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enthaply change

calculate the enthaply change given the values of enthaply of formation
ZnCO3 -812
CO2 -393
ZnO -348
ZnCO3 (s) -> ZnO (s) + CO2(g)
would the answer be -71 kjmol-1?
Reply 1
Avatar for Woody.
Woody.
12
Draw a Hess' diagram and notice where the arrows go

My answer:

Spoiler

Reply 2
Avatar for FethR.B.Lue
FethR.B.Lue
negative ?

Mine is positive...= =...
Reply 3
Avatar for hot angel 02
hot angel 02
OP
Im probably wrong :smile:
What did you do to get yor answer Feth?
Reply 4
Avatar for shengoc
shengoc
12
hot angel 02
calculate the enthaply change given the values of enthaply of formation
ZnCO3 -812
CO2 -393
ZnO -348
ZnCO3 (s) -> ZnO (s) + CO2(g)
would the answer be -71 kjmol-1?


enthalpy change = enthalpy formation of products - enthalpy formation of reactants

Remember to multiply by the stoichiometric coefficient as it appears in you equation, also if that value is for 2ZnCO3 for example, your final answer should be divided by 2 to give you units of energy/mol

In this case, enthalpy change = 348(1) + (-393)(1) - (-812)(1) = ?
Reply 5
Avatar for Kyle_S-C
Kyle_S-C
12
Think of it like this, these are the definitions of the standard formation for each of those compounds:

1) Zn(s) + C(s) + 3O(g) --> ZnCO3(s) then ΔHf=−812kJ mol−1\Delta H_f = -812 \text{kJ mol}^{-1}

2) Zn(s) + O(g) --> ZnO(s) then ΔHf=−348kJ mol−1\Delta H_f = -348 \text{kJ mol}^{-1}

3) C(s) + 2O(g) --> CO2(g) then ΔHf=−393kJ mol−1\Delta H_f = -393 \text{kJ mol}^{-1}

What you're doing is essentially this:

ZnCO3(s) --> Zn(s) + C(s) + 3O(g) --> ZnO(s) + CO2(g)

Think about which way around you have to do each of those reactions (1-3) to get from the start to the finish. If you reverse the direction of a reaction, reverse the sign of the enthalpy. You should end up with a positive number (maybe think about how the bonding has changed). This reaction is entropically driven by the CO2(g) coming off at the end.

N.B. If you actually wanted to do the reactions (1-3) you'd have to go through a number of different reactions...there's no single reaction that will do any of those, they're just representative.
Reply 6
Avatar for Woody.
Woody.
12
My method:

Spoiler

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