Any Chem ppl here???

[emeraldgirl08]

I have a good feeling that there might be an expert in Chem here.....so I'm in need of a little assistance with a Chemistry problem. The problem involves Hess's law and is set up as:

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From the enthalpies of reaction

2H2(g)+O2(g)--> 2H2O(g)     Delta H: -483.6 kj
3O2(g)--> 2O3(g)     Delta H: +284.6 kj

Calculate the heat of the reaction

3H2(g)+O3(g)-->3H2O(g)

If anyone could guide me in the right direction I'd truly appreciate it

[killer]

Blimey, when I did stinks (chemistry) at school we had none of this stuff!

Do you have a formula on which we can base a calculation?

[sir_synthsalot]

Is that a typo in the first equation? Mercury gas? Also the first equation is not balanced.

The enthalpy of a reaction can be determined from the sum of the individual steps.

It's very easy. You just reverse the second equation so O2 will cancel out (heat of reaction also reverses), balance the two equations so they add up to the third one, and the then add the heats of reaction. Whatever you do to an equation you also do to the heat of reaction, so for example if you multiply an equation by 2 to make it cancel out you also multiply the heat of reaction by 2.

[emeraldgirl08]

Is that a typo in the first equation? Mercury gas? Also the first equation is not balanced.

The enthalpy of a reaction can be determined from the sum of the individual steps.

It's very easy. You just reverse the second equation so O2 will cancel out (heat of reaction also reverses), balance the two equations so they add up to the third one, and the then add the heats of reaction. Whatever you do to an equation you also do to the heat of reaction, so for example if you multiply an equation by 2 to make it cancel out you also multiply the heat of reaction by 2.

lol glad you caught that! I corrected it!

Still perplexed about this b/c aren't the two molecules on opposite sides of the reaction:product sides supposed to be identical when cancelling? The O2 on one side is 1 mole while the other side is 3 moles.

[sir_synthsalot]

You have to multiply each one so they cancel out.

So the first equation:

2H2 + O2 -> 2H2O (-483.6 kj) | x3

becomes

6H2 + 3O2 -> 6H2O (-1450.8)

add the second equation reversed

2O3 -> 3O2 (-284.6)

you get

6H2 + 2O3 -> 6H2O (-1735.4)

then divide by 2 to get the final equation and enthalpy

3H2 + O3 -> 3H2O (-867.7)

Alternatively you can multiply the first equation by 3/2 and divide the second one by 2 and the 3/2 O2 will cancel and give you the identical answer.

[emeraldgirl08]

Thank you! You have a great night

[RealBlackStuff]

I'd rather learn how to speak Swahili

[jdrou]

This problem is basically just algebra with a slightly different notation. It's been probably 25 years since I did anything with chemistry and I could have solved this.

[virge]

I'd rather learn how to speak Swahili

This problem is basically just algebra with a slightly different notation. It's been probably 25 years since I did anything with chemistry and I could have solved this.

It's only been about 10 years since my wife and I took O-chem together. I stared at the problem like it WAS Swahili. Thought it would be interesting to show my wife and laugh about how much we'd forgotten. She promptly started muttering something about multiplying by 3 on each side.... Guess which one of us graduated with honors?