You are Here: Home

# Hess cycles watch

1. so here's my working what have i done wrong and why?
Attached Images

2. Can you try and re upload the picture because it's too blurry to see anything.
3. (Original post by B_9710)
Can you try and re upload the picture because it's too blurry to see anything.
better?
4. (Original post by thefatone)
better?
Much better.
At the bottom of the cycle should be the gaseous atoms with the bonds all broken. So at the bottom of the cycle should be N and 3H and the arrows should be going towards the bottom of the cycle - if you get what I mean - so to work out the enthalpy of formation you should be doing sum of enthalpy of bonds broken - sum of enthalpy of bonds made.
5. (Original post by B_9710)
Much better.
At the bottom of the cycle should be the gases atoms with the bonds all broken. So at the bottom of the cycle should be 2N and 6H and the arrows should be going towards the bottom of the cycle - if you get what I mean - so to work out the enthalpy of formation you should be doing sum of enthalpy of bonds broken - sum of enthalpy of bonds made.
why do the arrows go down? surely it's a formation enthalpy so the arrows go up from 2C and 6H not down?
my understanding if down is that, that would be an enthalpy of combustion which includes oxygen as one of the reactants in the main equation and what would be produced is water and CO2
but the equation doesn't show that so surely it's a formation enthalpy change and the arrows go up from 2C and 6H?
6. (Original post by thefatone)
why do the arrows go down? surely it's a formation enthalpy so the arrows go up from 2C and 6H not down?
my understanding if down is that, that would be an enthalpy of combustion which includes oxygen as one of the reactants in the main equation and what would be produced is water and CO2
but the equation doesn't show that so surely it's a formation enthalpy change and the arrows go up from 2C and 6H?
Yes but this is slightly different as we are using the mean bond enthalpies rather than enthalpies of combustion. So we break bonds of the N2 and 1.5H2 to form 2N and 3H (so the arrow go down). And then we breaking the bonds to go from NH3 to N and 1.5H2 (so the arrow goes down). Then you just reverse the arrow on the right - which is where bonds broken - bonds made comes from.
Upload a combustion cycle question if you want me to show you the slight differences.
7. (Original post by B_9710)
Yes but this is slightly different as we are using the mean bond enthalpies rather than enthalpies of combustion. So we break bonds of the N2 and 1.5H2 to form 2N and 3H (so the arrow go down). And then we breaking the bonds to go from NH3 to N and 1.5H2 (so the arrow goes down). Then you just reverse the arrow on the right - which is where bonds broken - bonds made comes from.
Upload a combustion cycle question if you want me to show you the slight differences.
i'll have to come back to this a bit later since i'm going out shopping for some waterproofs and stuff xD
btw thanks a lot for the help
8. (Original post by B_9710)
Yes but this is slightly different as we are using the mean bond enthalpies rather than enthalpies of combustion. So we break bonds of the N2 and 1.5H2 to form 2N and 3H (so the arrow go down). And then we breaking the bonds to go from NH3 to N and 1.5H2 (so the arrow goes down). Then you just reverse the arrow on the right - which is where bonds broken - bonds made comes from.
Upload a combustion cycle question if you want me to show you the slight differences.
ok what i don't get is where the oxygen comes from in the first cycle

9. (Original post by thefatone)
ok what i don't get is where the oxygen comes from in the first cycle

Well you are given combustion data - so you just burn each of the reactants in oxygen and you get an enthalpy change and you get given the value in the table. The oxygen though is not important because the enthalpy of formation and combustion of oxygen is 0 (well there is no value for oxygen - you can't burn oxygen in oxygen). So you combust C2H4 and H2 to CO2 and H2O and so the arrow goes from the reactants to CO2 and H2O.
Same for C2H6 - you burn it in oxygen to CO2 and H2O so the arrow goes from C2H6 to CO2 and H2O. You may want to look at this on chemguide.
10. (Original post by B_9710)
Well you are given combustion data - so you just burn each of the reactants in oxygen and you get an enthalpy change and you get given the value in the table. The oxygen though is not important because the enthalpy of formation and combustion of oxygen is 0 (well there is no value for oxygen - you can't burn oxygen in oxygen). So you combust C2H4 and H2 to CO2 and H2O and so the arrow goes from the reactants to CO2 and H2O.
Same for C2H6 - you burn it in oxygen to CO2 and H2O so the arrow goes from C2H6 to CO2 and H2O. You may want to look at this on chemguide.
ok understood thanks
11. (Original post by thefatone)
so here's my working what have i done wrong and why?
i had the same problem myself

for b)ii) you took it away wrong way -- this is how it is
=[(3/2 x436) + (1/2 x944)]- (388x3)
=-38KJmol-1

i'd love to show you my working but my phone's not working hope this helps

and for c just follow the working in the mark scheme carefully and you'll get it soon
12. (Original post by hideNfreak)
i had the same problem myself

for b)ii) you took it away wrong way -- this is how it is
=[(3/2 x436) + (1/2 x944)]- (388x3)
=-38KJmol-1

i'd love to show you my working but my phone's not working hope this helps

and for c just follow the working in the mark scheme carefully and you'll get it soon
yup thanks

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 26, 2016
The home of Results and Clearing

### 2,088

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. Sheffield Hallam University
Tue, 21 Aug '18
2. Bournemouth University
Wed, 22 Aug '18
3. University of Buckingham