A-level Chemistry/AQA/Module 2/Energetics

Energy Change

Energetics is all about the energy transfer involved in the making and breaking of bonds. The breaking of a bond involves the injection of energy into the system so that the bond can no longer hold the parts together; the creation of bonds involves the removal of energy from the system so that components are captured by the binding forces. In a typical reaction some bonds are broken and some are created so some energy is injected and some is removed. Whether the total(net) of these amounts to an injection or removal determines whether the reaction is endothermic (involving an net injection of energy) or exothermic (involving a net removal of energy.)

Enthalpy change (ΔH)

The simple idea described above is complicated by the interaction of the system(reaction) being studied and its environment. This interaction amounts to the changes in volume of the system brought about by the reaction. To deal with this complexity we introduce the idea of enthalpy(given the symbol H). Enthalpy is not measured directly: we concern ourselves instead with CHANGES in enthalpy (ΔH) resulting from a reaction.

For a given reaction change in Enthalpy is equal to the heat energy change under conditions of constant pressure.

Exothermic

• negative ΔH

Such reactions include the combustion of a fuel like methane CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l) = -890kJ mol⁻¹ Also including the oxicdation of carbohydrates such sa glucose C₆H₁₂O₆

Endothermic

• positive ΔH

Such reactions include the thermal decomposition of calcium carbonate CaCO₃ (s) → CaO (s) + CO₂ (g) = +178kJ mol⁻¹

Enthalpy change

The heat energy transferred under constant pressure.

ΔH = ΔH products – ΔH reactants

Standard conditions

We often deal with enthalpy change under standard conditions. i.e.

100kPa, 298K

Standard enthalpy of formation

$\Delta H_{f}^{\ominus }$

The enthalpy change when 1 mole of compound in its standard state is formed from its elements in their standard states (under standard conditions).

Standard enthalpy of combustion

$\Delta H_{c}^{\ominus }$

The enthalpy change when 1 mole of substance is completely burned in oxygen under standard conditions.

Calorimetry

Calculating energy changes

• $q=mc\Delta T$
  • heat energy = mass of substance x specific heat capacity x temperature change
• $\Delta H={\frac {q}{n}}$
  • enthalpy change = heat energy / moles

Hess's Law

Hess's Law

The overall enthalpy change is independent of the route the chemical reaction takes place.

Bond Enthalpies

Bond Enthalpy

• Energy/enthalpy needed to break/dissociate a bond averaged over different molecules.
• Always positive because breaking bonds requires energy.
• $\Delta H_{r}^{\ominus }=\Sigma \Delta H_{r}^{\ominus }(bonds\ broken)-\Sigma \Delta H_{r}^{\ominus }(bonds\ made)$