Comparing Energy Released by Fuels

1. Miss Chohan FSG C3 3.1 Comparing the Energy Released by fuels

2. How much energy comes from burning fuels? At the end of the lesson, you should be able to: • Describe how we can measure the energy produced by different fuels • Be able to state the unit of energy • Be able to calculate the energy given off when a fuel is combusted (in kJ/mole) from given data

3. Method • Put 100 cm3 of water in the calorimeter. • Record the temperature of the water. • Clamp the calorimeter in position about 10 cm above the protective mat. • Find the mass of a spirit burner containing an alcohol. • Place the burner under the calorimeter and light the wick. • Place the heat shield around the apparatus. • Stir the water with the thermometer. • Stop heating when the temperature has risen by 30 °C. • Replace the cap on the burner. • Reweigh the burner as soon as possible. • Keep noting the temperature – record the highest value reached. • If you can, repeat the experiment with other alcohols. • Try to keep all the factors the same

4. What is Really Happening? • Write a word equation for the reaction between the alcohol and oxygen (the first has been done for you) methanol + oxygen  carbon dioxide + water CH3OH (l) + • Where did the energy come from that was transferred to the water? 1½ 2 O2 (g) CO2 (g) + H2O(l) All of the bonds in the reactant molecules are broken and all of the bonds in the product molecules are formed.

5. The Energy in our Food… • We can find the energy in our food in a very similar way, if we can make it burn: What nutrient type is present in the three most ‘high energy’ foods? What sort of problems do scientists face, when trying to measure these values accurately?

6. Example Step 1: Heat change = m x C x ΔT = 100 x 4.2 x 30 Jezebel burned some propanol (C3H7OH). She set up her equipment. She added 100g of water to a copper calorimeter (100ml). She weighed her mass of fuel and the burner before the experiment; it had a mass of 193.45g. She allowed the fuel to raise the temperature of the water by 30.C. She weighed her mass of fuel and the burner after the experiment; it had a mass of 192.55g. How can she calculate a ΔH for the fuel (in kJ/mole)? = 12600 J = 12.6 kJ Step 2: Mass of fuel burned = 193.45 – 192.55 = 0.90 g

7. Example Step 3: Moles of fuel burned Mr (propanol) = 60 g/mole Jezebel burned some propanol (C3H7OH). She set up her equipment. She added 100g of water to a copper calorimeter (100ml). She weighed her mass of fuel and the burner before the experiment; it had a mass of 193.45g. She allowed the fuel to raise the temperature of the water by 30.C. She weighed her mass of fuel and the burner after the experiment; it had a mass of 192.55g. How can she calculate a ΔH for the fuel (in kJ/mole)? n = m/M = 0.90/60 = 0.015 moles Step 4: Heat change (kJ/mole) = 12.6/0.015 = 840 kJ/mole

8. YOUR TURN… These are the techniques that you can use to work out the heat changes (in kJ/mole) for chemical reactions. It is a classic Higher Tier CHEM3 skill. Have a go at a few examples yourself now. Prove these answers are correct! Q1: 2565 kJ/mole Q2: 2800 kJ/mole Q3: 458.2 kJ/mole