Energy In Reactions

1. Noadswood Science, 2013 Energy In Reactions

2. Energy In Reactions • To understand how energy in reactions can be calculated

3. Reactions • Most chemical reactions involve energy change – often this energy is in the form of heat: - • Exothermic reaction - energy is given out (temperature increases) • Endothermic reaction - energy is taken in (temperature decreases) • Exothermic - e.g. combustion of fuel (heat given out by the chemicals to the surroundings) • Endothermic - e.g. ice cube melting in your hand (heat taken away from the surroundings to the chemicals (which is why your hand feels cold when you hold an ice cube and it melts))

4. Energy Energy Energy • Energy must be supplied to break bonds • Energy is released when new bonds are made • A reaction is exothermic if more energy is released than supplied • If more energy is supplied than is released then the reaction is endothermic

5. Exothermic & Endothermic • Exothermic reactions transfer energy to the surroundings – the energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter, e.g. • Burning • Neutralisation reactions between acids and alkalis • The reaction between water and calcium oxide • Endothermic reactions take in energy from the surroundings – the energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder, e.g. • Electrolysis • The reaction between ethanoic acid and sodium carbonate • Thermal decomposition of calcium carbonate in a blast furnace

6. Exothermic & Endothermic • Exothermic reaction between sodium hydroxide and hydrochloric acid, and an endothermic reaction between sodium carbonate and ethanoic acid: -

7. Fuel Energy • Different fuels produce different amounts of energy – the energy released by a fuel can be calculated using water and a thermometer… • How can this be done?!

8. Calorimetry • Calorimetry calculates the fuel energy as folows: - • Put a known quantity of water in a container (e.g. 100g) and record the temperature • Weigh the fuel contents (including spirit burner) • Heat the water until the temperatures reaches a known point, e.g. 100oC • Extinguish the flame and weigh the fuel contents (including spirit burner) • Using the specific heat capacity (energy needed to heat 1g of material up by 1oC) you can then use the following equation to work out the fuel energy: - Q = mcT

9. Experiment • Complete the calorimetry experiment measuring the energy contained in the different spirit burners… • Methane = 50 MJ per kg • Ethane = 48 MJ per kg • Propane = 46 MJ per kg • Butane = 45 MJ per kg • Hydrogen = 142 MJ per kg • Petrol = 47 MJ per kg

10. Q = mcT Q = mcT Energy transferred (J) = mass of water (g) x specific heat capacity of water (c) x temperature change (oC) • In your exam you will be given the specific heat capacity of water (4.2J) • For example to work out how much fuel energy is in meths… • Mass of spirit burner before heating = 70g, mass of spirit burner after heating = 65.1g ∴4.9g of meths burnt to heat the 100g of water • Temperature water at start = 21oC, temperature at finish = 100oC ∴79oCtemperature change • 4.2J needed to heat 1g of water by 1oC (specific heat capacity of water) • Energy produced = 100 x 4.2 x 79 = 33,180 joules • 4.9g produces 33,180J • 1g produces 6771J (33,180 ÷ 4.9) of energy (6.7kJ)

11. Q = mcT • This method can be used to calculate the amount of energy in foods, however as with the fuels a lot of energy is wasted in heating the air / the container / the environment etc… so the figure is often lower than the actual energy content of the fuel / food