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.

Example:

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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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.