Chapter 17.2, Problem 20LC

Interpretation Introduction

Interpretation: The thermochemical equation for the combustion reaction is to be written.

Concept introduction: Thermochemical equations are balanced stoichiometric chemical equations that are accompanied by information about heat change.

Endothermic reactions are the ones in which heat is absorbed and enthalpy change is positive.

The exothermic reactions are the ones in which heat is released and enthalpy change is negative.

Answer to Problem 20LC

The thermochemical equation for the combustion reaction will be ${\text{2Mg(s)+O}}_{\text{2}}\text{(g)}\to \text{2MgO(s) }\Delta \text{H= -1204kJ}$

Explanation of Solution

The given reaction is 2 mol of solid magnesium $\text{(Mg)}$ combined with 1 mol of oxygen gas ${\text{(O}}_{\text{2}}\text{)}$ to form 2 mol of solid magnesium oxide $\text{(MgO)}$ and 1204 kJ of heat is released.

The reactants and products are accompanied by their physical states with (s) representing solid, (l) representing liquid, (g) representing gas, and (aq) representing the aqueous state.

As the heat is released, the reaction will be exothermic and the enthalpy change for the reaction will be negative.

Thus, the thermochemical equation will be ${\text{2Mg(s)+O}}_{\text{2}}\text{(g)}\to \text{2MgO(s) }\Delta \text{H= -1204kJ}$