Heats of Reaction

Thermochemistry

Thermochemistry, also referred to as Energetics, is a branch of physical chemistry that deals with energy changes occurring during chemical reactions. These energy changes can be either positive or negative and are denoted by H.

Enthalpy of reaction (heat of reaction)

Definition: Enthalpy is the energy change that takes place during a chemical reaction.

Common enthalpies of reactions

a. Standard enthalpy of combustion

This is the heat energy given out when 1 mole of a substance is completely burnt in a given amount of oxygen at standard state. If H for a combustion reaction is negative, the reaction is exothermic.

Example:

$C + O_2 \rightarrow CO_2$

How to balance combustion reactions:

1. Balance elements other than hydrogen and oxygen first.
2. Then balance hydrogen.
3. Finally, balance oxygen.

b. Standard enthalpy of formation ($\Delta H^\circ_f$)

This deals with the formation of compounds. It is the heat change that occurs when 1 mole of a substance is formed from its elements at standard state. $\Delta H^\circ_f$ can be positive (endothermic) or negative (exothermic).

Examples:

$C + O_2 \rightarrow CO_2 \quad \Delta H^\circ_f$

$C + 2H_2 \rightarrow CH_4$

c. Standard enthalpy of neutralization

The heat energy given out when one mole of water is formed from the reaction between an acid and a base at standard state.

Example:

$HCl + NaOH \rightarrow NaCl + H_2O$

d. Atomization energy

Energy absorbed when a molecule or element is converted to gaseous atoms. This is denoted by $E_{at}$.

Example:

$Br_2(g) \rightarrow 2Br(g)$

e. Sublimation energy

Energy absorbed when one mole of solid atoms is converted into gaseous atoms, denoted as $E_s$.

Example:

$Al(s) \rightarrow Al(g)$

f. Ionization energy

Energy used to remove an electron from the outermost shell of an atom or ion to form a cation. Denoted by $E_I$.

Example:

$Al(g) \rightarrow Al^+(g) + e^-$

g. Electron affinity

Energy change when one mole of non-metallic gaseous atoms combine with one mole of electrons. Denoted by $E_{aff}$.

Example:

$Cl(g) + e^- \rightarrow Cl^-$

h. Lattice energy

Energy given out when one mole of an ionic compound is formed from its ions. Denoted by $E_L$.

Example:

$Na^+(g) + Cl^-(g) \rightarrow NaCl(s)$

Calculations in thermochemistry

i. Based on combustion data

1. Identify the required equation.
2. Present the data.
3. Manipulate the data as needed.
4. Draw a conclusion.

Example:

Calculate the enthalpy of formation of methane given the following:

• Enthalpy of combustion of C: -394 kJ/mol
• Enthalpy of combustion of $H_2$: -286 kJ/mol
• Enthalpy of combustion of $CH_4$: -891 kJ/mol

Solution:

Required Equation:

$C + 2H_2 \rightarrow CH_4$

Data Manipulation:

$C + O_2 \rightarrow CO_2 \quad \Delta H = -394 \text{ kJ/mol}$

$H_2 + \frac{1}{2}O_2 \rightarrow H_2O \quad \Delta H = -286 \text{ kJ/mol (multiplied by 2)}$

The enthalpy of formation of methane is:

$\Delta H = -75 \text{ kJ/mol}$

ii. Based on bond energies

The enthalpy of a reaction can be calculated as the difference between bond breaking energies (BBE) and bond formation energies (FBE):

$\Delta H = \text{BBE} - \text{FBE}$

Example:

Calculate the enthalpy of hydrogenation of ethyne to ethane:

Solution:

BBE:

• C≡C: 839 kJ/mol
• H-H: 436 kJ/mol

FBE:

• C-C: 348 kJ/mol
• C-H: 412 kJ/mol

$\Delta H = \text{BBE} - \text{FBE}$

Final Answer:

$\Delta H = -132 \text{ kJ/mol}$