Label each of the following reactions as exothermic or endothermic ("exo" or "endo"), and according to whether work is done on or by the system ("on" or "by")? Note that no "en-on" cases appear here, as these are always thermodynamically unfavourable. There is no need to do any calculation, use chemical arguments to predict the sign of the enthalpy change.


3 NaN3(s) --> Na3N(s) + 4 N2(g)
(Simplified version of the chemical reaction in air bags. NaN3 is sodium azide.)

2 Cl(g) --> Cl2(g) H2O(l) --> H2O(g)

2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g)

H2(g) + O2(g) --> 2 H2O(l)

i would say that all of them are exothermic except for the last one...is that right?

but i don't know how to find out whether work is done on or by the system.

2 Cl(g) --> Cl2(g) exo-on

H2O(l) --> H2O(g) endo-by
2 H2(g) + O2(g) --> 2 H2O(l) exo-on
2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g) endo-by
3 NaN3(s) --> Na3N(s) + 4 N2(g)endo-by

To determine whether work is done on or by the system, you need to consider the conditions under which the reaction occurs. Here's how you can analyze each reaction:

1. 3 NaN3(s) --> Na3N(s) + 4 N2(g)
This reaction involves the formation of a solid and a gas. When a solid is formed, it releases energy and is exothermic. Therefore, this reaction is exothermic.

Regarding work, if a system expands against an external pressure, work is done by the system. If the system contracts, work is done on the system. In this case, the formation of 4 moles of gas (N2) indicates expansion. Therefore, work is done by the system.

Conclusion: This reaction is exothermic and work is done by the system.

2. 2 Cl(g) --> Cl2(g) H2O(l) --> H2O(g)
This reaction can be divided into two parts: the conversion of Cl gas to Cl2 gas and the evaporation of water from liquid to gas.

The conversion of Cl gas to Cl2 gas involves bond breaking, so energy is required and the process is endothermic.

The evaporation of water from liquid to gas requires energy to overcome intermolecular forces, so it is also endothermic.

Regarding work, without more information about the conditions, it is difficult to determine whether work is done on or by the system.

Conclusion: The reaction is endothermic, but it is uncertain whether work is done on or by the system.

3. 2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g)
The reaction involves the formation of an aqueous solution (LiOH) and the production of a gas (H2). When a substance dissolves in water, energy is released, indicating an exothermic process. Additionally, gas formation is typically exothermic.

Regarding work, if a system expands against an external pressure, work is done by the system. If the system contracts, work is done on the system. In this case, the production of gas (H2) indicates expansion. Therefore, work is done by the system.

Conclusion: This reaction is exothermic, and work is done by the system.

4. H2(g) + O2(g) --> 2 H2O(l)
This reaction involves the formation of a liquid from gases. The formation of a liquid typically releases energy, indicating that this reaction is exothermic.

Regarding work, without more information about the conditions, it is difficult to determine whether work is done on or by the system.

Conclusion: This reaction is exothermic, but it is uncertain whether work is done on or by the system.

In summary, reactions 1 and 3 are exothermic, and work is done by the system in both cases. Reactions 2 and 4 are also exothermic, but it is unclear whether work is done on or by the system without additional information.