I'm often confused with how to draw a Hess's law cycle and how you work out the alternate route for the reaction and then calculate the enthaply change of something. Can someone explain and please teach me by using the following example.
C3H8 has the standard enthaply change of combustion of -2220 kJmol-1
C has the standard enthaply change of combustion of -394 kJmol-1
H2 has the standard enthaply change of combustion of -286 kJmol-1
The equation that represents the standard enthaply change of formation of propane is shown below.
3C(s) + 4H2(g) -->C3H8(g)
Use Hess's law and the data about the enthaply change of combustion to calculate a value of the enthaply change of formation of propane.
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Hess's law cycle watch
- Thread Starter
- 12-06-2006 16:28
- 13-06-2006 13:10
Ok... enthalpy change = enthalpy change of combustion (reactants) - enthalpy change of combustion (products)
I just memorised that formulae, its the opposite for formation
therefore enthalpy change of formation of propane is:
(-394 + -286) - (-2220)
= -680 + 2220
= 1540 Kj per mol (and so is endothermic)
hope that helps! it took me ages to get that 2! xxx
- Thread Starter
- 13-06-2006 14:10
Thanks. There are formulae for the enthaply change?
What would the formula be if it were to claculate enthaply change of formation and reaction? Thanks.
- 13-06-2006 17:55
There isn't specific formulae but how i remeber it is when you construct it, the addition of the arrows going in the same direction = the value of the arrow going in the opposite direction. If you get what i mean
- 14-06-2006 16:30
Always remeber than if the arrows are going in the oppsite direction the value changes and you're sorted.
- 14-06-2006 19:41
I know this is very very obvious, but lots of ppl make the same mistake: - (-) gives + !!!!!
- 15-06-2006 12:39
I think you'll find i did put a plus thankyou
And the other formula is just switching the the reactants and products around.
Have you got the aqa modula books?
The formulas are in likle squicky boxes on the hess's law pages!