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.
Your answer is correct. If a reaction forms a gas, that gas expands into the surroundings and work is done.
To determine whether a reaction is exothermic or endothermic, you need to consider the sign of the enthalpy change (∆H). If ∆H is negative, the reaction is exothermic, meaning it releases heat to the surroundings. If ∆H is positive, the reaction is endothermic, meaning it absorbs heat from the surroundings.
Now, to determine whether work is done on or by the system, you need to consider the overall change in volume (∆V). If ∆V is negative, work is done on the system, meaning the surroundings are compressing or applying pressure to the system. If ∆V is positive, work is done by the system, meaning the system is expanding against the surroundings.
Now let's analyze each reaction:
1) 3 NaN3(s) --> Na3N(s) + 4 N2(g)
This reaction involves the formation of products from reactants. The formation of chemical bonds usually releases energy, resulting in an exothermic reaction (∆H < 0). Since there is no change in volume indicated, we cannot determine whether work is done on or by the system.
2) 2 Cl(g) --> Cl2(g) + H2O(l) --> H2O(g)
In this reaction, chlorine gas (Cl2) is converted to chlorine molecules, which is the reverse of the chlorine dissociation reaction. Dissociation of chlorine and vaporization of water both require energy, indicating an endothermic reaction (∆H > 0). Similar to the previous reaction, there is no information on the volume change, so we cannot determine the work.
3) 2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g)
This reaction involves the formation of an aqueous solution and the release of hydrogen gas. The formation of ionic compounds, such as LiOH, often releases energy, making it exothermic (∆H < 0). Again, there is no information about the volume change, so we cannot determine the work.
4) H2(g) + O2(g) --> 2 H2O(l)
This reaction represents the combustion of hydrogen gas with oxygen gas to form water. Combustion reactions are typically highly exothermic (∆H < 0). Since the number of molecules changes from 2 to 2 or 1 to 2, this reaction is known to produce gas. Hence, the system is expanding against the surroundings, meaning work is done by the system (work = P∆V > 0).
Therefore, your statement that all reactions are exothermic except for the last one is correct. However, we cannot determine whether work is done on or by the system in the first three reactions due to the lack of information about the volume change.
To determine whether work is done on or by the system, you can consider the pressure-volume work. If the volume of the system increases during the reaction, work is done on the system, and if the volume decreases, work is done by the system.
Let's analyze each reaction:
1. 3 NaN3(s) --> Na3N(s) + 4 N2(g)
This reaction involves the formation of products from reactants. Breaking the bonds in NaN3 requires energy, while forming new bonds in Na3N and N2 releases energy. Since energy is released overall, this reaction is exothermic. However, there is no mention of any changes in volume, so we cannot determine whether work is done on or by the system.
2. 2 Cl(g) --> Cl2(g) H2O(l) --> H2O(g)
The first part of the reaction involves the combination of two chlorine atoms to form a Cl2 molecule. Since a bond is formed, energy is released, making this reaction exothermic. Again, there is no information about changes in volume, so the work cannot be determined. The second part of the reaction involves the vaporization of water, which requires energy, making it endothermic.
3. 2 Li(s) + H2O(l) --> 2 LiOH(aq) + H2(g)
This reaction involves the reaction of lithium metal with water to form lithium hydroxide and hydrogen gas. Breaking the bonds in water and lithium requires energy, while forming new bonds in LiOH and H2 releases energy. Since energy is released overall, this reaction is exothermic. Additionally, the formation of the gas leads to an increase in volume, indicating that work is done by the system.
4. H2(g) + O2(g) --> 2 H2O(l)
This reaction involves the combustion of hydrogen gas in the presence of oxygen gas to form water. The formation of new bonds in water releases energy, making this reaction exothermic. Furthermore, the formation of the liquid water indicates a decrease in volume, suggesting that work is done on the system.
To summarize:
1. Exothermic, work unknown
2. Exothermic, work unknown
3. Exothermic, work done by the system (increase in volume)
4. Exothermic, work done on the system (decrease in volume)