AQA AS Chemistry Unit 2 3.2.1 Energetics

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3.2.1 Energetics

Enthalpy Change (H)

Enthalpy change ( H) is the amount of heat energy released or absorbed when a chemical or physical change occurs
at constant pressure.

Standard enthalpy changes refer to standard conditions, 100kPa and 298K.

Enthalpy change can be positive or negative (endothermic and exothermic)

Endothermic is…

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E.g. Use the following data to calculate the enthalpy of formation, Hf, of ethanol, C2H5OH.

HcC2H5OH = -1367.3 kJ mol-1
HcC = -393.5 kJ mol-1
HcH2 = -285.8 kJ mol-1

2C(s) + 3H2(g) + ½O2(g) à C2H5OH(l)



2CO2(g) + 3H2O(l)

Clockwise = anticlockwise
Hf-1367.3 =(-393.5x2) + (-285.8x3)
Hf= 1367.3 -…

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