9H Reactions and energy Energy drives every change that happens. That includes chemical reactions. Combustion reactions give out energy. We burn fuels . What’s special about a fuel? Fuels store chemical energy . They give out heat and light energy when they burn. The energy in fossil fuels came from the Sun . How did that happen? Most fuels contain carbon . Many fuels contain hydrogen . Complete this combustion word equation: carbon + oxygen ? energy given out
9H Fuels or fools? Fuel fact or fuel fantasy? 1. Combustion is an oxidation reaction. 2. Many fuels produce acidic gases when they burn. 3. Hydrogen fuel is a very clean fuel. 4. Coal is mainly sulphur and tar. 5. Coal, oil and natural gas are all fossil fuels. 6. Respiration and the combustion of hydrocarbon fuels are very similar reactions. True or False? The fuel combines with oxygen from the air – it is oxidised. TRUE! This causes acid rain. Sulphur impurities in the fuel burn to form acidic sulphur dioxide. TRUE! Hydrogen burns to form water. The exhaust from a hydrogen-powered vehicle would be just steam! TRUE! Coal is over 90% carbon with perhaps 2 or 3% sulphur. FALSE! Can you explain how they got that name? TRUE! Both use oxygen and produce carbon dioxide and water. But they are different – food doesn’t burn inside you! TRUE!
9H And the winner is… Different fuels are good in different ways. Can you think of some advantages and disadvantages? Which do you think is the better fuel? And why? fuel hydrogen gas fuels based on carbon (coal, coke, charcoal) produces no carbon dioxide or sulphur dioxide highly explosive when mixed with air difficult to store – needs to be pressurised advantages disadvantages easy to store, cheap produces carbon dioxide heavy to transport
9H Where does the energy come from? Energy is given out in combustion. Where does it come from? The same atoms are there at the end of a reaction as at the beginning. They are just arranged differently. The energy release must be to do with the way the atoms are arranged , not which atoms are present . For burning methane (natural gas): For burning carbon (coal): carbon + oxygen carbon dioxide + methane + oxygen carbon dioxide + water + +
9H Breaking up is hard to do If atoms are joined, they need to separate before they can react and join with new atoms. This takes energy .
9H Come together… So, the energy you put in breaks the bonds joining the atoms. But how did the energy get into the fuel? Think about how fuels are formed. The separated atoms then form the products and energy is given out . A fuel gives out more energy as it burns than you put in at the start. This is what makes fuels useful. How do you put the energy in to burn a fuel?
9H Putting it together We can show the energy changes during a combustion reaction on a graph. So, why must you light a fuel to make it burn? as reaction goes on … energy methane + oxygen + energy in more energy out separated atoms carbon dioxide + water +
9H Exothermic and endothermic Some reactions give out energy, like combustion reactions. These are exothermic reactions. Other reactions take in energy. These are endothermic reactions. One example is using salt to clear snow on roads. How do we explain this? When the atoms join to make the products, they give out so much energy that some comes out as heat. Come on, you have a go at explaining it!
9H Quiz 1. Combustion is a good example of… a) a respiration reaction b) an oxidation reaction c) someone losing their temper. 2. The main advantage of wood as a fuel is… a) it’s good for barbecues b) it’s renewable c) it burns with a yellow flame .
9H Quiz 3. Which of the following cannot be made from crude oil? a) lots of different trees b) lots of different fuels c) lots of different plastics. 4. An exothermic reaction… a) gives out heat b) takes in heat c) only works out of doors.
9H Quiz 5. To separate the atoms in a methane molecule we have to… a) take energy out b) put energy in c) use an atomic saw. 6. The law of conservation of energy says… a) energy cannot be created or destroyed b) energy can only be created in small steps c) greenhouses lose energy through the glass roof.
9H Love a chemist! Love a chemist! Why? Well, because they make so many useful things for us. The next slide shows a picture of a room. Try to work out how many things depend on a chemist to make them. And just to make it easy – only look for items that can be made from crude oil. Ready?
All these things are made from crude oil. How many did you get? Remember to thank a chemist for every one! Plastics were also used to make the kettle, microwave, washing machine, iron, oven and cupboard doors. vinyl floor plastic washing-up liquid bottle plastic chopping board plastic plug and socket cover plastic knife handles drainer and washing-up bowl work surfaces kitchen bin and bin bag plastic spoons plastic laundry basket 9H Made from crude oil?
9H Where do we start? How do chemists make new substances? Research chemists try to make a new material. Their reactions may not work, or the new material may not be very good. So they use small-scale experiments: grams of reactants. Breakthrough! Once a useful material has been made, development chemists find a cheap way of making it on a larger scale. They use kilograms at a time. Ready to roll : Now production chemists and engineers work together to design a chemical plant. This will produce tonnes of the useful material every day.
9H What you put in…you get out! This means that the mass stays the same. Scientists say: Mass is conserved. Chemical reactions change things. But they cannot destroy or create atoms. Whatever atoms you start with, you still have the same atoms left at the end. The atoms are just arranged differently.
9H A little less conservation? Mass is conserved. Work out the missing masses. How could you collect the hydrogen to measure its mass?
9H Burning magnesium Look at this experiment with burning magnesium. Mass of magnesium at start = 4.8 g Mass of magnesium oxide = 8.0 g Mass change = 3.2 g We have just measured a change in mass. So how can mass be conserved? Weigh a crucible, put in the magnesium and weigh it again. Heat the crucible with the lid on, lifting the lid occasionally to let air in. When the crucible is cool weigh it again.
9H Explaining the change in mass Mass of magnesium at start = 4.8 g Mass of magnesium oxide = 8.0 g Mass change = 3.2 g Think about the burning reaction: We have measured the mass of the magnesium and the mass of the magnesium oxide . So where did the 3.2 g come from? You’ve guessed it – oxygen from the air. magnesium + oxygen magnesium oxide Where did the 3.2 g come from?
9H Quiz 1. During a chemical reaction, the total mass… a) always increases b) always decreases c) stays the same. 2. When a heavy piece of wood burns, only a little light ash is left. This is because… a) mass is not conserved in this reaction b) most of the products are gases c) wood is a renewable fuel.
9H Quiz 3. When you burn magnesium, the mass seems to increase. This is because… a) carbon dioxide is given off into the air b) oxygen is used from the air c) some extra solid magically appeared in the crucible. 4. You lift the crucible lid to… a) let in oxygen b) let in carbon dioxide c) let out extra ash.