Specification Point 3.4:
Calculate the molar enthalpy change (△ h) from the heat energy change, q
ΔH is the symbol that represents the amount of energy lost or gained in a reaction.
+ΔH is endothermic (because it gains heat from the surroundings)
-ΔH is exothermic (because it loses heat to the surroundings)
The unit of ∆H is kJ/mol.
Molar Enthalpy Change
Data from a calorimetry experiment can be used to calculate the molar enthalpy change of a reaction.
The energy from burning 0.5 g of propane was transferred to 100 cm3 of water to raise its temperature by 20°C. Calculate the molar enthalpy change (in kJ / mol).
Mass of Water = 100 cm3
Heat capacity of Water = 4.2 j / g
Temperature rise = 20°C
Energy transferred = 100 x 4.2 x 20 = 8400 J
*1000 J = 1 kJ
So 8400 J = 8.4 kJ
Mr of Propane = 44
Moles of Propane burned = 0.5 ÷ 44 = 0.01136
Molar Enthalpy change = 8.4 ÷ 0.01136 = 739
This Reaction is Exothermic so Enthalpy Change Needs to be Negative
Molar Enthalpy change = – 739 kJ / mol
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