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Chapter 1.2 Energetics and Enthalpy Changes
Breaking bonds requires energy (endothermic)
Forming bonds releases energy (exothermic)
A + B C + D
Enthalpy change of reaction is the energy releases to the surroundings in an exothermic reaction.
Enthalpy is energy. It's represented by a `H'
H = H of products H of reactants
The H is positive for an endothermic reaction, as energy is gained.
The H is negative for an exothermic reaction, as energy is lost.
The heat capacity, C of an object is the amount of heat required to raise its temperature by 1K. Its
measured n joules per kelvin (J K¯¹)
The specific heat capacity, c is the amount of energy in joules (J) needed to raise the temperature of
one kg of a particular substance by 1 kelvin (K).
Energy transferred (J) Mass (kg) × Specific heat capacity (J kg K¯¹) × Temperature change (K)
E = mcT
Cup calorimetry To find out the enthalpy change of a reaction you can use an insulated container
called a calorimeter, and then measure the temperature change. A `coffee-cup calorimeter' uses
two expanded polystyrene cups as it's an excellent insulator and has a low specific heat capacity.
A bomb calorimeter is a more accurate way to determine the energy change during a reaction.
Standard conditions are 1 atm and 298 K (25°C)
An enthalpy change measured under standard conditions is referred to as the standard enthalpy
change of reaction and is represented by H .
The standard enthalpy changes of combustion vary because of :-
The number of bonds that need to be broken and made. This depends on the size of the
molecule. E.g. methane CH has less bonds than hexane CH, so there are more bonds
being broken and made in one mole of hexane.
The type of bonds involved. E.g. it takes more energy to break a C-H bond than a C-C bond.
Standard enthalpy change of formation H f of a compound is the enthalpy change where one mole
of the compound is formed from its elements under standard conditions.
E.g. formation of methane CH C (graphite) + 2H CH
It's measured in kJ mol¯¹. The per mole refers to the formation of one mole of the compound.
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Standard enthalpy change of atomisation H at of an element is the enthalpy change when one
mole of its atoms in the gaseous state is formed from the element under standard conditions.
E.g. C (graphite) C (g)
It's always endothermic since it involves increasing the separation between atoms, which requires
Standard enthalpy change of neutralisation H n is the energy change when an acid and a base
react to from 1 mole of water under standard conditions.
Here's a taster:
HO (l) HO (l) + ¹/ O (g)
H (g) + O (g) H HO (l) + ¹/ O (g)
To find H you do H - H, so
-285.8 - - 187.8 = -98 kJ mol¯¹
Bond dissociation enthalpy (bond energy) The energy needed to break a particular covalent bond,
or the energy released when the bond is formed.…read more