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# Enthalpy Change... how do I know whether my final value is + or -? watch

1. Hey guys,

This will probably turn out to be a really stupid question, but I'm currently doing some revision on enthalpy change and am completing some Chemsheet questions

E.g.
1. In an experiment, 0.600 g of propane (C3H8) was completely burned in air. The heat evolved raised the temperature of 100 g of water by 64.9°C. Use this data to calculate the enthalpy of combustion of propane (the specific heat capacity of water is 4.18 J g-1 K-1).

2. .0 cm3 of 1.00 mol dm-3 hydrochloric acid was added to 50.0 cm3 of 1.00 mol dm-3 sodium hydroxide solution. The temperature rose by 6.8°C. Calculate the enthalpy of neutralisation for this reaction. Assume that the density of the solution is 1.00 g cm-3, the specific heat capacity of the solution is 4.18 J g-1 K-1.

I do the whole q = mc△T method, divide my number of moles etc and got the right answer on both
However, I completely missed the -△H value on both questions
I understand the basis of if a reaction is exothermic, there is a -△H value and if it is endothermic, a +△H value, but just cannot for the life of me figure out how the - sign came into play, using the information given in the question alone

I'm probably just being stupid, but would appreciate some kind of explanation if possible!

Cheers
2. Hey, so from the question you know that the temperature of the water increases after the reaction takes place. This means that the reaction must have gave out energy to raise up the temperature so the reaction is Exothermic as exothermic reactions give out heat hence the - sign.Same thing with the second question: temperature rose = energy given out = exothermic reaction = - sign
3. (Original post by Audrius,C)
Hey, so from the question you know that the temperature of the water increases after the reaction takes place. This means that the reaction must have gave out energy to raise up the temperature so the reaction is Exothermic as exothermic reactions give out heat hence the - sign.Same thing with the second question: temperature rose = energy given out = exothermic reaction = - sign
Thanks mate

It's hella obvious once you point it out like that- I think I've been revising for too many hours now lol

Clearly losing my mind

Thanks again!
4. (Original post by Mooodle)
Thanks mate

It's hella obvious once you point it out like that- I think I've been revising for too many hours now lol

Clearly losing my mind

Thanks again!
No problem dude. Take it easy

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Updated: February 19, 2018
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