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Calculation of Enthalpy Values Using E = c m D T

Calculation of Enthalpy Values Using E = c m D T. Calculation of D H requires 3 steps. Use E = cm D T to calculate the energy change from the experiment. A scaling up of this value to obtain the energy change for 1 mole.

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Calculation of Enthalpy Values Using E = c m D T

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  1. Calculation of Enthalpy Values Using E = c m DT

  2. Calculation of DH requires 3 steps • Use E = cmDT to calculate the energy change • from the experiment • A scaling up of this value to obtain the energy change • for 1 mole 3. Checking to make sure the sign of the DH is correct

  3. Use E = cmDT to calculate the energy change On some occasions this may already be done for you There will be no temperature but there will be a value in kJ The mass of water in kg (litres) Calculate the litres of water and use this for m 1 litre weighs 1 kg Do not use the mass of chemical this is used in step 2 E = c m DT The change in temperature Specific heat capacity of water This will be found on the last page of the data book It does not matter what the liquid is, use the value for water

  4. 21.5 - 20.1 = 1.4 5 litre = 5kg 4.18 from databook When 1g of ethanol C2H5OH was burned the heat produced warmed 5litres of water from 20.1 oC to 21.5 oC Calculate the ethalpy of combustion of ethanol E = c m DT E = 4.18 x 5 x 1.4 E = 29.26 kJ Now do step 2

  5. 46g 1345.96 kJ 1g 29.26 kJ 46g 29.26 x 46 kJ When 1g of ethanol C2H5OH was burned the heat produced warmed 5litres of water from 20.1 oC to 21.5 oC From step 1 E = 29.26 kJ Scale up the value to obtain the energy change for 1 mole 2 x C = 2 x 12 = 24 6 x H = 6 x 1 = 6 1 x O = 1 x 16 = 16 Gram Formula Mass = 46 g Gram Formula Mass of ethanol = 46 g DH = 1345.96 kJ Now do step 3

  6. When 1g of ethanol C2H5OH was burned the heat produced warmed 5litres of water from 20.1 oC to 21.5 oC From step 2 DH = 1345.96 kJ Check to make sure the sign of the DH is correct Heat was produced in the reaction making it exothermic DH will have a negative value DH = - 1345.96 kJ mol-1

  7. 2g 2.09 kJ 40g 41.8 kJ When 2g of a compound ( formula mass 40) is dissolved in 50 cm3 of water the temperature rises by 10 oC Calculate the enthalpy of solution Step 1 E = c m DT E = 4.18 x 0.05 x 10 E = 2.09 kJ Step 2 Step 3 Temperature rise so exothermic DH = - 41.8 kJ mol-1

  8. When asked to calculate the enthalpy of neutralisation some minor changes must be made to the method. • All liquids, both the acid and alkali volumes, are heated • and so are included when calculating the mass of water • We will not be give a mass of acid • We will be given a concentration and a volume • we use these to calculate the number of moles of acid used. No. of moles = concentration x volume in litres c) We scale up (or down) the number of moles to 1

  9. 0.08 3.8456 kJ 1 mole 48.07 kJ When 100cm3 of hydrochloric acid concentration 0.8 mol l-1 is neutralised by 100 cm3 of an alkali, both at 12 oC the temperature of the salt solution rises to 16.6 oC Calculate the enthalpy of neutralisation of hydrochloric acid Step 1 E = c m DT E = 4.18 x 0.2 x 4.6 E = 3.8456 kJ No. of moles = conc x vol in litres No. of moles = 0.8 x 0.1 = 0.08 Step 2 Step 3 Temperature rise so exothermic DH = - 48.07 kJ mol-1

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