Rates of reaction

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Lesson notes

What is Rate of Reaction?

**Rate of reaction** is how quickly a chemical reaction takes place.
It depends on the **frequency of successful collisions** between reacting particles.
A successful collision has enough energy and correct orientation to react.

Factors Affecting Rate of Reaction

**Temperature**: Higher temperature → particles move faster → more frequent and energetic collisions → faster rate.
**Concentration/Pressure**: Higher concentration or pressure → more particles in same volume → more collisions → faster rate.
**Surface Area**: Breaking solid into smaller pieces increases **surface area to volume ratio** → more collisions on surface → faster rate.
**Catalyst**: A substance that increases rate without being used up; provides an alternative pathway with **lower activation energy**.

Measuring Rate of Reaction

**Change in mass**: For reactions producing a gas, measure mass loss over time (e.g., magnesium + hydrochloric acid → hydrogen gas escapes).
**Volume of gas**: Collect gas in a gas syringe and measure volume produced over time.
**Precipitation**: Time how long for a precipitate to form (e.g., sodium thiosulfate + HCl → yellow sulfur; use a black cross to observe cloudiness).
**Colour change**: Time how long for a colour change to occur.

Example: Magnesium and Hydrochloric Acid

Reaction: Mg + 2HCl → MgCl₂ + H₂ (hydrogen gas produced).
Rate can be measured by **mass loss** (escaping hydrogen) or **gas volume** in a syringe.
Using **magnesium powder** (higher surface area) gives a faster rate than large pieces.

Example: Sodium Thiosulfate and Hydrochloric Acid

Reaction: Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O (yellow sulfur precipitate).
Place flask over a black cross; time until cross is no longer visible due to cloudiness.
Increasing **concentration** or **temperature** of HCl increases rate (faster precipitation).

Particle arrangement in solids, liquids, and gases. In solids, particles are closely packed in a regular pattern; in liquids, they are close but irregular; in gases, they are far apart and move randomly. Higher temperature increases particle speed and collision frequency.

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Practice questions

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1.What is meant by the term 'rate of reaction'?
Easy
- AHow quickly a chemical reaction takes place
- BThe amount of product formed in a reaction
- CThe temperature at which a reaction occurs
- DThe energy released during a reaction
2.Which of the following statements is true?
Easy
- AParticles at a higher temperature move more quickly.
- BA catalyst is used up during a chemical reaction.
- CBreaking a solid into smaller pieces decreases its surface area to volume ratio.
- DThe rate of reaction does not depend on the frequency of successful collisions.
3.What is a catalyst?
Easy
- AA substance that increases the rate of reaction without being used up
- BA substance that slows down a reaction
- CA substance that is consumed in a reaction
- DA substance that increases the activation energy
4.Which of the following methods can be used to measure the rate of a reaction that produces a gas?
Medium
- AMeasuring the change in mass
- BObserving a colour change
- CUsing a light sensor to detect precipitate formation
- DTiming the appearance of a precipitate
5.In the reaction between sodium thiosulfate and hydrochloric acid, what is observed as the reaction proceeds?
Medium
- AA yellow sulfur precipitate forms, making the solution cloudy
- BA colourless gas is produced
- CThe solution turns blue
- DThe mass of the reaction mixture increases
6.Increasing the concentration of a reactant increases the rate of reaction because:
Medium
- AThere are more particles in the same volume, leading to more frequent collisions
- BThe particles move faster
- CThe activation energy is lowered
- DThe particles have more energy
7.Which of the following would increase the rate of reaction between magnesium and hydrochloric acid?
Medium
- AUsing magnesium powder instead of large pieces
- BDecreasing the temperature
- CUsing a more dilute hydrochloric acid
- DUsing a larger volume of the same concentration of acid
8.In an experiment to measure the rate of reaction between magnesium and hydrochloric acid, which method would be most appropriate?
Hard
- AMeasuring the volume of hydrogen gas produced over time using a gas syringe
- BObserving the colour change of the solution
- CUsing a light sensor to detect precipitate formation
- DMeasuring the increase in mass of the reaction mixture

Revision notes

## Rates of Reaction - **Rate of reaction** = how quickly a chemical reaction takes place. - Depends on **frequency of successful collisions** between reacting particles. ### Factors Affecting Rate - **Temperature**: Higher temperature → particles move faster → more frequent and energetic collisions → faster rate. - **Concentration/Pressure**: Higher concentration (solution) or higher pressure (gas) → more particles in same volume → more collisions → faster rate. - **Surface Area**: Smaller solid pieces (e.g., powder) → larger surface area to volume ratio → more collisions on surface → faster rate. - **Catalyst**: Substance that increases rate without being used up. Provides alternative pathway with **lower activation energy**. ### Measuring Rate - **Change in mass**: For reactions producing gas (e.g., Mg + HCl → MgCl₂ + H₂). Mass decreases as gas escapes. - **Volume of gas**: Collect gas in a gas syringe and measure volume over time. - **Precipitation**: Time for precipitate to form (e.g., sodium thiosulfate + HCl → yellow sulfur). Use a black cross under flask; time until cross is no longer visible. - **Colour change**: Time for colour change to occur. ### Key Points - True/False: (a) True, (b) False, (c) True, (d) False, (e) True. - Increasing temperature increases particle speed and collision energy. - Increasing concentration increases particle number and collision frequency. - Powder has higher rate than large pieces due to larger surface area. - Experiment: Use balance to measure mass loss over time for Mg + HCl.

Rate of reaction: How quickly a chemical reaction takes place.
Successful collisions: Collisions between reacting particles that result in a reaction.
Catalyst: A substance that increases the rate of reaction without being used up.
Activation energy: The minimum energy needed for a collision to result in a reaction.
Precipitate: A solid formed in a solution during a chemical reaction.

Flashcards

What does the phrase 'rate of reaction' mean?

It means how quickly a chemical reaction takes place.

What does the rate of reaction depend on?

It depends on the frequency of successful collisions between reacting particles.

How does temperature affect the rate of reaction?

Higher temperatures cause particles to move more quickly, increasing the frequency and energy of collisions, thus increasing the rate of reaction.

How does concentration affect the rate of reaction?

In a more concentrated solution, there are more particles in the same volume, leading to a higher rate of collisions and increasing the rate of reaction.

How does pressure affect the rate of reaction for gases?

A gas at higher pressure has more particles in the same volume, leading to a higher rate of collisions and increasing the rate of reaction.

How does surface area affect the rate of reaction?

Breaking a solid into smaller pieces increases its surface area to volume ratio, allowing more collisions on the surface and increasing the rate of reaction.

What is a catalyst?

A catalyst is a substance that increases the rate of reaction without being used up in the process.

How does a catalyst increase the rate of reaction?

It provides an alternative pathway with a lower activation energy, increasing the rate at which collisions result in a reaction.

Name one method to measure the rate of reaction that produces a gas.

Measuring the change in mass (loss of mass) as gas escapes.

Give an example of a reaction where measuring change in mass is used to find the rate.

The reaction between magnesium and hydrochloric acid to form magnesium chloride and hydrogen gas.

How can the volume of gas produced be used to measure rate of reaction?

Capture the gas in a gas syringe and measure the volume of gas produced over time.

What is precipitation in the context of rate of reaction?

Precipitation is the formation of a solid in a solution during a chemical reaction.

How can precipitation be used to measure rate of reaction?

By observing the time it takes for the precipitate to form, either visually or using a light sensor to detect cloudiness.

Give an example of a reaction where precipitation is used to measure rate.

The reaction between sodium thiosulfate and hydrochloric acid, which forms a yellow sulfur precipitate.

Describe the experiment using sodium thiosulfate and hydrochloric acid to measure rate.

Place the reaction mixture in a conical flask above a piece of paper with a black cross. Measure the time for the black cross to no longer be visible due to the precipitate.

How can colour change be used to measure rate of reaction?

Time how long it takes for the colour change to occur.

True or false: Particles at a higher temperature move more quickly.

True.

True or false: A catalyst is used up during a chemical reaction.

False.

True or false: The rate of reaction depends on the frequency of successful collisions.

True.

True or false: Breaking a solid up into smaller pieces decreases the surface area to volume ratio.

False.

True or false: A gas at a higher pressure has more particles in the same volume.

True.

Explain why increasing the temperature of reactants increases the rate of reaction.

Higher temperatures cause particles to move more quickly, increasing the frequency and energy of collisions, so more collisions are successful.

What effect does increasing the concentration of hydrochloric acid have on the reaction with sodium thiosulfate?

It increases the rate of reaction because there are more particles in the same volume, leading to a higher frequency of collisions.

Describe an experiment to investigate how the rate of reaction depends on the temperature of hydrochloric acid using sodium thiosulfate.

Use a conical flask above a black cross. Add sodium thiosulfate and hydrochloric acid at different temperatures. Measure the time for the cross to disappear. Repeat at several temperatures.

Which has a higher rate of reaction: magnesium powder or large pieces of magnesium with hydrochloric acid?

Magnesium powder has a higher rate because it has a larger surface area to volume ratio, allowing more collisions on the surface.

Describe how to experimentally verify which form of magnesium (powder or large pieces) has a higher rate of reaction with hydrochloric acid.

Use a balance to measure mass loss over time. Add the same mass of magnesium powder and large pieces separately to excess hydrochloric acid. The faster mass loss indicates a higher rate.

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