Standard enthalpy of formation question

[Peptides]

Hi guys, I need some help working out this question

It says, calculate the standard enthalpy of formation of NH3 from the following data

 

H2 + 1/2 O2 = H2O. DeltaH=-286kJmol^-1

4NH3 + 3O2 = 2N2 + 6H2O. DeltaH=-1537kJmol^-1

 

 

My first thought was to draw out a thermochemistry cycle, but I'm not sure whether the enthalpy values given are for formation or combustion. Does it matter which of the two they are? Also, how would I go about working this out? The previous two questions asked for the equation of the first law of thermodynamics, which is deltaU=q+w and and equation that relates internal energy, enthalpy, pressure and volume, which would be deltaH=deltaU+PdeltaV

Any help would be much appreciated

Edited May 1, 2012 by Peptides

[hypervalent_iodine]

The first question you need to ask yourself is what [math]\Delta[/math] H_f actually means; more specifically, what does the use of the delta symbol signify?

 

Secondly, what you know about the heats of formation for oxygen and nitrogen gas?

[Peptides]

Enthalpy of formation is the enthalpy change when one mole of compound is formed from its constituent elements under standard conditions, all reactants and products in their standard states. The delta symbol signifying a change

The heats of formation for nitrogen is negative, and wouldn't the formation of oxygen be positive? Just the opposite of what's written if the reaction were to go in the reverse direction?

[hypervalent_iodine]

H_f of N₂ and O₂ is 0. You will probably need to remember that.

 

The delta symbol in this means that you are looking for the difference. So, how do you think you could calculate the difference of the heats of formation of a reaction, given that you know have the H_f values for each of its components?