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AS Chemistry Enthalpy

Just wondering if anyone could help me with the first question here?

I'm guessing it's something to do with how some reactions are endothermic and some are exothermic and so the energy produced in one is used to break the bonds from another?

Or something along those lines I just need a little clarifying, thanks in advanceeImageUploadedByStudent Room1392918550.730051.jpg


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Reply 1
Avatar for Joan5626
Joan5626
When the Enthalpy change(delta H) is negative the reaction is exothermic (i.e. more energy is released in forming new bonds than is absorbed when breaking old bonds) and when it is positive, the reaction is endothermic (i.e. more energy is absorbed in breaking old bonds than is released in forming new bonds).

Hope I helped in some way!
Original post by Joan5626
When the Enthalpy change(delta H) is negative the reaction is exothermic (i.e. more energy is released in forming new bonds than is absorbed when breaking old bonds) and when it is positive, the reaction is endothermic (i.e. more energy is absorbed in breaking old bonds than is released in forming new bonds).

Hope I helped in some way!


Yes thanks!


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In an exothermic reaction will the temperature of the surroundings will increase?
Reply 4
Avatar for Joan5626
Joan5626
Original post by Rayabelle
In an exothermic reaction will the temperature of the surroundings will increase?

Yes and so it will get hotter. And in an endothermic reaction, the temperature will decrease, so it gets colder.
Reply 5
Avatar for DashStrike
DashStrike
The enthalpy change indicates if energy is released by the forward reaction (exothermic, indicated by a negative value) or if energy is need in order for the forward reaction to take place (endothermic, indicated by a positive value). These values can be calculated by setting up a Hess equation: formation equation, combustion equation. Hess's law states that ''The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs.''

This link explain very well the two types of reactions based on Hess's Law: http://www.chemguide.co.uk/physical/energetics/sums.html

Combustion looks at breaking bonds, formation at forming them.

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