Question

Given the following thermochemical equations:
$Zn+\frac{1}{2}{O}_2\to ZnO+84,000cal$
$Hg+\frac{1}{2}{O}_2\to HgO+21,700cal$
Accordingly the heat of reaction for the following reaction
$Zn+HgO\to Hg+$ heat is:

• A. 105,700 cal
• B. 61,000 cal
• C. 105,000 cal
• D. 62,300 cal

Answer 1

Answer: (D)

Use Hesse's law Given
$Zn+\frac{1}{2}{O}_2\to ZnO+84000cal$
$Hg+\frac{1}{2}{O}_2\to HgO+21,700cal$
$Zn+HgO\to Hg+ZnO+$ heat
$\frac{1}{2}OZn\to ZnO+84,000cal$
$\frac{HgO\to Hg+\frac{1}{2}{O}_2-21,000cal}{Zn+HgO\to ZnO+Hg+62,300cal}$
$\therefore$ Heat in reaction $=+62,300$