## 2000 - Written Paper - Q12(c) If someone could help please :)

### YIC

Posted 27 May 2005 - 05:50 PM

Question 12c) in 2000 section B about the question with Hess's Law

To get enthalpy change of the equation you first hav to manipulate the enthalpies of combustion of the C, H and the Hydrazine (equation given for that one).
For the equation for hydrogen I got: (the following is the original equation I keep mentioning)

2H2 + O2 --> 2H2O Enthalpy = -286

I need 3H2 instead of 2H2 so I multiply everything by 1.5 to get exactly the same equation shown in the answers and so I get an enthalpy change of (-286 x 1.5) -429. However, the anwers have it as -858. I understand that they have multiplied the original enthalpy by 3 to get -858, but this doesn't make sense to me as I thought you would have to multiply the original equation and enthalpy by 1.5 to get the desired equation.
I don't see where I have made the mistake but it could be that the original equation could be H2 + O2 --> H20 and that the 'answers' have multiplied everything by 3 but I don't think that is possible because the target equation has to be balanced (i.e it can't be 3H2 + 3O2 --> 3H20 and even the answers does not show this).

Could someone please help me. I understand if I have given a question that seems hard to follow but I just do not understand

YIC

### Ally

Posted 27 May 2005 - 06:19 PM

You're simply balancing the equation for the combustion of Hydrogen.

The balanced equation is:

3H2 + 1.5O2 ---> 3H2O

6 Hydrogen + 3 Oxygen ---> 6 Hydrogen. 3 Oxygen

The enthalpy of combustion for H2 = -286

So for 3H2 = -858.

### YIC

Posted 27 May 2005 - 06:30 PM

But isn't 2H2 = -286 since that is the balanced combustion equation

### YIC

Posted 27 May 2005 - 06:32 PM

So I shouldn't balance equations at the start (before I start manipulating them?)

I think I understand now, thanks

Edited by YIC, 27 May 2005 - 06:33 PM.

### Ally

Posted 27 May 2005 - 06:38 PM

QUOTE(YIC @ May 27 2005, 07:32 PM)
So I shouldn't balance equations at the start (before I start manipulating them?)

I think I understand now, thanks

Nope.

For 1 mole of H2 the balanced equation is:

H2 + 0.5O2 ---> H2O

For 2 moles of H2 the balanced equation is:

2H2 + O2 ---> 2H2O

For 3 moles (the one in this question) the balanced equation is:

3H2 + 1.5O2 ---> 3H2O

### YIC

Posted 27 May 2005 - 09:28 PM

Ok NOW I understand