Join TSR now and get all your revision questions answeredSign up now
    Offline

    3
    ReputationRep:
    Green chemistry bores me so much :boring:
    Offline

    3
    ReputationRep:
    I really don't understand hess's law or born harber cycles too so I would also appreciate any help on this topic thanks
    Offline

    9
    ReputationRep:
    (Original post by SubwayLover1)
    Guys...
    If CCl2 has a bond angle of 118 then why does it change so dramatically when you have two lone pairs... I thought if it was V-shaped then it would be 115
    If it has 2 lone pairs and 2 bonding pairs then it ought to have bond angle of 104.5o and it'll be a bent shape.
    Offline

    9
    ReputationRep:
    (Original post by TeaAndTextbooks)
    I really don't understand hess's law or born harber cycles too so I would also appreciate any help on this topic thanks
    Here's a site that should help you.
    Offline

    2
    ReputationRep:
    (Original post by TeaAndTextbooks)
    I really don't understand hess's law or born harber cycles too so I would also appreciate any help on this topic thanks
    (Original post by TheMadHatteress)
    Hey guys I'm really struggling.
    I think O understand the Hess's cycle but when I'm solving past papers I never seem to solve them. So could someone please explain the hess's cycle? Thanks much appreciated.

    Just to mention, the cycle for Combustion and the route is ALWAYS the same for every combustion-related question. The values just differ from question to question.
    1) Begin by forming an equation to form the products.
    2) Substitute numbers into the cycle and calculate the Sum of Reactants.
    3) As the cycle suggests to form the combustion products you can directly combust the products, we go against this arrow as suggested by the route which needs to be taken - and so, the value of the product changes from NEGATIVE to POSITIVE. Enthalpy change of reactants + products = answer.

    Hope that helped, I'll try and find a question for Enthalpy change of Formation.
    Attached Images
     
    Offline

    3
    ReputationRep:
    (Original post by Disney0702)
    If it has 2 lone pairs and 2 bonding pairs then it ought to have bond angle of 104.5o and it'll be a bent shape.
    I don't understand why it goes from 118 to 104.5 though?
    Offline

    3
    ReputationRep:
    (Original post by S.Ahmad)

    Just to mention, the cycle for Combustion and the route is ALWAYS the same for every combustion-related question. The values just differ from question to question.
    1) Begin by forming an equation to form the products.
    2) Substitute numbers into the cycle and calculate the Sum of Reactants.
    3) As the cycle suggests to form the combustion products you can directly combust the products, we go against this arrow as suggested by the route which needs to be taken - and so, the value of the product changes from NEGATIVE to POSITIVE. Enthalpy change of reactants + products = answer.

    Hope that helped, I'll try and find a question for Enthalpy change of Formation.


    Posted from TSR Mobile

    Thankyou that helps alot I finally get the hess's law stuff, born haber cycles are still killing me though but thanks anyway
    Offline

    2
    ReputationRep:
    (Original post by TeaAndTextbooks)
    Posted from TSR Mobile

    Thankyou that helps alot I finally get the hess's law stuff, born haber cycles are still killing me though but thanks anyway
    What specification are you doing?
    Offline

    3
    ReputationRep:
    (Original post by SubwayLover1)
    I don't understand why it goes from 118 to 104.5 though?
    the lone pair-lone pair repulsion is bigger and greater than the electron-electron bond
    Offline

    3
    ReputationRep:
    (Original post by sara_ara98)
    the lone pair-lone pair repulsion is bigger and greater than the electron-electron bond
    But for each electron pair they are repeled by 2.5 so I dont get why it goes from 118 to 104.5
    Offline

    3
    ReputationRep:
    (Original post by S.Ahmad)
    What specification are you doing?
    Edexcel you?

    Posted from TSR Mobile
    Offline

    3
    ReputationRep:
    (Original post by haemo)
    I don't know if this will help you, but this works for me:

    If given formation data: enthalpy = sum of products - sum of reactants

    If given combustion data: enthalpy = sum of reactants - sum of products


    I know that quite a few students use energy cycles, but I find that the formulas are easier to use.
    This makes life alot easier thank you

    Posted from TSR Mobile
    Offline

    2
    ReputationRep:
    (Original post by TeaAndTextbooks)
    Edexcel you?

    Posted from TSR Mobile
    OCR, can't help you with Haber cycle - sorry! Although, I can help with alternative overlaps.
    Offline

    2
    ReputationRep:
    Can someone help me out here, The spec says this:
    a) Describe the reactions of alkanes in terms of combustion and substitution by chlorine showing the mechanism of free radical substitution in terms of initiation, propagation and termination, and using curly half-arrows in the mechanism to show the formation of free radicals in the initiation step using a single dot to represent the unpaired electron.


    To me this makes no sense, Alkanes don't undergo substitution reaction?? Plus the book has nothing on it, somone help.
    Offline

    2
    ReputationRep:
    (Original post by Rstlss)
    Can someone help me out here, The spec says this:
    a) Describe the reactions of alkanes in terms of combustion and substitution by chlorine showing the mechanism of free radical substitution in terms of initiation, propagation and termination, and using curly half-arrows in the mechanism to show the formation of free radicals in the initiation step using a single dot to represent the unpaired electron.


    To me this makes no sense, Alkanes don't undergo substitution reaction?? Plus the book has nothing on it, somone help.


    Posted from TSR Mobile

    Alkanes do undergo substitution reactions.UV radiation breaks a C-h bond and makes radicals.
    The pics below should help.
    Attached Images
       
    Offline

    2
    ReputationRep:
    (Original post by Kadak)
    Posted from TSR Mobile

    Alkanes do undergo substitution reactions.UV radiation breaks a C-h bond and makes radicals.
    The pics below should help.
    Ahh yes, I jst came here to say I found it, it's jsut me being an iodiot i was busy trying to find it in the big pictures, it's not a picture in my textbook xD
    Offline

    2
    ReputationRep:
    (Original post by Rstlss)
    Can someone help me out here, The spec says this:
    a) Describe the reactions of alkanes in terms of combustion and substitution by chlorine showing the mechanism of free radical substitution in terms of initiation, propagation and termination, and using curly half-arrows in the mechanism to show the formation of free radicals in the initiation step using a single dot to represent the unpaired electron.


    To me this makes no sense, Alkanes don't undergo substitution reaction?? Plus the book has nothing on it, somone help.
    Radical Substitution - Alkane
    Initiation:
    Cl2 -> Cl + Cl
    Propagation:
    CH4 + Cl -> CH3 + HCl
    CH3 + Cl2 -> CH3Cl + Cl
    Termination/Overall:
    Cl + Cl -> Cl2
    CH3 + CH3 -> C2H6
    Cl + CH3 -> CH3Cl
    Offline

    16
    ReputationRep:
    Can anyone summarise mass spectrometry especially the fragment ion part?
    Offline

    3
    ReputationRep:
    (Original post by Kadak)
    Posted from TSR Mobile

    Alkanes do undergo substitution reactions.UV radiation breaks a C-h bond and makes radicals.
    The pics below should help.
    You mean UVbreaks Cl-Cl bond

    Posted from TSR Mobile
    Offline

    3
    ReputationRep:
    (Original post by rm_27)
    Green chemistry bores me so much :boring:
    literally
    why does it exist in the spec
 
 
 
Poll
Which Fantasy Franchise is the best?

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

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