Enthalpy 18/04/2019

Enthalpy H Enthalpy is the energy content of a substance ( potential energy). We can calculate the enthalpy for a reaction by ∆H = Hp – H r Exothermic reactions – energy is released to the surroundings. In an exothermic reaction the products have less potential energy than the reactants. Therefore ∆H = Hp – H r is negative. 18/04/2019

Endothermic reactions – energy is absorbed from the surroundings. The products have more potential energy that the reactants. Therefore ∆H = Hp – H r is positive. 18/04/2019

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Examples Exothermic reactions Combustions, neutralising acids with alkali’s. Endothermic reactions Dissolving some salts (NH4NO3) 18/04/2019

Enthalpy of Combustion This is the enthalpy change when 1 mole of a substance is burned completely in oxygen. We use the equation Eh = c m ΔT ΔT= change in Temperature C = specific heat capacity of water this is a constant – 4.18kJKg-1C-1 18/04/2019

Enthalpy of Combustion m = mass of water that is absorbing the heat. ( e.g. 1 cm3 has a mass of 0.001kg,100cm3 = 0.1 kg etc) Worked Example Calculate the Enthalpy of combustion of Ethanol. 18/04/2019

Example Information Mass of burner at start = 80.63g Mass of burner at end = 80.48g T at start = 20.5 oC T at end = 30.5 oC Volume of water = 100 cm3 Mass of Ethanol = 0.15g Rise in T = 10 oC 18/04/2019

Calculation Eh = c m ΔT 4.18 x 0.1 x 10 =4.18 kJ 0.15g of Ethanol produces 4.18kJ when burned. Mass of 1 mole of Ethanol = 46g So – 0.15g -------- 4.18 kJ 46g ----------4.18 x 46/0.15 =1282 kJ mol/l 18/04/2019

Enthalpy of Solution This is the enthalpy change when 1 mole of a substance is dissolved in water. Again we use: Eh = c m ΔT Example When 2g of NaOH is dissolved in 50 cm3 of water the temperature changes from 20 oC to 30 oC. 18/04/2019

Calculation c= 4.18 m = 0.05 ΔT = 10 Eh= c m ΔT 4.18 x 2 x 10 = 2.09 kJ mol/l 18/04/2019

The Answer ! Mass of 1 mole of NaOH = 40g 2g ----------2.09 kJ 40g --------40/2 x 2.09 = 41.8 kJ mol/l 18/04/2019

Enthalpy of Neutralisation This is the energy released when 1 mole of water is produced in a neutralisation reaction. Example Calculate the enthalpy of neutralisation when 50 cm3 of Na OH ( 1 mol/l) neutralises 50 cm3 of H Cl. (1 mol/l) The temperature rise is 6 oC. 18/04/2019

The calculation! H Cl + Na OH —> Na Cl + H2O Eh = c m ΔT 4.18 x 0.1 x 6 ( add mass of NaOH and HCl to get mass of water.) 2.52 kJ Number of Moles of Water From equation 1 mol H+ ions will give 1 mole H2O. 18/04/2019

More! n=cv 1 x 0.05 = 0.05 Heat released per mole of water 0.05 moles = 2.52 kJ I mole = 1/0.05 x 2.52 = 50.4 kJ Since heat is given out the reaction is exothermic – ΔH neutralisation = - 50.4 Kj 18/04/2019