Chapter U5.109, Problem 2E

Interpretation Introduction

Interpretation: Indication of heat of formation of metals needs to be explained.

Concept Introduction: Exothermic reactions are best source of energy. Exothermic reaction discharges energy into the surrounding from the system.

Answer to Problem 2E

The energy discharge when one mole of a compound is formed from elements is called the heat of formation. If the $\Delta H_{\text{f}}$ value is negative then the reaction is exothermic.

Explanation of Solution

Oxidation reactions are good source of energy because they are always exothermic. Heat of formation is denoted by symbol $\Delta H_{\text{f}}$ . The $\Delta H_{\text{f}}$ value indicates how much heat is transferred when one mole of compound is formed from its element. To explore the usefulness of metal oxidation as an energy source, compare the amount of thermal energy transferring during the reactions. With the help of $\Delta H_{\text{f}}$ , we can decide the reaction is exothermic or endothermic.

Conclusion

The heat of formation value of Magnesium oxide is $-602\text{kJ}$ and iron (III) oxide is $-826\text{kJ}$ . From the $\Delta H_{\text{f}}$ value it is clear that iron (III) oxide produces more energy than $\text{MgO}$ .