aqa AS nthalpy question

OP

But there are 2HN03, you've only accounted for one..

Reply 3

34 person

H20 + 2NO2 + (1/2)O2 ------> 2HNO3 enthalpy change = -128KJ mol -1

Standard enthaplipes of formation:
H20 = -286
NO2 = +34
O2 = 0

Use the data to caluclate the standard enthaply of formation of nitric acid.

Okay, from Elements to the left hand side, is +(-286) +2(34) =
-218 kJ mol-1

Therefore, by Hess' Law, the standard enthalphy of formation of nitric acid is (-218 +(-128))/2 = -173 kJ mol-1

Reply 4

Well, if you don't divide by 2 then you're finding the enthalphy of formation for 2 moles of HNO3, as 2 are formed in that reaction. So you divide by 2 to get the kJ mol-1 for the formation of one mole of HNO3.

Not sure what you mean by not taking into account HNO3, you know the enthalpy change in going from elements to the left hand side, and you know the enthalphy change in going from the left hand side to the right hand side (-128 kJ mol-1), so it's just a simple Hess's Law to get to HNO3.

Reply 5

OP

No I know sorry I was being stupid and deleted it lol.

I thought Hess law was products - reactants though :frown:

but you've added it...? Did you mean to put a minus instead of a plus sign there?

Reply 6

Hess's law states that the thermodynamic change is the same independant of the route taken, so A --> B --> C -- > D, has the same enthalpy change as A --> D, so Hess's law can be used in a few ways, in this case you're wanting to find the enthalpy of formation (from elements to the product nitric acid/HNO3), so if we call the elements A, the left handside B and the right handside C, we want A --> C, but we can get that by adding A --> B and B --> C.

Hope that wasn't too confusing and hope that's been helpful :smile:

Reply 7

OP

Wait I'm just gonna show the way I worked it out.

Enthalpy change = enthalpy formation(products) - enthalpy formation (reactants)

= (2(-128)) - ((-286)+2(34))
= -256 - -218
= -38KJ mol -1

Reply 8

OP

James G

Hess's law states that the thermodynamic change is the same independant of the route taken, so A --> B --> C -- > D, has the same enthalpy change as A --> D, so Hess's law can be used in a few ways, in this case you're wanting to find the enthalpy of formation (from elements to the product nitric acid/HNO3), so if we call the elements A, the left handside B and the right handside C, we want A --> C, but we can get that by adding A --> B and B --> C.

Hope that wasn't too confusing and hope that's been helpful :smile:

I get that. But I just can't DO hess law, anyway you can stop helping me now cos it's just getting embarrassing lol!

Reply 9

Well it's addition because you're wanting to find out the enthalpy change in forming nitric acid from its consituent elements, so you're wanting the enthalphy change in forming reactants, and the enthalphy change in going from reactants to products so your equation should be:

Enthalphy change = enthalpy of formation (products) + enthalpy formation (reactants)

= (-128) - ((-286)+2(34))
= -128 + -218
= - 346

But as that's for 2 moles, divide by 2 so

-346/2 = -173 kJ mol-1

Reply 10

OP

OHHHHH it's all cool now. Finally I get it. I'm going to blame my complete incompetance on the lack of having a scientific calculator and having to work out minus numbers in my head. Thankyou! :smile:

Reply 11

You're welcome :smile:

Reply 12

OP