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# enthalpy change? watch

1. I'm a little bit confused as im trying to work out the enthalpy change for the reaction between methane and chlorine free radical -
CH4 + CL. ---> CH3CL + H.

I worked it out to be 74 kjmol-1 but this cant be right
Help!
2. Surely we need to know the mass of water and heat change of the reaction as enthaply change is, q= m x c x delta T
3. what she said ^^
4. which method are you using?
5. (Original post by Vanny17)
Surely we need to know the mass of water and heat change of the reaction as enthaply chnage is, q= m x c x delta T
We dont need to do anything that complicated thank goodnesss! :P
I just did it by working out the bond enthaply of the reactants and products then added them together to find out the energy change ?
6. (Original post by gemstar!)
I'm a little bit confused as im trying to work out the enthalpy change for the reaction between methane and chlorine free radical -
CH4 + CL. ---> CH3CL + H.

I worked it out to be 74 kjmol-1 but this cant be right
Help!
Isn't the original equation supposed to be CH4 + Cl. --> CH3. + HCL?

Anyway if you're using bond enthalpy to work it out then add up all the bonds on the left hand side 4(C-H) and take away all the bonds on the right side (3(C-H) + (H-Cl)) then you should have your answer
7. (Original post by gemstar!)
We dont need to do anything that complicated thank goodnesss! :P
I just did it by working out the bond enthaply of the reactants and products then added them together to find out the energy change ?
. I have never worked out enthalpy change like that though . It would be a negative answer if energy is given out e.g combustion or burning meanwhile heat will be taken in and would therefore be positive for an endothermic reaction e.g photosynthesis.
8. (Original post by tateco)
Isn't the original equation supposed to be CH4 + Cl. --> CH3. + HCL?

Anyway if you're using bond enthalpy to work it out then add up all the bonds on the left hand side 4(C-H) and take away all the bonds on the right side (3(C-H) + (H-Cl)) then you should have your answer
Thats exactly what I'm doing but im ending up with a postive enthalpy change for an exothermic reaction :s
Anyway for this reastion - N2H4 + 02 ----> N2 + 2H20
I got the enthalpy change to be 38 kjmol-1???
Whats the correct answer, i cant see where im going wrong
9. Energy is taken in when bonds are broken and then energy is given out when the bonds are made so:

Enthaply Change = (Bond Enthalpies of Reactants) - (Bond enthalpies of Products)

I know it sounds silly but did you do it the right way round?
10. (Original post by soutioirsim)
Energy is taken in when bonds are broken and then energy is given out when the bonds are made so:

Enthaply Change = (Bond Enthalpies of Reactants) - (Bond enthalpies of Products)

I know it sounds silly but did you do it the right way round?
hmm now its coming out as 4058kjmol-1 which is still positive...grrrr!
11. (Original post by gemstar!)
Thats exactly what I'm doing but im ending up with a postive enthalpy change for an exothermic reaction :s
Anyway for this reastion - N2H4 + 02 ----> N2 + 2H20
I got the enthalpy change to be 38 kjmol-1???
Whats the correct answer, i cant see where im going wrong
Unfortunately I haven't had the time to memorise all the average bond enthalpies so don't know the correct answer, if you're doing it the way I said then it's right
12. (Original post by gemstar!)
hmm now its coming out as 4058kjmol-1 which is still positive...grrrr!
How do you know it's supposed to be exothermic?
13. C-H 412
C-Cl 339

energy change = 412-339 = +73kJ

These values may vary slightly according to source.

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