Question

Given the following thermochemical equations:
$Z n+\frac{1}{2} O_{2} \rightarrow Z n O+84,000\, cal$
$Hg +\frac{1}{2} O _{2} \rightarrow HgO +21,700\, cal$
Accordingly the heat of reaction for the following reaction
$Zn + HgO \rightarrow Hg +$ 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
$Z n+\frac{1}{2} O _{2} \rightarrow Z n O+84000 \,cal$
$ Hg +\frac{1}{2} O _{2} \rightarrow HgO +21,700\, cal $
$ Zn + HgO \rightarrow Hg + ZnO +$ heat
$\frac{1}{2} OZn \rightarrow ZnO +84,000\, cal $
$\frac{HgO \rightarrow Hg +\frac{1}{2} O _{2}-21,000 \,cal}{ Zn + HgO \rightarrow ZnO + Hg +62,300\, cal}$
$\therefore $ Heat in reaction $=+62,300$