- DGraph D: steeper initial gradient for ribbon, higher final volume
23.A catalyst provides an alternative reaction pathway with a lower activation energy.
Easy
True or false?
24.In an experiment to investigate the effect of concentration on reaction rate, sodium thiosulfate solution is mixed with hydrochloric acid. The time taken for a cross to disappear is measured. Which change would decrease the time for the cross to disappear?
Medium
- AUsing a more dilute sodium thiosulfate solution
- BUsing a lower temperature
- CUsing a more concentrated hydrochloric acid
- DUsing a smaller volume of sodium thiosulfate solution
25.Arrange the following steps in the correct order to investigate the effect of a catalyst on the decomposition of hydrogen peroxide.
Medium
- Add manganese(IV) oxide to the hydrogen peroxide
- Measure the volume of oxygen produced over time
- Set up apparatus with a conical flask and gas syringe
- Repeat the experiment without the catalyst for comparison
26.In an experiment, hydrochloric acid reacts with calcium carbonate chips and the volume of carbon dioxide produced is measured over time. Which change would give a steeper initial gradient but the same final volume of gas?
Medium
- AUsing a lower concentration of acid
- BUsing smaller chips (larger surface area)
- CUsing a lower temperature
- DUsing the same mass of chips but with a larger piece
27.In an experiment to investigate the effect of concentration on the rate of reaction, sodium thiosulfate solution is reacted with dilute hydrochloric acid. The time taken for a cross drawn on paper to become invisible is recorded. Which change would decrease the time taken for the cross to disappear?
Medium
- AUsing a lower concentration of sodium thiosulfate
- BUsing a lower temperature
- CUsing a higher concentration of sodium thiosulfate
- DUsing a larger volume of water to dilute the sodium thiosulfate
28.During a chemical change, new chemical substances are formed that have different properties from the reactants.
Easy
True or false?
29.Which of the following is a sign that a chemical reaction has occurred?
Easy
- AA change of state
- BDissolving
- CEffervescence (fizzing)
- DMaking a mixture
30.Which of the following factors affect the rate of a chemical reaction? (Select all that apply)
Medium
- AConcentration of reactants
- BColour of reactants
- CTemperature
- DSurface area of solid reactants
31.Match each description to the type of change: physical or chemical.
Easy
- Melting of ice
- Rusting of iron
- Dissolving sugar in water
- Burning of wood
- Physical change
- Chemical change
- Physical change
- Chemical change
32.Arrange the following steps in the correct order to investigate the effect of temperature on the rate of reaction between sodium thiosulfate and hydrochloric acid.
Medium
- Draw a cross on paper under the flask
- Add hydrochloric acid to the flask and start stopwatch
- Heat sodium thiosulfate solution to the desired temperature
- Stop stopwatch when cross disappears
- Place the flask over the cross
33.A catalyst increases the rate of a chemical reaction by providing an alternative reaction pathway with a lower activation energy.
Medium
True or false?
34.Which of the following factors does NOT affect the rate of a chemical reaction?
Easy
- AConcentration of reactants
- BSurface area of solid reactants
- CColour of reactants
- DTemperature
35.Which of the following statements about catalysts are correct? (Select all that apply)
Medium
- ACatalysts are consumed in the reaction
- BCatalysts provide an alternative reaction pathway with lower activation energy
- CCatalysts increase the rate of reaction
- DCatalysts are specific to certain reactions
36.The graph shows the volume of gas produced over time for the reaction of hydrochloric acid with calcium carbonate chips. Which change would produce the curve with the steeper initial gradient and the same final volume?
Hard
- AUsing smaller chips of calcium carbonate
- BUsing a lower concentration of hydrochloric acid
- CDecreasing the temperature of the reaction
- DAdding a catalyst that increases the final volume
37.Increasing the temperature of a reaction increases the rate because particles have more kinetic energy, leading to more frequent collisions and a higher proportion of collisions with energy greater than the activation energy.
Medium
True or false?
38.In an experiment to investigate the effect of concentration on the rate of reaction, sodium thiosulfate solution is mixed with hydrochloric acid. The time taken for a cross to disappear is recorded. Which of the following would give the shortest time?
Hard
- A50 cm³ of 0.1 mol/dm³ sodium thiosulfate + 5 cm³ of 0.1 mol/dm³ acid
- B50 cm³ of 0.2 mol/dm³ sodium thiosulfate + 5 cm³ of 0.1 mol/dm³ acid
- C50 cm³ of 0.1 mol/dm³ sodium thiosulfate + 5 cm³ of 0.2 mol/dm³ acid
- D50 cm³ of 0.2 mol/dm³ sodium thiosulfate + 5 cm³ of 0.2 mol/dm³ acid
39.A student investigates the rate of reaction between magnesium and hydrochloric acid. They use the same mass of magnesium but in two forms: magnesium ribbon and magnesium powder. Which statement is correct?
Medium
- AThe powder reacts faster because it has a larger surface area
- BThe ribbon reacts faster because it has a larger surface area
- CBoth forms react at the same rate because the mass is the same
- DThe powder reacts faster because it has a higher temperature
40.Match each experimental setup to the factor being investigated.
Medium
- Measuring mass loss on a balance
- Using a water bath to heat acid before adding magnesium
- Using different concentrations of sodium thiosulfate with hydrochloric acid
- Effect of temperature
- Effect of concentration
- Effect of surface area
41.A chemical change always results in the formation of new substances.
Easy
True or false?
42.The rate of a reaction approximately doubles for every 10 °C rise in temperature. If the rate at 20 °C is 2.0 cm³/s, what is the approximate rate at 50 °C?
Medium
43.The rate of a reaction at 20 °C is 2.5 cm³/s. Using the rule that the rate approximately doubles for every 10 °C rise, calculate the rate at 50 °C.
Medium
44.Complete the sentence.
Medium
In terms of collision theory, increasing the temperature increases the rate of reaction because particles have more ____ energy and collide more ____.
45.State the effect of increasing the pressure of a gas on the rate of reaction and explain why in terms of collision theory.
Medium
46.In an experiment, 50 cm³ of gas is collected in 20 seconds. What is the average rate of reaction in cm³/s?
Easy
47.Complete the sentence about collision theory.
Easy
For a chemical reaction to occur, particles must ____ and have sufficient ____.
48.Name one sign that a chemical change has occurred.
Easy
49.The rate of a reaction at 30 °C is 3.0 cm³/s. Using the rule that the rate approximately doubles for every 10 °C rise, calculate the rate at 50 °C.
Easy
50.Complete the sentences about factors affecting reaction rate.
Easy
Increasing the concentration of a reactant ____ the rate of reaction because there are more particles per ____, leading to more frequent ____.
51.Describe how you would investigate the effect of a catalyst on the rate of decomposition of hydrogen peroxide. Include the method and the expected result.
Medium
52.Explain why increasing the temperature has a greater effect on reaction rate than increasing the concentration, in terms of collision theory.
Hard
53.Describe how you would investigate the effect of temperature on the rate of reaction between magnesium and hydrochloric acid. Include the method and the expected result.
Medium
54.The rate of a reaction approximately doubles for every 10 °C rise in temperature. If the rate at 20 °C is 2.0 cm³/s, what is the approximate rate at 50 °C?
Medium
- A15 cm³/s
- B18 cm³/s
- C16 cm³/s
- D17 cm³/s
55.The rate of a reaction at 20 °C is 2.5 cm³/s. Using the rule that the rate approximately doubles for every 10 °C rise, calculate the rate at 50 °C.
Medium
- A18 cm³/s
- B22 cm³/s
- C20 cm³/s
- D24 cm³/s
56.In an experiment, 50 cm³ of gas is collected in 20 seconds. What is the average rate of reaction in cm³/s?
Easy
- A1.5 cm³/s
- B2.5 cm³/s
- C3.5 cm³/s
- D4.5 cm³/s
57.The rate of a reaction at 30 °C is 3.0 cm³/s. Using the rule that the rate approximately doubles for every 10 °C rise, calculate the rate at 50 °C.
Easy
- A11 cm³/s
- B12 cm³/s
- C13 cm³/s
- D14 cm³/s

Flashcards

What is a physical change?

A physical change does not produce new chemical substances; it is often easy to reverse, e.g., melting or dissolving.

What is a chemical change?

A chemical change produces new substances with different properties; it is usually difficult to reverse.

List four signs that a chemical reaction has occurred.

Colour change, temperature change, effervescence (fizzing), and formation of a precipitate.

Define exothermic reaction.

An exothermic reaction gives out heat/thermal energy, causing a temperature increase.

Define endothermic reaction.

An endothermic reaction takes in heat/thermal energy, causing a temperature decrease.

State five factors that affect the rate of a reaction.

Concentration, pressure (for gases), surface area, temperature, and presence of a catalyst.

How does increasing concentration affect the rate of reaction?

Increasing concentration increases rate because there are more particles per unit volume, leading to more frequent successful collisions.

How does increasing pressure affect the rate of a gas reaction?

Increasing pressure increases rate because particles are in a smaller volume, causing more frequent successful collisions.

How does increasing surface area affect the rate of reaction?

Increasing surface area increases rate because more solid surface is exposed, leading to more frequent successful collisions.

How does increasing temperature affect the rate of reaction?

Increasing temperature increases rate because particles gain kinetic energy, leading to more frequent and more energetic successful collisions.

What is a catalyst?

A catalyst is a substance that speeds up a reaction without being consumed; it provides an alternative pathway with lower activation energy.

State collision theory.

Collision theory states that particles must collide with sufficient energy (≥ activation energy) for a reaction to occur.

What is activation energy?

Activation energy is the minimum energy colliding particles must have to react.

How does a catalyst increase reaction rate according to collision theory?

A catalyst lowers the activation energy, so a higher proportion of collisions have sufficient energy to be successful.

Describe a method to investigate the effect of concentration on rate using sodium thiosulfate and acid.

Measure 50 cm³ sodium thiosulfate into a flask, add 5 cm³ dilute HCl, time how long it takes for a cross under the flask to disappear. Repeat with different concentrations.

Describe a method to investigate the effect of surface area on rate using magnesium and acid.

Add dilute HCl to a conical flask, add magnesium ribbon, collect gas by downwards displacement, measure volume in fixed time. Repeat with different sized pieces (same total mass).

Describe a method to investigate the effect of temperature on rate using magnesium and acid.

Heat dilute HCl to a set temperature using a water bath, add magnesium, time until magnesium disappears. Repeat at different temperatures.

Describe a method to investigate the effect of a catalyst on rate using hydrogen peroxide.

Add hydrogen peroxide to a flask, add manganese(IV) oxide, collect oxygen by downwards displacement, measure volume in fixed time. Repeat without catalyst.

How can the rate of a reaction producing a gas be measured by mass loss?

Place the reaction flask on a balance; cotton wool in the flask mouth allows gas out but prevents splashing. Record mass loss over time.

How does the rate of reaction change over time on a graph of product vs. time?

Rate is fastest at the start (steepest gradient), decreases as reactants are used up, and becomes zero when reaction stops (horizontal line).

How do you calculate the rate of reaction at a specific time from a graph?

Draw a tangent at that time, calculate its gradient: rate = change in y / change in x.

If the concentration of a reactant is doubled, how does the initial rate and final product amount change?

Initial rate increases (steeper gradient), and final product amount doubles (if it is the limiting reactant).

What is the effect of increasing temperature on the energy of collisions?

Temperature increases the kinetic energy of particles, so a greater proportion of collisions have energy ≥ activation energy.

Give an example of an exothermic reaction.

Reaction of calcium oxide with water (produces calcium hydroxide and heat).

Give an example of an endothermic reaction.

Dissolving ammonium chloride in water (absorbs heat, used in cold packs).

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