This is not mine, it is from this lovely website here:

They are from the AQA section but are useful to anyone that does energetics in their A-level

 These notes go into more detail than is necessary, and I am hoping to upload condensed versions soon :)

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Preview of Energetics

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Topic 2.1
Measuring and Calculating Enthalpy Changes
Mean Bond Dissociation Enthalpies
Hess' Law

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1. Exothermic and endothermic reactions
When a chemical reaction takes place, the products and reactants have different stabilities and
thus there is a change in potential energy. However since total energy is always conserved, any
change in potential energy must be balanced by an equal and opposite change in kinetic energy.
The lower the chemical potential energy of a given chemical species, the more stable it
is. This means that stable species have a lower potential energy than unstable species.…read more

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Standard enthalpy changes
The change in chemical potential energy during a chemical reaction is known as the enthalpy
change for that reaction. It is given the symbol H.
By convention, if the reaction is exothermic (ie heat is given out) the enthalpy change is said to
be negative: H = ve. If the reaction is endothermic (ie heat is absorbed) the enthalpy change is
said to be positive: H = +ve.
The enthalpy change of a reaction depends on the reaction conditions.…read more

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Standard enthalpies of formation and combustion
The term "standard enthalpy of reaction" can be used to describe any chemical reaction, but
there are some important reactions which have special names:
The standard enthalpy of formation of a substance is the enthalpy change when one mole of
that substance is formed from the most stable allotropes of its elements in their standard states
under standard conditions.
It is given the symbol Hof.
Eg C(s) + 2H2(g) CH4(g), H = 74.8 kJmol1.…read more

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Calculating enthalpy changes from temperature changes
Enthalpy changes are generally measured by carrying out a reaction under controlled conditions
in a laboratory and measuring the temperature change.
The amount of heat required to change the temperature of a system by 1K is known as the heat
capacity of a system (Hc). It is measured in JK1.
The heat energy change for a given reaction can therefore be calculated from the equation:
q = T x Hc.…read more

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During a chemical reaction, the bonds in the reactants are broken. This is an endothermic
process energy is required to do this. After the bonds have been broken, however, the bonds
in the products are formed. This is an exothermic process energy is released when this
The enthalpy change for a chemical reaction can be deduced from consideration of the energy
required to break bonds in the reactants and the energy released when the bonds in the
products are formed.…read more

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These bond enthalpies are mean values the exact strength of a bond depends on its
environment. Even the strengths of the same type of bond in the same molecule may vary:
Eg in water: HOH(g) HO(g) + H(g) H = +502 kJmol1
HO(g) H(g) + O(g) H = +427 kJmol1
It is necessary therefore to take an average = ie (502 + 427)/2 = +465 kJmol1.…read more

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Thus bond enthalpies calculated from different reactions may vary slightly. They also do not take
intermolecular forces into account. Mean bond enthalpies thus only give you approximate
values for enthalpy changes.
b) Solids and liquids
In giant covalent substances, all the covalent bonds have to be broken before free gaseous
atoms can be formed.
In either case, the energy required to separate completely the atoms in one mole of the
substance is known as the atomisation energy (Hat).…read more

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Determining mean bond enthalpies from given data:
If the enthalpy change for a reaction is known, and most of the bond enthalpies are known, it is
possible to calculate the mean bond enthalpy of a particular bond:
Eg The enthalpy of formation of methane is known to be 76 kJmol1. The bond enthalpy of a
HH bond is +436 kJmol1, and the enthalpy of atomization of carbon (C(s) C(g)) is +713
kJmol1.…read more

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Hess' Law states that "the enthalpy change for a chemical reaction depends only on the
initial and final states and is independent of the path followed".
In other words whichever route, however direct or indirect, by which the reaction proceeds, the
overall enthalpy change for the reaction will be same. It is an application of the principle of
conservation of energy.
Hess' Law can be used to calculate many enthalpy changes which cannot be measured directly.…read more


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