Enthalpy Change (ΔH) -> This refers to a change is heat energy at constant pressure
Standard Enthalpy Change -> Change in heat energy at constant pressure, under standard conditions
Standard conditions refers to 298K, 100kPa and 1 mol/dm3
solutions
Reaction Profiles:
This shows an endothermic reaction profile. The energy of the
products is above the energy of reactants, so ΔH is positive
(products -> reactant), giving an endothermic reaction. The
reactants take energy in from their surroundings to form the
higher energy products. Examples include thermal
decomposition & cracking of alkanes
This shows an exothermic reaction profile. The energy of the
products is below the energy of reactants, so ΔH is negative
(products -> reactant), giving an exothermic reaction. The
reactants give out energy to their surroundings to form the
lower energy products. Examples include neutralisation &
combustion
The activation energy (minimum energy required to trigger a
reaction) is shown by a vertical line from reactants to the
peak of the curve
The enthalpy change is show by a vertical line from the reactants to the products
This shows a reaction profile with the use of a catalyst. The
catalyst just provides an alternative chemical pathway, with
a lower Ea, but doesn’t get changed or used up in the
process. The enthalpy change remains the same.
Enthalpy Definitions:
Enthalpy of Reaction (ΔrH) -> The enthalpy change that takes place when a reaction occurs. This is
unique to all reactions
Enthalpy of Combustion (ΔcH) -> The enthalpy change that occurs when 1 mole of a compound is
completely combusted, in a plentiful supply of O2, under standard conditions
Enthalpy of Formation (ΔfH) -> The enthalpy change that occurs when 1 mole of a compound is
formed from its constituent elements in their standard states, under standard conditions
Enthalpy of Neutralisation (ΔnH) -> The enthalpy change that occurs when 1 mole of water is formed
when an acid and a base react together, neutralising one another, under standard conditions
Remember to put the ‘under standard conditions’ on the end if the enthalpy change is standard (298k,
100kPa).
Calorimetry:
The mass must only be the mass of the solution, you do
not take into account the mass of the solid. You do not
need to convert between degrees and kelvin here as we
are looking at the temperature change and as this is the
same in both degrees and Kelvin, it is unnecessary to
change between the 2.
If the temperature increases, put a negative in front as this
enthalpy change is exothermic
Sometimes, you may have to calculate the mass from the density and volume. Use the following formula;
You may have to calculate the enthalpy change per mole. This is simple, just calculate the number of moles
that have reacted, and divide the value of ‘q’ by this number to find the ΔH per mole.
Ensure that you use the limiting reagent when calculating the enthalpy change per mole. Find which
reagent is limiting by using the ICE table.
The main problem with all calorimetry experiments is heat loss. We can reduce the heat loss in 2 main
ways; use a lid to insulate the calorimeter and use an insulated beaker. This will help to reduce the heat
loss as much as possible giving the most accurate enthalpy change per mole.
Hess’ Law:
Hess’ Law states ‘The enthalpy change for a reaction or process depends upon the initial and final states,
and is independent of the route taken’. We can use Hess cycles to calculate any enthalpies;
The arrows in a Hess cycle must go underneath the reactants and products. Combustion arrows point
downwards, and formation arrows point upwards. Always put on the clockwise & anticlockwise arrows
before calculating anything. You can then use the following formula to calculate any enthalpy changes;
Remember to multiply any enthalpy values by their coefficient!
An elements enthalpy of formation should be 0kJ/mol. If it isn’t this, it is because the element is not in its
standard state.
Bond Enthalpies:
For a bond to be broken, the reactants must be in the gaseous state. If they’re not in the gaseous state,
you must take into account the enthalpy of vaporisation.
Specific bond enthalpy -> The enthalpy change that occurs when 1 mole of covalent bonds is broken,
forming 2 moles of free-radicals
Mean bond enthalpy -> The enthalpy change that occurs when 1 mole of covalent bonds is broken,
forming 2 moles of free radicals, averaged over a range of compounds.
Some bonds only exist in 1 type of molecule (Cl-Cl), so for these we use the specific bond enthalpy.
However, other bonds such as the C-C bonds, exist is many different types of molecule, each with a slightly
different bond enthalpy, so we use the mean bond enthalpy here.
Sometimes, you may have to use a Hess cycle to find the enthalpy change, then divide by the number of
bonds present to find the bond formation enthalpy of a specific bond.