Collins Cambridge IGCSE Combined Science Sample by Collins

Rate of reaction INTRODUCTION

Some chemical reactions take place extremely quickly. For example, when petrol is ignited it combines with oxygen almost instantaneously. Reactions like these have a high rate. Other reactions are much slower, for example when an iron bar rusts in the air; reactions like these have a low rate. Chemical reactions can be controlled and made to be quicker or slower. This can be very important in situations like food production, either by slowing down or increasing the rate at which food ripens, or in the chemical industry where the rate of a reaction can be adjusted to an optimum level.

Δ Fig. 2.34 Petrol igniting.

KNOWLEDGE CHECK ✓ Know the arrangement, movement and energy of the particles in the three states of matter: solid, liquid and gas. ✓ Understand how the course of a reaction can be shown in an energy level diagram. ✓ Be able to write and interpret balanced chemical equations.

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RATE OF REACTION

LEARNING OBJECTIVES ✓ Be able to describe a practical method for investigating the rate of a reaction involving the evolution of a gas. ✓ Be able to interpret data obtained from experiments concerned with rate of reaction. ✓ Be able to describe the effects of concentration, particle size, catalysts and temperature on the rates of reactions. ✓ EXTENDED Be able to suggest apparatus, given information, for experiments, including collection of gases and measurement of rates of reaction. ✓ EXTENDED Be able to describe and explain the effect of changing concentration in terms of collisions between reacting particles. ✓ EXTENDED Be able to explain that an increase in temperature causes an increase in collision rate and more of the colliding particles have sufﬁcient energy (activation energy) to react, whereas an increase in concentration only causes an increase in collision rate.

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SCIENCE LINK

BIOLOGY  ENZYMES

• the factors that aﬀect how quickly a chemical reaction happens link directly to the role of enzymes in the maintenance of body processes • describing how the energy of the particles changes at higher temperatures also allows us to explain why enzymes will not work above a certain temperature

PHYSICS  SIMPLE KINETIC MODEL • explaining why the diﬀerent factors aﬀect the rate of a chemical reaction uses the same particle model that gives us the simple structure of solids, liquids and gases • thinking about the forces between the particles and the energy involved in the interactions between particles leads to a common explanation in terms of particle speed and kinetic energy EXTENDED

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Δ Fig. 2.35 Particles must collide with sufﬁcient energy to make an effective collision.

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PHYSICAL CHEMISTRY

COLLISION THEORY For a chemical reaction to occur, the reacting particles (atoms, molecules or ions) must collide. The energy involved in the collision must be enough to break the chemical bonds in the reacting particles – or the particles will just bounce off one another. A collision that has enough energy to result in a chemical reaction is an effective collision.

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Extended: B3.2d Water potential 1. Write a definition for water potential.

2. What is the water potential of pure water?

3. Does a solution have a positive or a negative water potential? Explain your answer.

4. The diagram shows the water potential of two cells in a plant. (Water potential is measured in MPa, megapascals.)

A –2.5 MPa

B –2.1 MPa

a) Which cell has the higher water potential? Explain your answer.

b) In which direction will osmosis take place?

c) Explain your answer to part b).

Cambridge IGCSE® Combined Science Teacher Pack

B3.2 Food tests In this activity you will use the tests for reducing sugars (glucose), starch, proteins, and fats or oils on a range of foods to see which contain these food groups.

Apparatus food samples for testing Benedict’s solution measuring pipettes test tubes mortar and pestle distilled water white spotting tile iodine solution hot water from a kettle, beaker and insulation heatproof test-tube holder potassium hydroxide solution copper sulfate solution ethanol small spoons or spatulas safety goggles SAFETY INFORMATION Wear safety goggles. Be careful using the hot water. Do not taste foods in a science laboratory. Avoid foods that produce known allergies (such as peanuts). Take care when using potassium hydroxide solution. Wash off any spillages immediately.

Method Preparing the food samples 1. Put on safety goggles. 2. Use the mortar and pestle to grind up a small amount (about a teaspoonful) of the food, adding drops of distilled water to help produce a thick paste. 3. Place a teaspoonful of the paste into a test tube, then add 4 cm3 of distilled water and shake or stir to disperse the paste. 4. Prepare all the solid food samples this way. Liquid food samples do not need preparation.

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Benedict’s test for glucose (reducing sugar) 5. Measure 1 cm3 of the food extract into a clean test tube. 6. Add 1 cm3 of Benedict’s solution and shake to mix the solutions. 7. Using a test-tube holder to hold the test tube, place the tube into a boiling water bath and gently shake the solution in the tube to mix it until it reaches the temperature of the water bath. 8. If glucose (a reducing sugar) is present, the blue Benedict’s solution will turn green, then yellow, and then orange. If there is a large amount of glucose in the solution, a red-brown precipitate may form. Starch test 9. Place a drop of iodine solution in each of two dimples in a spotting tile. 10. Add a drop or two of a food solution to one dimple. 11. If starch is present in the food solution, the yellow iodine solution will turn blue-black. (Comparing the food dimple with the iodine-only dimple will help you see even a very slight change in colour.) Biuret test for protein 12. Measure 1 cm3 of the food extract into a clean test tube. 13. Measure 1 cm3 of potassium hydroxide solution to the test tube and mix the solutions. 14. Add 2 drops of copper sulfate solution and mix. If protein is present in the food, a mauve or purple colour will develop slowly. Emulsion test for fats and oils 15. Measure 2 cm3 ethanol into a clean test tube. 16. Add 1–2 cm3 of the food extract to the tube. Cover the end of the tube and shake it vigorously to dissolve the food in the ethanol. Allow the solid particles to settle. 17. Pour the ethanol into another clean test tube that is half-full with water, taking care to leave any solid particles in the first test tube. If fats or oils are present, the water will go cloudy white.

Handling experimental observations and data 18. Draw up a table to record the results of the tests on each of the food samples you analysed, and complete it by adding your results. 19. Use your table to identify which foods contain each of the main food groups. 20. Identify any patterns in the results, such as whether all the samples that contain glucose come from plant sources or from animal sources.

Planning and evaluating investigations 21. Your results are qualitative, which means they only identify the presence or absence of particular food molecules. Suggest how the method could be adapted to produce a quantitative test, where you could compare the amount of a particular food molecule in different foods. 22. You should also consider whether the tests are specific to those food groups (for example, will the test for glucose give positive results with other carbohydrates?).

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Cambridge IGCSE® Combined Science Teacher Pack