# enthalpy changes

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#1
help! i can do enthalpy change reaction questions but i do the arrows and box method and never know which way the arrows should go for combustion or formation reactions. can ne body help me please!
0
18 years ago
#2
(Original post by wath)
help! i can do enthalpy change reaction questions but i do the arrows and box method and never know which way the arrows should go for combustion or formation reactions. can ne body help me please!
Give an example question and ill do my best to help.
0
18 years ago
#3
(Original post by wath)
help! i can do enthalpy change reaction questions but i do the arrows and box method and never know which way the arrows should go for combustion or formation reactions. can ne body help me please!
By the box method do you mean hess law? with Hess' triangle?....
If its combustion then the arrow will go from the products/reactacnts to the products of combustion (elements)

If it is formation then the arrow will be in the opposite direction.

When you do the end calculation the enthpy change = enthlpy change of reactants - enthpy change of products
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#4
thanku!
0
18 years ago
#5
(Original post by oanjum)
for combustion, the arrow goes from the product to the elements.
for formation..from elements to product.

eg..

ch4------> c02+2h20 = combustion
c+2h2---->ch4 = formation.

CO2 and H2O aren't elements.
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#6
i think thats just an example of a combustion reaction etc
0
18 years ago
#7
(Original post by wath)
i think thats just an example of a combustion reaction etc
yes?
0
18 years ago
#8
A nice trick I always use is get your arrows the correct way then follow the other root. If you go forwards down one of the arrows, add the enthalpy change on. If you go the wrong way through an arrow, take the enthalpy change off (be careful of enthalpy changes which are negative, like most combustion reactions and some formation reactions)
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