IB Chemistry Sub-topic R1.2 Notes

Types of Enthalpy

You previously learned about the standard enthalpy change of a reaction (ΔH^ø). Remember that the formula for this is:

$\Delta H = \Delta H _{products} - \Delta H_{reactants}$

However, in the HL syllabus, you need to appreciate that this is an umbrella term used to describe the enthalpy change that occurs during any reaction. However, there are seven main subtypes you need to be aware of:

1. Standard enthalpy change of formation (ΔH^ø_f) - this is the enthalpy change when one mol of a substance forms from its constituent elements in their standard states at STP. For example: 

   C_(s) + O_{2 (g)} → CO_{2 (g)}

   Note if O₂ was liquid, this would not be at STP and thus incorrect. Additionally the formation of any element by itself has no enthalpy change! For example:

   O + O → O₂ ΔH^ø_f = 0 kJmol^-1

2. Standard enthalpy change of combustion (ΔH^ø_c) - this is the enthalpy change when one mol of a substance completely burns in oxygen at STP. For example: 

   CH₄ + 2O₂ → CO₂ + 2H₂O

3. Standard enthalpy change of neutralization (ΔH^ø_n) - this is the enthalpy change when one mol of water is produced when an acid reacts with an alkali at STP. For eaxmple:

   NaOH + HCl → NaCl + H₂O the you learned about three types of enthalpy change. In the HL curriculum, you need to learn an additional six types:

4. Standard enthalpy change of atomization (ΔH^ø_hyd) – this is the enthalpy change when one mole of gaseous atoms are formed from its element in its standard state. This is typically highly endothermic. For example: 

   Li_(S) → Li_(g) or 0.5Cl_2(g) → Cl_(g)

5. Standard lattice enthalpy change (ΔH^ø_latt) – this is the enthalpy change when one mole of a solid ionic lattice is broken into its individual gaseous ions. This process is endothermic. For example: 

   BeF_{2 (s)} → Be²⁺_(g)­ + 2F^-_(g)

6. Ionization energy (ΔH^ø_ie) – Remember that this is the enthalpy change when one mole of electrons are removed from one mole of gaseous atoms. For example: 

   Na_(g) → Na⁺_(g) + e^-

7. Electron affinity (ΔH^ø_ea) – Remember that this is the enthalpy change when one mole of electrons is added to one mole of gaseous atoms. It is usually an exothermic reaction. For example: 

   Ca²⁺_(g) + e^- → Ca⁺_(g).

Enthalpy of Aqueous Solutions 

An additional two types of enthalpy changes are used in a separate energy cycle relating to aqueous solutions. These are:

1. Standard enthalpy change of hydration (ΔH^ø_hyd) -  this is enthalpy change when one mole of gaseous ions are dissolved in water to form an infinitely dilute solution. It is always exothermic. For example: 

   Cl^-_(g) → Cl^-_(aq)

2. Standard enthalpy change of solution (ΔH^ø_sol) - this is the enthalpy change when one mole of a solute is dissolved in excess pure solvent. This is typically highly endothermic. For example: 

   LiBr_(s) → Li⁺_(aq) + Br^-_(aq)