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Qualification => GCE AS & A2 Level => Queries => Topic started by: thecandydoll on May 29, 2010, 05:00:01 am

Title: Chemistry doubts.
Post by: thecandydoll on May 29, 2010, 05:00:01 am

I need help solving the enthalpy questions.
How do we do it.
Any tips?

Title: Re: Chemistry doubts.
Post by: CHEMMASTER6000 on May 29, 2010, 05:06:57 am

entalphy = heat evolved / moles of reactant

heat evolved =m.c. temp change

m mass of soln total soln

c= specific heat capacity

temp change = well temp change

so devide heat evolved by moles of ractants gives you enaltalphy change

Title: Re: Chemistry doubts.
Post by: thecandydoll on May 29, 2010, 05:11:25 am

I meant chemical energetics.
Like find the enthalpy change of reaction.
THat is enthalpy change of products- reactants.
yoohoo :)

Title: Re: Chemistry doubts.
Post by: destructor on May 29, 2010, 05:23:02 am

so listen
while doing these enthalpy reactions
u wil be given the kj/mol for the formation of two bonds like for example if u want to find the enthalpy for the formation of HCl
it would be like this
H-H =678 kj
Cl-Cl = 343 kj
and  H-Cl = 478kj
then u add H-H and Cl-Cl
and subtract from H-Cl
over here it will be (478-(678+343))
which will result in a -ve value
hence when the value is negative it is an exothermic reaction
dont worry lol
u will be given the table for the energies
so yeah thats basically it!

Title: Re: Chemistry doubts.
Post by: beacon_of_light on June 02, 2010, 06:36:20 am

Are you talking about hess's law??

Title: Re: Chemistry doubts.
Post by: Igcseboy on June 04, 2010, 11:00:42 am

Quote from: thecandydoll on May 29, 2010, 05:00:01 am

I need help solving the enthalpy questions.
How do we do it.
Any tips?

CIE or Edexcel...if edexcel so its in unit 1 which is already finished :S