Question

In which of the following reactions the system performs work on surrounding?

• A. 2H$_2$O$_{2(l)}$ $\longrightarrow$ 2H$_2$O$_{(l)}$ + O$_{2(g)}$
• B. NH$_{3(g)}$ + HCl$_{(g)}$ $\longrightarrow$ NH$_4$Cl$_{(s)}$
• C. 2C$_{(s)}$ + 3H$_{2(g)}$ $\longrightarrow$ C$_2$H$_{6(g)}$
• D. 2SO$_{2(g)}$ + O$_{2(g)}$ $\longrightarrow$ 2SO$_{3(g)}$

Answer 1

Answer: (A)

2H$_2$O$_{2(l)}$ $\longrightarrow$ 2H$_2$O$_{(l)}$ + O$_{2(g)}$

Answer 2

For the system to do expansion work on the surroundings, the reaction must result in an increase in the number of gas moles (i.e. expansion). In option (A), the production of gaseous O$_2$ from liquid H$_2$O$_2$ causes an expansion in volume. In (B), the gaseous reactants form a solid; in (C) and (D), the reaction leads to a decrease in the number of gas moles.