9H Using chemistry Reactions and energy Making useful substances Reactions and mass

9H Using chemistry Reactions and energy

Complete this combustion word equation: 9H Reactions and energy Energy drives every change that happens. That includes chemical reactions. Fuels store chemical energy. They give out heat and light energy when they burn. Combustion reactions give out energy. We burn fuels. What’s special about a fuel? Most fuels contain carbon. The energy in fossil fuels came from the Sun. How did that happen? Many fuels contain hydrogen. Complete this combustion word equation: carbon + oxygen ? energy given out

Fuel fact or fuel fantasy? 9H Fuels or fools? True or False? Fuel fact or fuel fantasy? 6. Respiration and the combustion of hydrocarbon fuels are very similar reactions. 4. Coal is mainly sulphur and tar. 5. Coal, oil and natural gas are all fossil fuels. 3. Hydrogen fuel is a very clean fuel. 1. Combustion is an oxidation reaction. 2. Many fuels produce acidic gases when they burn. 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! Coal is over 90% carbon with perhaps 2 or 3% sulphur. FALSE! This causes acid rain. Sulphur impurities in the fuel burn to form acidic sulphur dioxide. TRUE! The fuel combines with oxygen from the air – it is oxidised. TRUE! Hydrogen burns to form water. The exhaust from a hydrogen-powered vehicle would be just steam! TRUE!

Which do you think is the better fuel? And why? 9H And the winner is… Different fuels are good in different ways. Can you think of some advantages and disadvantages? disadvantages fuel hydrogen gas fuels based on carbon (coal, coke, charcoal) advantages produces no carbon dioxide or sulphur dioxide highly explosive when mixed with air difficult to store – needs to be pressurised easy to store, cheap produces carbon dioxide heavy to transport Which do you think is the better fuel? And why?

Energy is given out in combustion. Where does it come from? 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. For burning carbon (coal): carbon + oxygen carbon dioxide + For burning methane (natural gas): methane + oxygen carbon dioxide + water + The energy release must be to do with the way the atoms are arranged, not which atoms are present.

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.

So, the energy you put in breaks the bonds joining the atoms. 9H Come together… So, the energy you put in breaks the bonds joining the atoms. The separated atoms then form the products and energy is given out. How do you put the energy in to burn a fuel? A fuel gives out more energy as it burns than you put in at the start. This is what makes fuels useful. But how did the energy get into the fuel? Think about how fuels are formed.

So, why must you light a fuel to make it burn? 9H Putting it together We can show the energy changes during a combustion reaction on a graph. separated atoms carbon dioxide methane + oxygen + water + + energy in more energy out energy as reaction goes on … So, why must you light a fuel to make it burn?

Come on, you have a go at explaining it! 9H Exothermic and endothermic Some reactions give out energy, like combustion reactions. These are exothermic reactions. When the atoms join to make the products, they give out so much energy that some comes out as heat. Other reactions take in energy. These are endothermic reactions. One example is using salt to clear snow on roads. Come on, you have a go at explaining it! How do we explain this?

1. Combustion is a good example of… a) a respiration reaction 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.

3. Which of the following cannot be made from crude oil? 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.

5. To separate the atoms in a methane molecule we have to… 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.

Making useful substances 9H Using chemistry Making useful substances

Well, because they make so many useful things for us. 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?

9H Made from crude oil? Time starts now!

All these things are made from crude oil. 9H Made from crude oil? All these things are made from crude oil. plastic plug and socket cover plastic knife handles plastic washing-up liquid bottle plastic spoons work surfaces drainer and washing-up bowl plastic chopping board kitchen bin and bin bag plastic laundry basket vinyl floor Plastics were also used to make the kettle, microwave, washing machine, iron, oven and cupboard doors. How many did you get? Remember to thank a chemist for every one!

How do chemists make new substances? 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 Using chemistry Reactions and mass

This means that the mass stays the same. Scientists say: 9H What you put in…you get out! 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. This means that the mass stays the same. Scientists say: Mass is conserved.

Work out the missing masses. 9H A little less conservation? Work out the missing masses. Mass is conserved. How could you collect the hydrogen to measure its mass?

Look at this experiment with burning magnesium. Heat the crucible with the lid on, lifting the lid occasionally to let air in. Weigh a crucible, put in the magnesium and weigh it again. When the crucible is cool weigh it again. We have just measured a change in mass. So how can mass be conserved? Mass of magnesium at start = 4.8 g Mass of magnesium oxide = 8.0 g Mass change = 3.2 g

Where did the 3.2 g come from? 9H Explaining the change in mass Where did the 3.2 g come from? 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: magnesium + oxygen magnesium oxide 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.

1. During a chemical reaction, the total mass… a) always increases 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.

a) carbon dioxide is given off into the air 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.