Endothermic Reactions

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Homework Statement

http://i4.minus.com/j7HwKoL8yhl96.JPG [Broken]

Homework Equations

The enthalpy is equal to the heat of the products minus the heat of the reactants.

The Attempt at a Solution

We are not supposed to use a bond enthalpy table to ascertain the enthalpy of the reactions. I know that B is clearly exothermic, as is C. B is a phase change and in the process energy is released from the gas into the environment as it transitions into the lower-energy liquid. C is a combustion reaction and that's obviously exothermic.

D is clearly endothermic. Heat must be added to change water from a solid to a liquid.

However, what about A? It's a composition or synthesis reaction. Forming bonds also releases energy. However, bonds have to be broken before the water can be formed. I'm not sure how I'm supposed to tell whether A is exothermic/endothermic from just looking at it. Is there a way?

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Rawrr!
A is very exothermic. Oxygen and hydrogen are often used together as a means of propulsion in rockets. (Although to save space, they're usually condensed into a liquid state). It's a synthesis reaction as you're turning two reactants into one product.

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Is there a general rule I can apply to figure out A?

Mentor
If you have hydrogen and oxygen mixed together and you create a spark, is heat given off in the ensuing explosion? Do you have to remove heat to get the water produced back to the original temperature?

You know that going from a solid to a liquid absorbs heat, so must be what?

Gold Member
I calculated the enthalpy of reaction using bond energies and I got a positive figure for reaction A; could I have been using bonds between atoms of the wrong states? The bond energy table I used didn't give states. I remember that the carbon oxygen double bond in carbon dioxide varies with states...

Yanick
There are a few ways of thinking about it. First is that you can think of the fact that it is a type of combustion reaction, which all tend to be exothermic. You are combusting hydrogen gas instead of a hydrocarbon in this case.

Also you can use everyday life and just observe that when you spark hydrogen gas in the presence of oxygen gas you'll get lots if heat out (its pretty much an explosion).

Finally if you consider Gibbs free energy, dG = dH - TdS, you can see that the entropy of the system is decreasing (3 mols of gas become 2). To make this reaction proceed (in other words dG < 0) you need dH < 0.

EDIT: Scratch that last paragraph. It was a rule of thumb I picked up in Gen Chem and may not apply. In fact I just re-read a part of a PChem text which says that oxygen and hydrogen gas produce water with an increase of entropy. Apologies.

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Do you have to remove heat to get the water produced back to the original temperature?

How can you tell? I understand that for a forward endothermic reaction if you reserve it, it'll become an exothermic reaction. How do I ascertain this in this case?

Gold Member
Oh wait, I thought of a cool new way to remember that hydrogen gas and oxygen gas react, exothermically.

Remember, remember, the explosion of the Hindenburg.