Clarification on Calculation Methods

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Linh Vo 2J
Posts: 61
Joined: Sat Apr 28, 2018 3:00 am

Clarification on Calculation Methods

Postby Linh Vo 2J » Mon Jan 28, 2019 11:35 am

Can someone explain to me the three methods and when to use which?

Kirsty Star 2H
Posts: 48
Joined: Fri Sep 28, 2018 12:24 am

Re: Clarification on Calculation Methods

Postby Kirsty Star 2H » Mon Jan 28, 2019 12:12 pm

The first method is Hess's Law, which you use when you know the deltaH of two different reactions and are able to combine them to find the deltaH of the new reaction. The second method uses bond enthalpies, since in chemical reactions, bonds in reactants and broken and bonds in products are formed. The bond enthalpies for certain elements are known average values that you can add and subtract to find the overall deltaH. The last method uses standard enthalpies of formation, which is the standard deltaH of 1 mol of a substance that is formed from its elements in their most stable form. The last method shows that the sum of all the standard enthalpies of formation of the products minus the sum of all the standard enthalpies of formation of the reactants gives the overall standard enthalpy of the reaction.


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