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# less exothermic? Watch

1. why is CH4(g) + 1½O2(g) -> CO(g) + 2H2O(g) less exothermic than CH4(g) + 1½O2(g) -> CO(g) + 2H2O(l), where water is a liquid?
2. (Original post by rainerised)
why is CH4(g) + 1½O2(g) -> CO(g) + 2H2O(g) less exothermic than CH4(g) + 1½O2(g) -> CO(g) + 2H2O(l), where water is a liquid?
Do you know the bond enthalpies? I would write out all the bonds, then take the enthalpy of the products away from the reactants.
3. (Original post by rainerised)
why is CH4(g) + 1½O2(g) -> CO(g) + 2H2O(g) less exothermic than CH4(g) + 1½O2(g) -> CO(g) + 2H2O(l), where water is a liquid?
you know when you draw energy change diagram, the "level" representing energy marks the "average"/rough amount of the actual energy the molecules have at a particular temperature

as you know, gaseous water (water vapour) would have higher energy than liquid water. since energy released during combustion (albeit incomplete in your case) will be needed to vaporise any water products - that tells you whether the overall energy released would be more or less, hence that answers your question ,yes?
4. (Original post by shengoc)
you know when you draw energy change diagram, the "level" representing energy marks the "average"/rough amount of the actual energy the molecules have at a particular temperature

as you know, gaseous water (water vapour) would have higher energy than liquid water. since energy released during combustion (albeit incomplete in your case) will be needed to vaporise any water products - that tells you whether the overall energy released would be more or less, hence that answers your question ,yes?
So because some of the energy from the combustion reaction is needed to vaporise the liquid water to water vapour, overall less energy is released and the reaction is less exothermic. Is that right?
5. (Original post by rainerised)
So because some of the energy from the combustion reaction is needed to vaporise the liquid water to water vapour, overall less energy is released and the reaction is less exothermic. Is that right?
qualitatively speaking yes

quantitatively, gaseous water has higher relative energies compared with liquid water, hence average energy of the products with gaseous water will be higher in energy than that producing liquid water (note the word average)

you can draw lines to represent this on your energy change diagram for the reaction (exothermic), then you can see the drop from the energy ("average") of the reactants to the products will be lesser for the one producing gaseous water.

simple diagram can tell you the whole story.
6. (Original post by shengoc)
qualitatively speaking yes

quantitatively, gaseous water has higher relative energies compared with liquid water, hence average energy of the products with gaseous water will be higher in energy than that producing liquid water (note the word average)

you can draw lines to represent this on your energy change diagram for the reaction (exothermic), then you can see the drop from the energy ("average") of the reactants to the products will be lesser for the one producing gaseous water.

simple diagram can tell you the whole story.
Ok thanks

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Updated: April 12, 2013
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