Question

1. Consider the generic reaction:A + 2BC AH = -55 kJDetermine the amount of heat emitted when each amountof reactant completely reacts (assume that there is morethan enough of the other reactant).(a) 1 mol A(b) 2 mol A(c) 1 mol B(d) 2 mol B

Answer 1

Answer:

(a) 55 kJ of heat are released when 1 mol of reactant A is used;

(b) 110 kJ of heat are released when 2 moles of reactant A are used;

(c) 27.5 kJ of heat are released when 1 mol of reactant B is used;

(d) 55 kJ of heat are released when 2 moles of reactant B are used.

Explanation:

The question requires us to determine the amount of heat released when the given amounts of reactants are used, considering the following balanced chemical equation:

$A+2B\rightarrow C\text{ }\Delta H=-55kJ$

When the enthalpy change for a reaction (or heat of reaction, ΔH) is given in units of energy, such as kilojoules (kJ), and not units of energy per mol (such as kJ/mol), we can consider that ΔH corresponds to the heat absorbed or released for the molar quantities of reactants as given in the balanced chemical equation. In the case given by the question, for example, we can say that 55 kJ of energy are released when 1 mol of A reacts with 2 moles of B.

Therefore, we can use the molar quantitites from the balanced chemical equation as a reference to determine the amount of heat released when different amounts of reactants are used.

Considering the information above, we can calculate:

(a) heat released when 1 mol of A reacts:

Note that 1 mol of A corresponds to the amount of reactant A given by the balanced chemical equation. Therefore, 55kJ of energy are released when 1 mol of A is used.

(b) heat released when 2 moles of A reacts:

Note that 2 moles of A corresponds to the double of the amount of reactant A given by the balanced chemical equation. Thus, we must multply ΔH by 2: 55 kJ x 2 = 110 kJ of energy are released when 2 moles of A are used.

(c) heat released when 1 mol of B reacts:

Note that 1 mol of B corresponds to half of the amount of reactant B given by the balanced chemical equation. Thus, we must divide ΔH by 2: 55 kJ / 2 = 27.5 kJ of energy are released when 1 mol of B is used.

(d) heat released when 2 moles of B reacts:

Note that 2 moles of B corresponds to the amount of reactant B as given by the balanced chemical equation. Therefore, 55 kJ of energy are released when 2 moles of B are used.