Given the thermochemical equations: #i.# #PrarrR# #;DeltaH=X# #ii.# #RrarrS# #;DeltaH=Y# #iii.# #1/2SrarrQ# #;DeltaH=Z# How do we assess the reaction....#Prarr2Q# #DeltaH_"rxn"^@=??#

1 Answer
Nov 8, 2017

The solution relies on #"Hess's law"# of heat summation....

Explanation:

We want #Prarr2Q# #;DeltaH_"rxn"^@=?#

We got....

#i.# #PrarrR# #;DeltaH=X#

#ii.# #RrarrS# #;DeltaH=Y#

#iii.# #1/2SrarrQ# #;DeltaH=Z#

And so we take #2xx(iii)+1xx(ii)+(i)#:

#P+ cancelS+cancelRrarr2Q+cancelS+cancelR#

And the enthalpies change sums up in the same way...

...we take #2Z+Y+X=DeltaH#