Unit 3 Science Investigation Skills

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Presentation transcript:

Unit 3 Science Investigation Skills Topic G: Energy content of fuels © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

Calorimetry: a method for measuring the energy released from a fuel

Calorimetry calculations Step 1: Energy change can be calculated using the following equation: q = mc∆T q = energy change in joules m = mass of substance being heated (often water) in grams c = specific heat capacity in joules per °C per gram (4.18 JK-1g-1 for water) ∆T = change of temperature in °C. (Final temperature – initial temperature) © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

Convert the energy released into kJ. q (J) = q (kJ) 1000 Step 3: Work out number of moles of chemical that causes the energy change. Use moles = mass/Mr Or use moles = concentration x volume © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

Heat energy released from a fuel in kJmol-1: ∆H = ______q (kJ)_______ Step 4: Heat energy released from a fuel in kJmol-1: ∆H = ______q (kJ)_______ no. of moles of reactant Step 5: Finally, insert the positive or negative sign. (If the temperature goes up, it is negative; if the temperature goes down, it is positive.) © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

The following measurements were made on burning propanol, C3H7OH: For the following question, assume c = 4.18Jg-1K-1 and density of water = 1.00gcm-3 The following measurements were made on burning propanol, C3H7OH: volume of water in calorimeter = 100cm3 temperature rise of water = 21.5 ºC mass of propanol burned = 0.28g Use this information to calculate ∆ Hc (kJ mol-1) for propanol. © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

Calorimetry – a method used to investigate the energy released from fuels 4. The water is heated up by a certain temperature (e.g. 50°C) 3. The spirit burner is lit 5. The spirit burner is extinguished 2. The start temperature of the water is measured 6. The end temperature is recorded 1. The spirit burner is weighed 7. The spirit burner is reweighed © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

Calorimetry calculations Step 1 Calculate the energy change of the water. q = energy change in joules q = mc∆T m = mass of water in grams c = specific heat capacity in joules per °C per gram (4.18 J °C -1g-1 for water) ∆T = change of temperature in (Final temperature – initial temperature) Example: q = 100 x 4.18 x 50 = 20900 J © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

Calorimetry calculations Step 2 Convert the energy change to kJ. q (J) = q (kJ) 1000 Example: q = 20900 J = 20.9 kJ 1000 Step 3 Work out how many moles of fuel were burnt. Number of moles = mass of fuel burnt mr of fuel Example: number of moles = 2.5 = 0.05426525 moles 46.07 © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

Calorimetry calculations Step 4 Calculate the energy released per mole of fuel. Energy released per mole = energy released (q) (kJ) number of moles burnt (kJmol-1) Example: energy released per mole = 20.6 = 379.6 kJ mol-1 0.05426525 = 380 kJ mol-1 (2sf) Step 5 Add a negative sign if energy was released (if the reaction was exothermic). Example: energy released per mole = -380 kJ mol-1 (2sf) © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.

Energy released (q=mcΔt) q = 100 x 4.18 x 21.5 = 8987 J 0.28g of propanol, C3H7OH, were burnt under 100cm3 of water in a calorimeter which increased in temperature by 21.5°C. Assume that c = 4.18Jg-1°C-1 and that 1cm3 of water = 1g. Using this information, calculate the energy released per mole of propanol (kJ mol-1). Energy released (q=mcΔt) q = 100 x 4.18 x 21.5 = 8987 J 2. Energy released in kJ = 8987 = 8.897 kJ 1000 3. Moles burnt = mass = 0.28 = 0.0046589 mr 60.10 4. Energy released per mole (kJmol-1) = 8.897 = 1909.7 0.0046589 (kJmol-1) 5. Add a sign (if exothermic). Energy released per mole (kJmol-1) = -1909.7 (kJmol-1) = -2000 (kJmol-1) (1sf) © Pearson Education Ltd 2016. Copying permitted for purchasing institution only.