Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    0
    ReputationRep:
    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!
    Offline

    19
    ReputationRep:
    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
    Offline

    1
    ReputationRep:
    what she said ^^
    Offline

    2
    ReputationRep:
    which method are you using?
    also I don't think you're far off with your answer.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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 ?
    I just assumed it had to be a negetive answer..
    • PS Helper
    Offline

    14
    ReputationRep:
    PS Helper
    (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
    Offline

    19
    ReputationRep:
    (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 just assumed it had to be a negetive answer..
    . 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.
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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
    Offline

    12
    ReputationRep:
    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? :p:
    • Thread Starter
    Offline

    0
    ReputationRep:
    (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? :p:
    hmm now its coming out as 4058kjmol-1 which is still positive...grrrr!
    • PS Helper
    Offline

    14
    ReputationRep:
    PS Helper
    (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
    Offline

    12
    ReputationRep:
    (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?
    • Community Assistant
    • Study Helper
    Offline

    14
    ReputationRep:
    Community Assistant
    Study Helper
    C-H 412
    C-Cl 339

    energy change = 412-339 = +73kJ

    These values may vary slightly according to source.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Has a teacher ever helped you cheat?
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Write a reply...
    Reply
    Hide
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.