Question

Given Thermochemical equation, $2H_{2(g)} + O_{2(g)} \rightarrow 2H_2O_{(l)} ; \, \Delta H = -571.6\, kJ$. Heat of decomposition of water is

• A. $- 571.6 \, kJ$
• B. $+ 571.6\, kJ$
• C. $-1143.2\, kJ$
• D. $+ 285.8 \,kJ$

Answer 1

Answer: (D)

Given

(i) $2 H _{2}(g)+ O _{2}(g) \longrightarrow 2 H _{2} O ; \Delta H=-571.6\, kJ$

At $ H _{2} O (I) \longrightarrow H _{2}(g)+\frac{1}{2} O _{2}(g) ; \Delta H=?$

If reaction (i) is reversed and divided by $2$ , the sign of $\Delta H$ changes, i.e.,

$\Delta H$ for decomposition of water

$=+\frac{571.6}{2}=285.8\, kJ$