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Page name: Hess's Law [Exported view] [RSS]

2007-06-18 17:59:33

Last author: Firenze
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Hess's Law

Hess's law states that the enthalpy change in converting reactants into products is the same regardless of the route taken. So the enthalpy change for two routes from the same reactants to the same products will be equal.

Example

<img:http://elftown.eu/img/drawing/46376_1127161540.jpg>

<img:http://elftown.eu/img/drawing/46376_1127161540.jpg>

If all but one of the above enthalpy changes are known then the remaining enthalpy change can be calculated.

<img:stuff/ETA_divider.jpg>

Example

The formation of ethane can be represented by the following equation:

<img:http://elftown.eu/img/drawing/46376_1127161846.jpg>

Calculate the enthalpy change for the reaction using the enthalpies of combustion of carbon, hydrogen and ethane given in the SQA data book.

<img:http://elftown.eu/img/drawing/46376_1127162070.jpg>

<img:http://elftown.eu/img/drawing/46376_1127162070.jpg>

<img:http://elftown.eu/img/drawing/46376_1127162369.jpg>

= 2 × enthalpy of combustion of carbon (2 × -394).

<img:http://elftown.eu/img/drawing/46376_1127162521.jpg>

= 3 × enthalpy of combustion of hydrogen (3 × -286).

<img:http://elftown.eu/img/drawing/46376_1127162647.jpg>

= reverse of enthalpy of combustion of ethane. The enthalpy of combustion of ethane has a negative value so the reverse will have a positive value (+1560).

<img:stuff/ETA_divider.jpg>

Using Hess's Law:

<img:http://elftown.eu/img/drawing/46376_1127162830.jpg>

<img:http://elftown.eu/img/drawing/46376_1127162982.jpg>

~Chemistry~

2005-09-28 [knight flyer]: Cool this is really helpful

2005-09-28 [Firenze]: Thank you I spent a while making this page

2005-11-28 [japegrin]: how does one go about becoming a student?

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