# AS Chemistry help- Enthalpy changes

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Thread starter 17 years ago
#1
Enthalpy change always rfefers to the system, but you take measurements of surroundings right?

but in q=mc(delta)T

is the m, mass, is the mass of all the reactants including water/acid etc and the solids or is it just the liquid?

for example:
A pellet of potassium hydroxide, KOH, of mass 0.166g, was added to 50g of water in an insulated container. temp of water rose from 19.4 to 20.2 degrees C.

specific heat capacity of water= 4.2 J/g/K

calculate heatchange q of water..
so is mass= 0.166g+50g or just 50g??
---------------

also can you reply with answers for a few of these please, i'm not sure about them

write down equations that correspond to the enthalpy change of formaion of eahc of the following compounds:
a) H2SO4
b) KMnO4
c) SO2
d) (NH4)2SO4
e) copper (II) sulphate
f) sodium nitrate

heh just that this thermochemistry is a "selflearning module" and i havent been taught much by our teacher.
0
17 years ago
#2
I think it's meant to be the sum of the masses of the reactancts technically, but in this case as the mass of solid is 0.166g it can be ignored.
0
17 years ago
#3
(Original post by sleepingbeauty)

write down equations that correspond to the enthalpy change of formaion of eahc of the following compounds:
a) H2SO4
b) KMnO4
c) SO2
d) (NH4)2SO4
e) copper (II) sulphate
f) sodium nitrate

heh just that this thermochemistry is a "selflearning module" and i havent been taught much by our teacher.
A: H2+S+2O2 ( I think)

B:K+Mn+2O2

etc.
possibly including standard starte symbols i.e. O2 (g)
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17 years ago
#4
When we were taught this we were told just to ignore the mass of the chemical you added and just use the mass of the water the reaction is taking place in.
As for those questions, for enthalpy of formation, you simly take the constituent elements from the compound and seperate them out, bear in mind some are molecules such as H2 and O2, however most are single atoms such as C. Bear in mind these are theoretical reactions and dont actually happen often in real life.
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