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IGCSE Chemistry

IGCSE Chemistry. Rates of Reactions Teacher Version. What is a rate?. Rate measures how fast or slow a chemical reaction occurs Who cares about rates of reactions? Chemical engineers must know how long a reaction takes and how much product is produced Chemical reactions in fireworks

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IGCSE Chemistry

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  1. IGCSE Chemistry Rates of Reactions Teacher Version

  2. What is a rate? • Rate measures how fast or slow a chemical reaction occurs • Who cares about rates of reactions? • Chemical engineers must know how long a reaction takes and how much product is produced • Chemical reactions in fireworks • Objects rusting • Concrete setting

  3. Measuring Rates of Reactions • During a reaction we can measure one of two things: • How quickly the reactants are used up • How quickly new products are produced What happens when we add zinc to sulphuric acid? • Identify the products • Write down the equation for the reaction • What makes this reaction easy to identify a rate?

  4. How can we measure the volume of gas given off?

  5. How can we measure the volume of gas given off?

  6. Measuring the mass of gas given off

  7. Measuring the time it takes for precipitate to produce

  8. Reaction between Mg and dilute HCl Where is the reaction speed the greatest? How does the reaction speed change as the reaction moves forward? How does the curve change as the reaction speeds up? Describe the graph shape once the reaction is complete (finished).

  9. Reaction between Mg and dilute HCl • The rate changes all through the reaction • The rate is greatest at the start but decreases as the reaction moves forward • The faster the reaction, the steeper the curve • When the reaction is over, the curve goes flat

  10. Can we change the rate of a reaction? • Don’t forget the COLLISION THEORY! • Particles can only react if they collide and have enough energy to break the bonds between reactants • More collision means a faster reaction • So … how can we produce more collisions?

  11. Changing the Rate of a Reaction • Four factors can affect the rate of a reaction: • Concentration • Temperature • Surface Area • Catalyst

  12. The Effect of Concentration • A reaction moves faster when the concentration of a reactant is increased • This increased concentration leads to more particles of the reactant in a given space, increasing the chances of collisions and therefore a faster rate • Ex: Dilute vs. concentrated hydrochloric acid and magnesium

  13. Concentration and Particle Collisions

  14. The Effect of Temperature • At a higher temperature, a reaction goes faster • When the temperature increases by 10°, the reaction rate almost doubles! • At an increased temperature, particles have more energy which means they move faster and the chance of collisions increases

  15. The Effect of Temperature Which reaction will proceed at a faster rate?

  16. Temperature: dilute hydrochloric acid and sodium thiosulphate solution This reaction produces a fine, yellow precipitate of sulphur

  17. Temperature: dilute hydrochloric acid and sodium thiosulphate solution • How to measure the rate: • A cross is marked on a piece of paper • A beaker containing some sodium thiosulphate solution is put on top of the paper (the cross should still be visible through the beaker) • HCl is added quickly to the solution and a timer starts • The cross becomes harder to see as the precipitate forms • The timer stops at the moment the cross is no longer visible

  18. Temperature and Food Rates • Food is kept in the refrigerator to slow down decomposition reactions • What might be some other examples of common temperature/ reaction rates?

  19. The Effect of Surface Area • In many reactions, one reactant is a solid. By breaking up the solid (while keeping the mass the same), the rate of reaction can be increased • Ex: hydrochloric acid and marble chips (calcium carbonate) • What is the balanced chemical equation for this reaction?

  20. The Effect of Surface Area

  21. The Effect of Surface Area • Marble chips and acid are placed in a flask and plugged with cotton wool to prevent liquid from splashing out during the reaction • Take the mass of the flask • Tip the flask to allow reactants to mix and simultaneously start a timer • Note the mass at regular intervals • Because the carbon dioxide can escape through the cotton wool, the mass of the flask decreases • Experiment repeated twice keeping everything the same EXCEPT the surface area of the marble chips

  22. The Effect of a Catalyst • A catalyst is a substance that changes the rate of a chemical reaction • The catalyst remains chemically unchanged at the end of the reaction • Catalysts work by giving reacting particles a surface to cling to, increasing collisions and thus reaction rate

  23. Surface of a Catalyst

  24. Biological Catalysts - Enzymes • Enzymes are large protein molecules which increase the rate of chemical reactions in the body

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