Question

For which of the following reactions is work done by the surroundings? (1) H2O(l) -> H2O(g) (2) Br2(l) -> Br2(s) (3) H2(g) + Cl2(g) -> 2 HCl(g) (4) 4 NO(g) + 5 H2O(l) -> 4 NH3(g) + 5 O2(g)

Answer 1

Work is done by the surroundings in the process where liquid water is converted to water vapor, as this is an endothermic process requiring the absorption of heat from the surroundings.

Step-by-step explanation:

To determine for which reactions the surroundings do work, we need to consider whether the process is endothermic or exothermic as well as the volume change of the system. Work is done by the surroundings on a system typically in endothermic processes or when a system's volume decreases as it goes from a less ordered to a more ordered state (like gas to liquid or liquid to solid), as per the first law of thermodynamics which relates changes in the internal energy of a system to heat added to the system and the work done by the system.

Out of the provided reactions, the one for which work is done by the surroundings is:

H₂O(l) -> H₂O(g): This is an endothermic process because heat is absorbed to convert liquid water to water vapor, increasing the disorder.

The other reactions involve either exothermic processes or changes in state that do not necessarily imply that work is done by the surroundings on the system. For example:

Br₂(l) -> Br₂(s): This process is exothermic and involves a phase change from liquid to solid.

H₂(g) + Cl₂(g) -> 2 HCl(g): An exothermic reaction where chemical bonds are formed.

4 NO(g) + 5 H₂O(l) -> 4 NH₃(g) + 5 O₂(g): This equation does not clearly show an endothermic process or work done by the surroundings.

Answer 2

To determine in which of these reactions work is done by the surroundings, we need to understand the concept of work in thermodynamics, particularly in relation to volume changes in a system during a reaction.

1. H2O(l) -> H2O(g) : This is a phase change from liquid to gas. When water evaporates, it expands, which means the system does work on the surroundings.

2. Br2(l) -> Br2(s) : This is a phase change from liquid to solid. When bromine solidifies, it contracts, which means the surroundings do work on the system.

3. H2(g) + Cl2(g) -> 2 HCl(g) : This is a chemical reaction with no change in the number of gas molecules (2 moles of gas reactants form 2 moles of gas products). There's minimal net change in volume, so minimal work is done by or on the surroundings.

4. 4 NO(g) + 5 H2O(l) -> 4 NH3(g) + 5 O2(g) : This reaction involves a liquid reacting to form gases. The generation of gas from a liquid will lead to an expansion of the system, meaning the system does work on the surroundings.

The reaction where work is done by the surroundings is Br2(l) -> Br2(s), as the system contracts and the surroundings must compress it.