Chemical Change And Rate Of Reaction

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Notes

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).

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**.

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.

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.

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.

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.

Particle arrangement in solid, liquid, and gas. Solids have fixed, closely packed particles; liquids have particles close but able to move; gases have particles far apart and moving rapidly.

Bohr model of sodium atom showing nucleus with 11 protons and 12 neutrons, and electron shells with 2, 8, and 1 electrons.

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.
Medium
4.The rate of a reaction approximately doubles for every 10 °C rise in temperature. If the rate at 20 °C is $2.0 cm^{3}/s$ , what is the approximate rate at 40 °C?
Medium
- A8
- B10
- C7
- D9
5.Complete the sentence using the correct word.
Medium
A catalyst provides an alternative reaction pathway with $a ____$ activation energy.
6.Match each factor to its effect on the rate of reaction.
Hard
- 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.
Hard
- Add hydrochloric acid to the flask and start the stopwatch.
- Draw a cross on paper and place it under the flask.
- Measure $50 cm^{3}$ 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 $1.0 mol/dm^{3}$ acid instead of $0.5 mol/dm^{3}$ acid
- DAdding a catalyst

Slides

Physical vs Chemical Changes

Physical changes do not produce new substances and are often easy to reverse (e.g. melting, dissolving).
Chemical changes form new substances with different properties and are usually difficult to reverse.
Signs of a chemical change include colour change, temperature change, effervescence (fizzing), and formation of a precipitate.

Which of the following is a sign of a chemical change?

Melting of ice
Dissolving sugar in water
Effervescence (fizzing)
Boiling water

Factors Affecting Rate of Reaction

The rate of a chemical reaction can be changed by: concentration (or pressure for gases), surface area of solids, temperature, and the presence of a catalyst.
Increasing any of these factors generally increases the rate of reaction.
Higher rates are often desirable for efficiency in industrial processes.

Which of these factors affect the rate of a reaction?

Concentration of reactants
Colour of reactants
Temperature
Surface area of solid reactants

Collision Theory (Extended)

For a reaction to occur, particles must collide with sufficient energy (≥ activation energy) and in the correct orientation.
A successful collision leads to product formation; an unsuccessful one means particles just bounce off.
The rate depends on collision frequency and the proportion of collisions with energy above the activation energy.

According to collision theory, particles must collide with energy at least equal to $the ____$ energy for a reaction to occur.

Effect of Concentration and Pressure

Increasing concentration (or pressure for gases) increases the number of particles per unit volume.
This leads to more frequent collisions per second, so the rate increases.
The number of collisions is proportional to the number of particles.

Increasing the pressure of a gas reaction increases the rate because:

Particles move faster
Particles are closer together, so collisions are more frequent
The activation energy is lowered
More particles have energy above the activation energy

Effect of Surface Area

Breaking a solid into smaller pieces increases its surface area.
More surface area means more particles are exposed to the other reactant, leading to more frequent collisions.
For example, powdered magnesium reacts faster than magnesium ribbon with acid.

Increasing the surface area of a solid reactant $____ the$ rate of reaction.

Effect of Temperature

Raising temperature increases the kinetic energy of particles.
Particles move faster, so collisions are more frequent.
More importantly, a greater proportion of collisions have energy ≥ activation energy.
A 10°C rise roughly doubles the rate for many reactions.

Why does increasing temperature increase reaction rate significantly?

Only because particles move faster
Only because more particles have energy above activation energy
Both because collisions are more frequent and a greater proportion have sufficient energy
Because the activation energy decreases

Effect of Catalysts

Catalysts speed up reactions without being used up.
They provide an alternative reaction pathway with a lower activation energy.
This means more collisions are successful, increasing the rate.
Enzymes are biological catalysts; iron is used in the Haber process.

Which statements about catalysts are true?

Catalysts are used up in the reaction.
Catalysts provide an alternative pathway with lower activation energy.
Catalysts increase the rate of reaction.
Catalysts change the amount of product formed.

Investigating Rate: Measuring Gas Volume

The rate can be measured by collecting gas produced (e.g., using a gas syringe or downward displacement of water).
Volume of gas is recorded at regular time intervals.
A steeper gradient on a volume-time graph indicates a faster rate.

In a reaction producing gas, how does the rate change as the reaction proceeds?

It stays constant
It increases
It decreases
It stops immediately

Interpreting Rate Graphs

The initial gradient is steepest because reactant concentration is highest.
As reactants are consumed, the gradient becomes less steep.
When the line becomes horizontal, the reaction has stopped (one reactant used up).
The final volume (or mass) of product depends on the limiting reactant.

To find the rate at a specific time, draw $a ____ to$ the curve and calculate its gradient.

Worked Example: Effect of Concentration

If the concentration of a reactant is doubled, the initial rate doubles (more particles per volume).
The total amount of product also doubles if the reactant is limiting.
On a graph, the curve for a higher concentration has a steeper initial slope and reaches a higher final volume.

In an experiment, $25 cm^{3}$ of $0.1 mol/dm^{3}$ H₂O₂ produces $40 cm^{3}$ of O₂. If the concentration is doubled to $0.2 mol/dm^{3}$ , what volume of O₂ is produced?

$20 cm^{3}$
$40 cm^{3}$
$80 cm^{3}$
$100 cm^{3}$

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 $50 cm^{3}$ sodium thiosulfate into a flask, add $5 cm^{3}$ 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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