Fast and Slow reactions Objectives Understand reactions can
Fast and Slow reactions
Objectives Understand reactions can be fast or slow describe a variety of methods of speeding up or slowing down chemical reactions
Explosions are chemical reactions that take place very quickly. Explosions also release a lot of heat, light and noise.
Rate of reaction The rate of a reaction is a measure of how quickly a chemical reaction occurs.
Speeding up a reaction with heat Why do reactions speed up while heating? Heating a substance adds energy to its particles. They move more rapidly and collide more frequently. Heating substances, therefore, usually causes the rate of a chemical reaction to increase.
Why food is stored in refrigerator or freezer? Food 'goes off' because micro-organisms cause chemical reactions in the food that make it rot. These chemical reactions can be slowed by lowering the temperature of the food
Catalyst A catalyst is a chemical that can speed up a chemical reaction but is still present once the reaction has finished. Why catalysts are not reactants? Catalysts are not reactants because they are not changed by the reaction.
Examples of Catalysts Catalyst Use in 1 Platinum In car exhaust – to convert toxic carbon monoxide to carbon dioxide 2 Enzymes Help digestion of food quickly Nitrogen Oxide reacts with toxic carbon monoxide to convert it to less harmful carbon dioxide Apples go brown when its chemicals react with oxygen in the air. Enzymes speed up the reaction
Changing speed of reaction Speed of reaction can be changed by Changing Temperature Adding Catalysts Increasing surface area Amount of reactants
End of the section activities q What is the rate of a reaction? q The rate of reaction is a measure of how quickly a reaction happens. q State five different methods of changing the rate of a reaction. q The rate of a reaction can be changed by: heating, cooling, adding a catalyst, adding more reactants or increasing the surface area of the reactants.
End of the section activities How does heating increase the rate of a reaction? Heating increases the rate of reaction because it causes the particles in the reactants to move around more quickly. They react when they collide; therefore, the more quickly they move, the more quickly they react.
End of the section activities q What is a catalyst? q A catalyst is a substance that speed up the rate of a chemical reaction without being used up or changed. q How do you know that a catalyst is not a reactant? q Catalysts are not reactants because they are not changed by the reaction.
End of the section activities q What is an enzyme? q An enzyme is a chemical found in living things. Enzymes speed up chemical reactions inside living things but they are not used up or changed in the reactions.
Thinking questions q Does a refrigerator stop food from rotting or does it just slow the rotting? Explain your answer. q A refrigerator slows the process of food rotting. This is because the lower temperature slows down the chemical reactions that cause food to spoil.
Thinking questions q What is the point of adding catalysts to washing powders? q Enzyme catalysts are added to washing powders so that the chemical reactions necessary to clean the clothing will occur more quickly while the clothes are in the washing machine.
Thinking questions q Which will dissolve more quickly — a sugar cube or the same amount of sugar in a teaspoon? Explain your answer q Granulated sugar in a spoon will dissolve faster than a cube of sugar as granulated sugar has a much greater surface area for reaction.
- Slides: 16