Chemistry Enthalpy Question

jumbogala

1. The problem statement, all variables and given/known data
Determine the standard enthalpy change for the reaction below.

4NH3 + 3O2 --> 2N2 + 6H2O

Given:
Standard heat of formation of NH3: -45.9 kJ/mol

Standard heat of formation of H2O: -285.83 kJ/mol

2. Relevant equations

3. The attempt at a solution
I had this question on a quiz and got points taken off, but I can't figure out why. Here's my solution:

{ (6 x -285.83 kJ/mol) + (2 x 0) } - { (3 x 0) + (4 x -45.9 kJ/mol)} = -1531 kJ/mol

alxm

Because the standard heat of formation of gaseous water is -241.826 kJ/mol.
I'm not sure where you got the -285.83 from, but I believe it's the liquid value.

Borek

Mentor
Reaction (as entered) doesn't state whether it should be gaseous or liquid water. My thermo is somewhat rusty, but isn't standard state of water liquid?

alxm

Reaction (as entered) doesn't state whether it should be gaseous or liquid water. My thermo is somewhat rusty, but isn't standard state of water liquid?
It is. But it wouldn't be wrong then! :)

I assumed the gas-phase states since, well, it's a gas-phase reaction.
If the problem was stated without specifying H2O (l) or H2O (g) then either would be correct, depending on whether you mean the 'standard enthalpy' in the strict sense, or the 'reaction enthalpy at standard pressure and temperature'.

Borek

Mentor
There is always a slight possibility that there was a mistake in the test

jumbogala

Just to clarify, the reaction was wriiten without states in the problem, and there were no states specified in the heat of formation given either.

Was I supposed to divide the value I got by four?

Borek

Mentor
That would be enthalpy change for a mole of ammonia, not for the reaction as written. I am not sure if there exeist some precise convention, I have seen it discussed (without any decisive conclusions) by chemistry teachers.

jumbogala

Yeah, I always thought you didn't divide by anything when it was for an actual reaction. It wouldn't make sense here because it doesn't specify per mole of ammonia vs. per mole of oxygen.

The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving