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April 23, 2026, 06:13:36 pm
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Author Topic: enthalpy  (Read 748 times)  Share 

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jasonn93

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enthalpy
« on: November 09, 2010, 03:33:44 pm »
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TSSM 2010:
Question 15
When 0.10 g of magnesium is burnt in oxygen, 540 joule of energy is released. The closest
ΔH value for the combustion of magnesium will be, in kJ mol-1
A. 540
B. -130
C. -260
D. -540

Can't the answer be B as well, if you used the equation Mg + 1/2 O2 -> MgO ?
Or do we assume whole numbers when dealing with these equations? Or only assume in MC?   :buck2:
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Martoman

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Re: enthalpy
« Reply #1 on: November 09, 2010, 06:43:32 pm »
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Strictly yes, its TSSM so they don't think of these things. They will usually stipulate it in the question otherwise ambiguities like this can arise.
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John President

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Re: enthalpy
« Reply #2 on: November 09, 2010, 06:49:29 pm »
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Yes, always work with whole numbers with these equations. Very easy trap to fall into on MC questions.

Hence in this case since the equation is 2Mg + O2 -> 2MgO, the answer is C (-260).

If the coefficient is 2 for the desired reactant , then the ΔH value represents the energy released from 2 moles of the reactant, even though ΔH is measured in kJ/mol (usually)



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John President

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Re: enthalpy
« Reply #3 on: November 09, 2010, 06:50:34 pm »
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Quote from: Martoman on November 09, 2010, 06:43:32 pm
Strictly yes, its TSSM so they don't think of these things. They will usually stipulate it in the question otherwise ambiguities like this can arise.
And yes, TSSM are shocking. Last TSSM exam I did (for Methods) had a continuous probability with an area of 1/4 under the curve instead of 1.
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The Finnish word pilkunnussija, which translates literally to “comma fucker,” is someone who risks his social wellbeing to annoyingly and persistently point out punctuation errors.

Completed VCE in 2010 - Now doing BSc at UoM

Martoman

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Re: enthalpy
« Reply #4 on: November 09, 2010, 06:50:49 pm »
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this confused me so much first time around >.<
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jasonn93

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Re: enthalpy
« Reply #5 on: November 09, 2010, 07:11:13 pm »
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fair enough, thanks guys! hopefully VCAA don't pull a shifty like this on us LOL :D
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