What is enthalpy?

of the energy stored in (or heat content of) a system. It cannot be measured directly.

The enthalpy change for a reaction is usually observed as a change in temperature, which can be measured or calculated.

During reactions, the enthalpy of the reactants and the products is not the same. This results in energy being either given out or taken in during the reaction. This energy is the enthalpy change, ∆H (‘delta H’).

exchange with its surroundings at constant pressure.

Enthalpy is the energy content of the reactants and is given the symbol H.

Standard enthalpy changes are measured at a standard pressure of 100 kPa and temperature of 298 K. Standard enthalpy changes are represented by ∆Hө298 but this is usually shortened to ∆Hө.

Therefore, enthalpy change is represented by ∆H. It has the units kilojoules per mole (kJ mol-1).

In science, change is represented by the upper case Greek letter delta, ∆.

represented by:

C(s, graphite) + 2H2(g) → CH4(g)

By definition, the standard enthalpy of formation of an element, in its standard state, must be zero.

∆Hfө = -74.9 kJ mol-1

The standard enthalpy of combustion of methane can be represented by:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

∆Hcө = -890 kJ mol-1

mc∆T

q = enthalpy change in joules

To work out the enthalpy of neutralization, the density and specific heat capacities of the acid and base used are taken to be the same as for pure water.

∆T = change of temperature in Kelvin.

c = specific heat capacity in joules per Kelvin per gram (4.18 JK-1g-1 for water)

m = mass of substance being heated (often water) in grams

conservation of energy. It is sometimes expressed in the following form: Energy cannot be created or destroyed, it can only change form.

This means that in a closed system, the total amount of energy present is always constant.

Hess’s law states that the overall enthalpy change for a reaction is independent of the route the reaction takes.

In 1840, the Russian chemist Germain Hess formulated a law which went on to be known as Hess’s Law.

This went on to form the basis of one of the laws of thermodynamics:

forming B directly, ΔH1, is the same as the enthalpy change for the indirect route, ΔH2 + ΔH3.

ΔH1

ΔH2

ΔH3

A → B

ΔH1

A → C → B

ΔH2 + ΔH3

Therefore: ΔH1 = ΔH2 + ΔH3

Hess’s law can be used to calculate the standard enthalpy change of a reaction from known standard enthalpy changes.

direct route

indirect route

to the surroundings.

There is therefore a net release of energy, which is measured as an increase in temperature.

This happens because more energy is released making new bonds in the products than is taken in breaking bonds in the reactants.

+

+

in from the surroundings.

This happens because more energy is used in breaking bonds in the reactants than is released when bonds are formed in the products.

There is therefore a net intake of energy, which is measured as a decrease in temperature.

+

+

dissociation enthalpy for a particular bond in a range of different compounds.

Using the mean bond enthalpy takes into account the different bond dissociation enthalpies of the particular bond in different compounds.

The H–H bond is only found in H2 and its bond dissociation energy is +436 kJ mol-1. However, bonds such as C–H exist in different compounds and have a slightly different bond dissociation enthalpy in each case.

Precisely, it is the average enthalpy change for breaking 1 mole of a particular bond in a range of different compounds in the gas phase.