Energetics (I)

Energetics means energy changes

Enthalphy
Change in energy under constant pressure

Enthalpy diagrams

In order to show a reaction, we need an enthalpy diagram. Most occur under normal pressure in open containers this is shown by $∆H$. Exothermic means negative values; Endothermic means positive ones.

Component Exothermic Endothermic
Energy Released Taken in
Enthalpy level of reactants Higher than products Lower than products
$∆H$ Negative Positive
Enthalpy diagram High to low Low to high
Enthalpy of products Lower than reactants Higher than reactants

1

Enthalpy change nomenclature

You need to consider reaction & surroundings, as the surroundings can cause a reaction to occur We don’t expect endothermic reactions Enthalpy changes vary according to conditions

Standard enthalpy

The standard conditions

  • 1 atmosphere ($1013hPa, 101kPa$)
  • 1 mol
  • 298k (25C)

Enthalpy of formation - $∆H_{f, 298}$

The enthalpy change when 1 mol of a substances is formed from its elements in their standard states at 100kPa pressure & at a stated temperature, normally 298K

Enthalpy of reaction - $∆H_{r, 298}$

The enthalpy change of a reaction is the change when the number of moles of the substances in the equation as written react under standard conditions of 100kPa & a stated temperature, normally 298K

Enthalpy of neutralisation - $∆H_{neut, 298}$

The enthalpy change when 1 mol of water is produced by the neutralisation of a solution of an acid by excess base under standard conditions with all solutions of concentration of 1 mol; the units are $kJ mol^{-1}$

Standard neutralisation equasion

\begin{equation}{H^+}_{\left(Aq\right)}{{+\ OH}^-}_{\left(Aq\right)}\rightarrow\ H_2O_{\left(l\right)}\end{equation}

Enthalpy of atomisation - $∆H_{at, 298}$

The enthalpy change when 1 mol of gaseous atoms is formed from the element in its standard state at 100kPa pressure & a stated temperature, normally 298K; the units are $kJ mol^{-1}$

Enthalpy of combustion - $∆H_{c, 298}$

The enthalpy change when 1 mol of the substance is burnt in excess oxygen under standard conditions of 100 kPa pressure & a stated temperature, normally 298K; the units are $kJ mol^{-1}$

Bond breaking and making

One can work out enthalpy change using average bond enthalpys, average as each bond is slightly different in energy.

  • Bond breaking is endothermic
  • Bond making is exothermic

Hess's law

Effectively states that whatever route taken to the same products will have the same enthalpy change overall.

For example, to make the enthalpy of formation for methane, you can use the enthalpy of combustion for hydrogen and oxygen, and then add the enthalpy change from the combustion of methane.