Enthalpy of Reaction: MDCAT Chemistry notes
Enthalpy of Reaction MDCAT notes: enthalpies of formation, combustion, neutralization, atomization and solution, their signs and food energy sums.
Enthalpy and enthalpy of reaction
Enthalpy is the heat content of a system. The heat exchanged at constant pressure equals the enthalpy change, $\Delta H = q_p$. The enthalpy of reaction is the enthalpy change when the moles of reactants shown in a balanced equation react completely.
- Exothermic: $\Delta H_{\text{products}} \lt \Delta H_{\text{reactants}}$, so $\Delta H$ negative.
- Endothermic: $\Delta H_{\text{products}} \gt \Delta H_{\text{reactants}}$, so $\Delta H$ positive.
Types of standard enthalpy change
| Type | Definition (standard conditions) | Sign |
|---|---|---|
| Formation, $\Delta H_f^\circ$ | 1 mole of a compound formed from its elements in their standard states | Usually negative; can be positive |
| Combustion, $\Delta H_c^\circ$ | 1 mole of a substance burnt completely in oxygen | Always negative |
| Neutralization, $\Delta H_n^\circ$ | 1 mole of water formed from acid and alkali | Always negative |
| Atomization, $\Delta H_{at}^\circ$ | 1 mole of gaseous atoms formed from the element | Always positive |
| Solution, $\Delta H_{sol}^\circ$ | 1 mole of solute dissolved in excess solvent | Positive or negative |
Formation
Only equations that make exactly one mole of a compound from elements in standard states are formation equations, e.g. $\mathrm{H_2(g) + \tfrac12 O_2(g) \rightarrow H_2O(l)}$, $\Delta H_f^\circ = -285.8\ \mathrm{kJ\,mol^{-1}}$. Equations starting from a compound (e.g. CO) are not. $\Delta H_f^\circ$ of every element in its standard state is zero. $\Delta H_f^\circ$ of CO is about $-110.5\ \mathrm{kJ\,mol^{-1}}$; it cannot be measured directly because burning carbon also forms $\mathrm{CO_2}$.
Neutralization
For any strong acid with any strong base, $\mathrm{H^+(aq) + OH^-(aq) \rightarrow H_2O(l)}$, $\Delta H_n^\circ = -57.4\ \mathrm{kJ\,mol^{-1}}$. The value is the same because the net reaction is always the same. For weak acids or bases it is slightly less, as some energy is used to ionize them. A diprotic acid neutralized by a dibasic base, e.g. $\mathrm{H_2SO_4 + Mg(OH)_2}$, forms two moles of water, so about twice the heat is released.
Atomization
Atomization always requires breaking bonds, so it is always endothermic.
Energy from food
Approximate energy values: carbohydrate $4\ \mathrm{kcal\,g^{-1}}$, protein $4\ \mathrm{kcal\,g^{-1}}$, fat $9\ \mathrm{kcal\,g^{-1}}$.
Worked example: a snack with 30 g carbohydrate, 5 g protein and 10 g fat supplies $30\times4 + 5\times4 + 10\times9 = 120 + 20 + 90 = 230\ \mathrm{kcal}$.
Key formulas
- $\Delta H = q_p$
- $\Delta H_{\text{reaction}} = \sum \Delta H_f^\circ(\text{products}) - \sum \Delta H_f^\circ(\text{reactants})$
- $q = mc\Delta T$ for calorimetry
Common MDCAT traps
- Enthalpy of neutralization is per mole of water, not per mole of acid.
- Always exothermic: combustion and neutralization. Always endothermic: atomization, fusion, sublimation.
- Enthalpy of formation is not always exothermic (some compounds have positive values).
- A formation equation must produce one mole of product from elements only.
- Heat at constant pressure is enthalpy; heat at constant volume is internal energy.
Quick revision
- $\Delta H_n^\circ$ for strong acid and strong base is $-57.4$ kJ/mol.
- $\Delta H_f^\circ$ of an element in its standard state is zero.
- $\Delta H_f^\circ$ of CO is about $-110$ kJ/mol.
- Fat gives 9 kcal/g; carbohydrate and protein give 4 kcal/g.
- Atomization always has a positive enthalpy.