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How do i know if a reaction is endothermic or exothermic?

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How do i know if a reaction is endothermic or exothermic?

Postby VxW » Sun Nov 03, 2013 6:39 pm

Consider the Reaction: 2Hg) (s) --> 2Hg (l) + O2 (g)

Bond:
Hg=O 340kJ/mol
O=O 498 kJ/mol

How do i solve this?

My answer was exothermic but it was wrong. It should be endothermic.

This is how i solved it:

Enthalpy Before: 2(340) = 680
Enthalpy After: 498

Change in Enthalpy: 498 - 680= -182

Therefore, the reaction is exothermic because energy is released from the bond.

Can you please tell me what i did wrong and how to solve it.

In additon, my textbook states that an endothermic reaction is a reaction which requires energy overall and absorbs heat. While exothermic reactions release energy overall and releases heat.

It also states that Catabolic reactions are exothermic reactions while anabolic reactions are endothermic reactions because it requires a net input of energy.

A reaction is exothermic when change in entalphy is less than 0 and endothermic when change in enthalpy is greater than 0.

How come in the reaction H2 + Cl2 --> 2HCl, it requires energy to break the bonds into H H Cl Cl when it is catabolic and releases energy when reforming the bonds to H-Cl and H-Cl when it is anabolic?

Thank You
VxW
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Re: How do i know if a reaction is endothermic or exothermic?

Postby jonmoulton » Mon Nov 04, 2013 4:07 pm

Change in enthalpy = sum of bond energies of reactants - sum of energies of products
= Total input energy – Total output energy

From: Enthalpy Changes in Chemical Reactions
http://faculty.ncc.edu/LinkClick.aspx?f ... tabid=1871

This reverses the sign of the enthalpy.

Enthalpy Before: 2(340) = 680
Enthalpy After: 498

Change in Enthalpy: 680 - 498= 182
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