Energetics
(a) Explain that some chemical reactions are accompanied by energy changes, principally in the form of heat energy; the energy changes can be exothermic (
D
H, negative) or endothermic
(b) Explain and use the terms:
(i) Enthalpy change of reaction and standard conditions, with particular reference to: formation, combustion, hydration, solution, neutralisation, atomisation
(ii) Bond energy (
D
H positive, i.e. bond breaking)
(c)
(d)
Calculate enthalpy changes from appropriate experimental results, including the use of the relationship
Enthalpy change = mc
D
T
Apply Hess’ Law to construct simple energy cycles, and carry out calculations involving such cycles and relevant energy terms, with particular reference to:
(i) determining enthalpy changes that cannot be found by direct experiment, e.g. an enthalpy change of formation from enthalpy changes of combustion
(ii) average bond energies
(e) Construct and interpret a reaction pathway diagram, in terms of the enthalpy change of the reaction and of the activation energy

Endothermic reactions require energy to form products (∆H is positive)
Exothermic reactions release energy as a product (∆H is negative)

Endothermic:
• Melting
• Vaporizing
• Chemical reactions that have a positive
∆H (absorb energy)
Exothermic
• Freezing
• Condensing
• Combustion
• Chemical reactions that have a negative
∆H (release energy)

Calculating the enthalpy (
D
H ) change of a reaction
D
H = m x c x
D
T
The specific heat capacity, c is the amount of heat needed to raise the temperature of 1 g of substance by 1 K. Its units are joules per gram per Kelvin, or Jg -1 K -1 .
For example, the specific heat capacity of water is 4.2 g -1 K -1 , so it takes
4.2 joules to raise the temperature of 1 gram of water by 1 degree kelvin.

Hess’s Law states that the total energy (or enthalpy) for a chemical reaction is the same, whatever route is taken, provided that the initial and final conditions are the same.
We can show this on a diagram called a thermochemical cycle
See book, 13.5.2 for calculations (p.132)

The standard molar enthalpy change of formation ,
D
H f is the enthalpy change when a mole of compound is formed from its elements in their standard states under standard conditions

The standard molar enthalpy of combustion
D
H c is the enthalpy change at standard state when a mole of substance is completely burned in oxygen