Question

Determine which of the following reactions taking place at constant pressure represent system that do work on the surrounding environment

(i) ${Ag}^{+}\left(aq\right)+{CI}^{-}\left(aq\right)⟶AgCI\left(s\right)$

(ii) ${NH}_{4}CI\left(s\right)⟶{NH}_3\left(g\right)+HCI\left(g\right)$

(iii) $2{NH}_3\left(g\right)⟶N_2\left(g\right)+3H_2\left(g\right)$

• A. I
• B. III
• C. II and III
• D. I and II

Answer 1

Answer: (A)

I

A system that does work on the surroundings has a negative value for work.

• $A{g}^{+}\left(aq\right)+C{l}^{-}\left(aq\right)\to AgCl\left(s\right)$
  

In this reaction 2 volumes of reagent combine to form 1 vol of product. The total volume falls, so the work done is negative (the atmosphere does work on the gaseous system).

• $N{H}_{4}Cl\left(s\right)\to N{H}_3\left(g\right)+HCl\left(g\right)$
  
In this reaction 1 volume of reagent combine to form 2 vol of product. The total volume rises, so the work done is positive.
• $N{H}_3\left(g\right)\to N_2+3H_2\left(g\right)$
  
In this reaction 1 volume of reagent combine to form 2 vol of product. The total volume rises, so the work done is positive.
Hence option A is correct answer.