Chapter 17, Problem 65A

(a)

Interpretation Introduction

Interpretation: The enthalpy change for the formation of 1 mole of a compound is to be calculated.

Concept Introduction: The standard enthalpy of formation is the amount of energy released or consumed when one mole of a substance is formed under standard conditions from its pure elements.

The standard enthalpy of formation of a pure substance in its elemental form or pure state is zero.

Answer to Problem 65A

The enthalpy change for the formation of 1 mole of aluminum oxide is $\text{-1676 kJ}$

Explanation of Solution

The given chemical reaction is:

  ${\text{2Al}}_2{\text{O}}_3\text{(s)}\to {\text{4Al(s)+3O}}_{\text{2}}\text{(g) }\Delta \text{H= 3352 kJ}$

To get the required reaction of the formation of aluminum oxide, the reaction has to be reversed and thus, the sign of enthalpy change will also be reversed.

  ${\text{4Al(s)+3O}}_{\text{2}}\text{(g)}\to {\text{2Al}}_2{\text{O}}_3\text{(s) }\Delta \text{H= -3352 kJ}$

Now, to get the reaction of the formation of 1 mole of aluminum oxide, the reaction has to be multiplied by half and thus the value of enthalpy change will also be halved.

  $\text{2Al(s)+}\frac{3}{2}{\text{O}}_{\text{2}}\text{(g)}\to {\text{Al}}_2{\text{O}}_3\text{(s) }\Delta \text{H= }\frac{1}{2}\times \text{-3352 kJ=-1676 kJ}$

(b)

Interpretation Introduction

Interpretation: Whether the reaction is endothermic or exothermic is to be found.

Concept Introduction:

Endothermic reactions are the reactions in which heat is absorbed and thus the enthalpy change for the endothermic reaction is positive. Exothermic reactions are the reactions in which heat is released and thus, the enthalpy change for the exothermic reaction is negative.

Answer to Problem 65A

The reaction is exothermic.

Explanation of Solution

  $\text{2Al(s)+}\frac{3}{2}{\text{O}}_{\text{2}}\text{(g)}\to {\text{Al}}_2{\text{O}}_3\text{(s) }\Delta \text{H= -1676 kJ}$

As the chemical reaction for the formation of 1 mole of aluminum oxide involves a negative sign of enthalpy change, it means the reaction is exothermic.