Thursday, May 19, 2011

Factors Affecting Enthalpy of Reaction

The various factors on which enthalpy of reaction depend are as follows:
1.            Physical State of Reactants and Products
                The enthalpy of reaction changes with change in physical state. This is because as the physical state changes, extent heat is evolved.
                Example:            
                                H2 (g)  +  1/2 O2 (g)  ------------>   H2O (l)  ;   DH   =   – 286 KJ.
                                H2 (g)  +  1/2 O2 (g) ------------>    H2O (g)  ;  DH  =  – 242 KJ.
2.            Quantity of Reactants                  
                                The change in enthalpy of reaction depends upon the quantity of reactants used. When the number of moles of reactants are doubled, the enthalpy change also becomes double.
3.            Allotropic Modification
For elements existing in different allotropic modifications, the heat of reaction is different if different allotropic form is involved in reaction.
                Example:            
C (graphite)   +    O2 (g)  ------------>    CO2 (g)   ;     DH  =  – 393.4 KJ
C (diamond)   +   O2 (g)  ------------>   CO2 (g)   ;    DH  =   – 395.4 KJ.
4.            Temperature and Pressure         
                                The enthalpy of reaction depends upon the temperature and pressure of reaction. Therefore, the values are generally expressed under standard conditions of temperature (298K) and pressure    (1 atm.)

38 comments:

  1. yeah, yeah, thanks t'was a tiny wee lil' bit helpful.

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  2. dude, "helpful"? are'nt you being a bit too modest? this crap is lame!

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    Replies
    1. aj ajajajajajajajajajajajajajajajajajajajajajajajajajajajajkaja

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  3. This was AMAZING! Thank you!!! :D

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  4. but in a question it says "In an experiment to measure the enthalpy change of a reaction involving gases, which of the following conditions must always be kept constant?"
    the answer is pressure.. NOT pressure and temperature. can you please explain why?
    ref.. edexcel a level chemistry question paper Jan 2013 Unit 1

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  5. THIS IS SO HELPFUL

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  6. How do you know that H is a function of pressure and temperature only ?! Why doesn't enthalpy depend on like, volume ?....

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  7. james o'grady

    fat piece of shit

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    Replies
    1. chip de chip cant talk cant mention my name in war why coz the sweet chilli riddim was poor

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  8. Tom judd, is it mad?

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  9. mad ting sad ting

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  10. everyone rush aj. but dont tell him

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  11. james ate tom judds poo

    ReplyDelete
    Replies
    1. i ate ur mom out as well

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  12. fwedy main is it a mad ting?

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  13. James sweats cooking oil

    ReplyDelete
  14. Replies
    1. i have a black ski mask

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    2. but i dont ski

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  15. i hate white skinheads

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  16. i love aj and his sillytoe

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    Replies
    1. i;love toms prickly head

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  17. i wish i could suck jmes dick

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  18. james why are your tits so sweaty?

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  19. Great answer.
    just know that you have helped one desperate student tonight.
    All the best.

    ReplyDelete