Question

What is the molar enthalpy (in kJ/mol) for the reaction
2 CaO (s) → 2 Ca (s) + O₂ (g)?

Answer 1

The molar enthalpy change for the reaction
`\(2 \text{CaO (s)} \rightarrow 2 \text{Ca (s)} + \text{O}_2 (g)\) is \(1270.2 \, \text{kJ/mol}\)`, signifying an endothermic process that absorbs heat from the surroundings.

The molar enthalpy change
`(\( \Delta H \))` for a chemical reaction can be determined using thermochemical principles and experimental data. The molar enthalpy change is the heat exchanged under constant pressure conditions and is expressed in units of kJ/mol.

The given reaction is
`\(2 \text{CaO (s)} \rightarrow 2 \text{Ca (s)} + \text{O}_2 (g)\)`.

To find
`\( \Delta H \)`, we can utilize Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction.

Here, we'll use the enthalpy of formation
`(\( \Delta H_f \))` values for the substances involved. The
`\( \Delta H_f \)` is the enthalpy change when one mole of a substance is formed from its elements in their standard states.

The standard enthalpy of formation for CaO (s) is
`\( -635.1 \, \text{kJ/mol} \)` because it is an exothermic reaction when calcium oxide is formed from its elements.

The standard enthalpy of formation for Ca (s) is
`\( 0 \, \text{kJ/mol} \)` because it is in its standard state.

The standard enthalpy of formation for
`\( \text{O}_2 (g) \) is \( 0 \, \text{kJ/mol} \)` because oxygen is diatomic in its standard state.

Now, applying Hess's Law:

`\[ \Delta H = \sum \Delta H_f(\text{products}) - \sum \Delta H_f(\text{reactants}) \]`

`\[ \Delta H = \left[2 * \Delta H_f(\text{Ca (s)}) + 1 * \Delta H_f(\text{O}_2 (g))\right] - \left[2 * \Delta H_f(\text{CaO (s)})\right] \]`

`\[ \Delta H = \left[2 * 0 + 1 * 0\right] - \left[2 * (-635.1 \, \text{kJ/mol})\right] \]`

`\[ \Delta H = 0 - (-1270.2 \, \text{kJ/mol}) \]`

`\[ \Delta H = 1270.2 \, \text{kJ/mol} \]`

Therefore, the molar enthalpy change
`(\( \Delta H \))` for the reaction
`\(2 \text{CaO (s)} \rightarrow 2 \text{Ca (s)} + \text{O}_2 (g)\) is \(1270.2 \, \text{kJ/mol}\).` This positive value indicates that the reaction is endothermic, meaning it absorbs heat from the surroundings.