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© Boardworks Ltd 2005 1 of 49 IGCSE Chemistry Rates of Reaction

© Boardworks Ltd 2005 1 of 49 IGCSE Chemistry Rates of Reaction

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Page 1: © Boardworks Ltd 2005 1 of 49 IGCSE Chemistry Rates of Reaction

© Boardworks Ltd 20051 of 49

IGCSE Chemistry

Rates of Reaction

Page 2: © Boardworks Ltd 2005 1 of 49 IGCSE Chemistry Rates of Reaction

© Boardworks Ltd 20052 of 49

Rates of Reaction

Effect of concentration

Effect of catalysts

Introduction

Summary activities

Effect of temperature

Contents

Effect of surface area

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Rates of reactions

The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow.

slow

very fast

very slow

fast

slow

rusting

explosion

chemical weathering of rocks

sodium and water

rotting fruit

Reaction Rate

What is the rate of these reactions?

The speed of a reaction is called the rate of the reaction.

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Reactions, particles and collisions

Reactions take place when particles of reactants collide with a certain amount of energy.

This energy is called activation energy, and is different for each reaction.

The rate of a reaction depends on three things:

the frequency of collisions between particles; the energy with which particles collide.

If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

the geometry with which particles collide.

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Increased temperature;

increased concentration of dissolved reactants, and increased pressure of gaseous reactants;

increased surface area of solid reactants;

use of a catalyst.

Changing the rate of reactions

Anything that increases the number of successful collisions between reactant particles will speed up a reaction.

What factors speed up reactions?

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Measuring rates of reaction

Measuring the rate of a reaction means measuring the rate of change over a period of time.

This means measuring the change in the amount of a reactant or the amount of a product.

What can you measure to calculate the rate of reaction between magnesium and hydrochloric acid?

The amount of magnesium used up (g/min).

The amount of magnesium chloride produced (g/min). The amount of hydrogen produced (cm3/min).

The amount of hydrochloric acid used up (cm3/min).

+magnesiumhydrochloric

acid +magnesium

chloridehydrogen

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Slower and slower!

Reactions do not proceed at a steady state. They start off at a certain speed, then get slower and slower until they stop.

As the reaction progresses, the concentration of reactants decreases. This reduces the frequency of collisions between particles and so the reaction slows down.

percentage completion of reaction

reactant A

reactant B

product0%

fast

25%

slower

75%

very slow

100%

stopped

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Rate of reaction and graphs

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Graphs and reactant-product mix

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Particles and rates of reaction

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Rates of Reaction

Effect of concentration

Effect of catalysts

Introduction

Summary activities

Effect of temperature

Effect of surface area

Contents

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The effect of temperature on collisions

How does temperature affect the rate of particle collision?

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Temperature

The higher the temperature, the faster the rate of a reaction. In many reactions, a rise in temperature of 10°C causes the rate of reaction to approximately double.

Why does increased temperature increase the rate of reaction?

At a higher temperature, particles have more energy. This means they move faster and are more likely to collide with other particles.

When the particles collide, they do so with more energy, and so the number of successful collisions increases.

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Temperature and particle collisions

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Temperature and food

Food goes off because chemical reactions take place.

Why does food remain usable for much longer if it is kept in a freezer?

The low temperature in the freezer means that particles will move much slower and with less energy than if they were at room temperature. This means that there are fewer successful collisions and so a slower rate of reaction.

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Temperature and cooking

Before microwave ovens were common, many people used pressure cookers to cook food more quickly.

In a pressure cooker, water doesn’t boil until it reaches about 115°C. How does this help cooking?

The higher temperature means that particles move more quickly and with more energy. This means that there are more successful collisions between particles, and the food cooks more quickly.

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Temperature and rate of reaction

The reaction between sodium thiosulfate and hydrochloric acid produces sulfur.

Sulfur is solid and so it turns the solution cloudy.

The effect of increasing temperature on the rate of reaction can be measured by comparing how long it takes the solution to turn cloudy at different temperatures.

hydrochloricacid

sodiumchloride

sulfursodium

thiosulfate + + watersulfur

dioxide ++

Na2S2O3

(aq)2HCl(aq)

2NaCl(aq)

S(s)++

SO2

(g)H2O(l)

+ +

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Sodium thiosulfate and hydrochloric acid

To run the experiment investigating the effect of temperature on the rate of reaction:

1. Mark a cross on a piece of paper.

2. Add a known amount of sodium thiosulfate to a beaker, and place it on the piece of paper.

3. Add a known amount of hydrochloric acid to the beaker and immediately start a stop-clock. The solution will begin to turn cloudy.

4. As soon as the cross can no longer be seen, stop the clock and note the time.

5. Repeat the experiment at different temperatures using the same volume of reactants. Compare how long it takes thecross to disappear.

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Sodium thiosulfate and hydrochloric acid

When looking down into the beaker, the cross will become fainter over time:

The time taken for the cross to disappear can be used as the time of the reaction.

increasing time

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Time of reaction vs. temperature graph

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True or false?

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Rates of Reaction

Effect of concentration

Effect of catalysts

Introduction

Summary activities

Effect of temperature

Effect of surface area

Contents

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Increasing concentration

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Concentration

The higher the concentration of a dissolved reactant, the faster the rate of a reaction.

Why does increased concentration increase the rate of reaction?

At a higher concentration, there are more particles in the same amount of space. This means that particles are more likely to collide with other particles.

low concentration high concentration

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Concentration and particle collisions

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Reactive metals such as magnesium react with acid to produce hydrogen gas.

Reaction between acid and metal

hydrochloricacid

magnesiumchloride

magnesium + + hydrogen

2HCl (aq)Mg (s) MgCl2 (aq) ++ H2 (g)

The effect of increasing concentration on the rate of reaction can be measured by comparing how quickly hydrogen is produced using different concentrations of hydrochloric acid.

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Mg + HCl: experiment set-up

What equipment do you need for the experiment investigating the effect of concentration on the rate of reaction?

magnesium ribbon

rubber bung

hydrochloric acid

rubber connector gas syringe

glasstube

conicalflask

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Magnesium and hydrochloric acid

To run the experiment investigating the effect of concentration on the rate of reaction:

1. Measure out a fixed volume of hydrochloric acid into the conical flask.

2. Add a known mass of magnesium to the flask, immediately attach the gas syringe and start a stop-clock.

3. Measure the volume of hydrogen collected in the syringe at regular intervals until no more gas is produced.

4. Repeat the experiment using a different concentration of hydrochloric acid but using the same volume of acid and the same mass of magnesium. Compare the rate at which hydrogen is produced.

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Mg + HCl: different concentrations

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The gas particles become closer together, increasing the frequency of collisions, and so increasing the rate of reaction.

Pressure

Why does increasing the pressure of gaseous reactants increase the rate of reaction?

As the pressure increases, the space in which the gas particles are moving becomes smaller.

The same number of particles but in a smaller space.

low pressure high pressure

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Rates of Reaction

Effect of concentration

Effect of catalysts

Introduction

Summary activities

Effect of temperature

Contents

Effect of surface area

Contents

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Surface area

Any reaction involving a solid can only take place at the surface of the solid.

If the solid is split into several pieces, the surface area increases.

The smaller the pieces, the larger the surface area. This means more collisions and a faster rate of reaction.

This means that there is an increased area for the non-solid reactant particles to collide with.

slow rate

fast rate

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Surface area and particle collisions

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Reaction between a carbonate and acid

Marble chips are made of calcium carbonate. They react with hydrochloric acid to produce carbon dioxide.

The effect of increasing surface area on the rate of reaction can be measured by comparing how quickly the mass of the reactants decreases using marble chips of different sizes.

hydrochloricacid

calciumchloride

calciumcarbonate + + water + carbon

dioxide

CaCO3 (aq)

2HCl(aq)

CaCl2

(aq)++

H2O(aq)

+ CO2

(g)

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CaCO3 + HCl: experiment set-up

What equipment do you need for the experiment investigating the effect of surface area on the rate of reaction?

cotton wool ‘plug’

hydrochloricacid

conicalflask

calciumcarbonate

chipsweighing

scales

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Calcium carbonate and hydrochloric acid

To run the experiment investigating the effect of surface area on the rate of reaction:

1. Measure out a fixed volume of hydrochloric acid into a conical flask and place the flask on weighing scales.

2. Add a fixed mass of calcium carbonate chips to the flask, and place a cotton wool plug in the neck. This stops the liquid from spitting while allowing the CO2 to escape.

3. Begin taking mass readings straight away, and continue until there is no further change in mass.

4. Repeat the experiment using the same mass of calcium carbonate but of a smaller chip size, and the same volume of hydrochloric acid. Compare the rate at which the mass of reactants decreases.

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CaCO3 + HCl: different surface areas

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Rates of Reaction

Effect of concentration

Effect of catalysts

Introduction

Summary activities

Effect of temperature

Effect of surface area

Contents

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What are catalysts?

What are catalysts?

Catalysts are substances that change the rate of a reaction without being used up in the reaction.

Catalysts are very important in industry because products can be made more quickly, saving time and money.They can also avoid having to use high temperatures, so they can save fuel and reduce pollution.

Catalysts are also very important in living cells. Biological catalysts are special types of protein called enzymes.

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Examples of catalysts

Nickel is a catalyst in the production of margarine (hydrogenation of vegetable oils).

Iron is a catalyst in the production of ammonia from nitrogen and hydrogen (the Haber process).

Many catalysts are transition metals or their compounds. Different reactions use different catalysts. For example:

Platinum is a catalyst in the catalytic converters of car exhausts. It catalyses the conversion of carbon monoxide and nitrogen oxide into the less polluting carbon dioxide and nitrogen.

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How do catalysts work?

How do catalysts work?

Different catalysts work in different ways, but most solid catalysts work by lowering the amount of energy needed for the reaction to take place.

For a chemical reaction to take place:

the reacting parts of particles need to be brought together.

energy is needed to break existing bonds, so new bonds can be formed;

Catalysts work by lowering the activation energy of a reaction.

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How a platinum catalyst works

When hydrogen and oxygen are mixed in a jar, there is no reaction. If a platinum wire is added, the gases react instantly with a loud pop, producing water.

How does platinum catalyse this reaction?

The gas molecules are brought together onto the surface of the platinum. They are adsorbed.

The molecules are much closer together and their bonds are weakened, lowering the activation energy of the reaction.

platinum wire

oxygen molecule

hydrogen molecule

The larger the surface area of the platinum, the quicker the reaction.

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Without a catalyst, this reaction is very slow, and can take months. With a catalyst such as manganese (IV) oxide, the reaction takes minutes.

Decomposition of hydrogen peroxide

Hydrogen peroxide decomposes into water and oxygen.

2H2O (l)2H2O2 (aq) + O2 (g)

waterhydrogenperoxide + oxygen

Catalysts never produce more product – they just produce the same amount but quicker.

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Rates of Reaction

Effect of concentration

Effect of catalysts

Introduction

Summary activities

Effect of temperature

Effect of surface area

Contents

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Glossary

activation energy – The amount of energy needed for a reaction to begin.

adsorption – The formation of a layer of molecules on the surface of a solid.

catalyst – A substance that changes the rate of a reaction without being used up.

concentration – The amount of particles in a given volume.

enzyme – A biological catalyst.

rate of reaction – The speed with which a particular chemical reaction progresses.

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Anagrams

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Increase or decrease?

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Stages of a reaction

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Multiple-choice quiz