Topic 5 – Chemical reactions • • • • • •
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TEMPERATURE CHANGES In chemical reactions, transfer of heat energy between the reactants and the surroundingsàchange in temperature Exothermic reactions: Exothermic reactions give out heat energy to the surroundingsàtemperature of the reaction mixture and its surroundings increases Most reactions are exothermic… Examples of exothermic reactions: o All combustion reactions e.g combustion of methane: § methane + oxygen à carbon dioxide + water o Explosions (release a lot of heat energy very quickly) Endothermic reactions: Endothermic reactions take in heat energy from the surroundingsàtemperature of the reaction mixture and of the surroundings decreases Few chemical reactions are endothermic… Examples of endothermic reactions: o Reaction of sodium hydrogencarbonate with hydrochloric acid (if you touch the test tube it feels cold because heat energy is taken from your hand) o Dissolving ammonium nitrate in water o Photosynthesis (takes in heat energy from the sun) Making and breaking bonds: In chemical reactions, bonds in the reactants are broken and new bonds are formed to make the products: o Breaking bonds requires energyàit’s an endothermic process o Making bonds releases energyàit’s an exothermic process In an exothermic reaction: o More heat energy is released making bonds (in the products) than is required to break bonds (in the reactants) o àheat energy is released In an endothermic reaction: o More heat energy is required to break bonds (in the reactants) than is released making bonds (in the products) o àheat energy is absorbed These energy changes can be shown using diagrams:
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RATES OF REACTION The rate of a chemical reaction is the speed at which it takes place – i.e it tells us how fast reactants are used up and products are made: o E.g explosions are very fast chemical (exothermic) reactions that release lots of heat energy and gas in a short space of time o E.g2 rusting of iron is an everyday example of a slow chemical reaction Collision theory: For a reaction to occur, particles of the reactants must collide with each other o àif particles collide more frequently, rate of reaction will increase However, not all collisions lead to a reaction – particles must collide with enough energy to break the bonds in the reactants: o àthe more energy particles have, the greater the chance of ‘successful collisions’ (i.e greater chance of particles reacting when they collide) o àrate of reaction will increase if there are more high-energy collisions between particles The effect of temperature: The higher the temperature of the reactants, the faster the reaction - e.g eggs cook faster in boiling water than in warm water Explanation: o As temperature is increased, particles move faster…à: § 1. collisions are more frequent § 2. particles have more energyàgreater chance of successful collisions § àincreased rate of reaction Sometimes we cool reactions to slow them down: o E.g some foods are put in a fridge to slow down chemical reactions that make food go off The effect of concentration: The higher the concentration of a reactant in a solution, the faster the reaction Explanation: o The more concentrated a solution, the more solute particles there are in a given volume: § àmore likely that reactant particles will collide with one another § àincreased rate of reaction The effect of surface area: The greater the surface area of a solid, the faster the rate of reaction Explanation: o Increasing the surface area increases the number of particles exposed on the surface (that can collide and react) o àmore frequent collisions between reactant particles o àincreased rate of reaction To increase surface area, solids are crushed into lots of small pieces: o E.g in power stations, coal is ground up into a fine powder to help it burn faster CATALYSTS Catalysts are substances that speed up chemical reactions without being used up in the reactions (note: they are neither reactants nor products)
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Catalysts speed up chemical reactions by increasing the probability of successful collisions - i.e when particles collide they don’t need as much energy to react Many chemical processes use catalysts to increase the rate at which products are generated Using catalysts means that reactions can be done at lower temperatures and pressures than they would otherwiseàless energy is usedàsaves money Catalytic converters: The combustion of petrol in car engines produces carbon monoxide (toxic) and unburned hydrocarbons àcars are now built with catalytic converters: o Catalytic converters help to combine carbon monoxide and unburned petrol with oxygen from the air to form carbon dioxide and water vapouràreducing pollutants in exhaust gases o Catalytic converters contain the transition metals platinum, rhodium or palladium (all very expensive), which act as catalysts o The catalysts have a high surface area (‘honeycomb structure’)…: § 1. àrate at which carbon monoxide and unburned petrol react with oxygen from the air to form carbon dioxide and water vapour is increased § 2. àless of the expensive metals are usedàcosts are lower o Catalytic converters work best at high temperatures (as particles collide more frequently and with more energy)…à: § When a car engine is first started, a catalytic converter is cool and doesn’t work well § However, the hot gases from the engine quickly heat it up