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Hess’s Law

Hess’s Law. By: Jordan Gower and Kylie Truong 4 th period 12/6/13. What is it?. Definition: Law which states that the energy change in an overall chemical reaction is equal to the sum of the energy changes in the individual reactions comprising it. A Little background.

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Hess’s Law

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  1. Hess’s Law By: Jordan Gower and Kylie Truong 4th period 12/6/13

  2. What is it? • Definition: Law which states that the energy change in an overall chemical reaction is equal to the sum of the energy changes in the individual reactions comprising it.

  3. A Little background • This law was developed by Russian chemist Germain Henri Hess (1802 - 1850) who was important primarily for his thermochemical studies. I’m Russian

  4. Confusing, right? Understanding Hess's law • If you add two or more thermochemical equations, then you can also add the heats of reaction to give the final heat of the reaction • It allows you to determine the heat of the reaction indirectly • Use Hess’s Law to find the enthalpy change for the conversion of one chemical into another Enthalpy Change is the change in heat content of a system at constant pressure. It’s symbolized by ∆H. Heat and enthalpy change are the SAME THING.

  5. Addin’ em all up • Hess’s Law is the overall enthalpy change in a reaction and is equal to the sum of the enthalpy changes for the individual steps in the conversion of one chemical into another Initial Stage 4 ∆H4 Final Stage 2 ∆H2 Stage 1 ∆H Stage 3 ∆H3 See how there are different stages from start to finish throughout the conversion? You ad these up to get the overall enthalpy change ∆H1+∆H2+∆H3+∆H4…=∆Htotal

  6. Time to Practice!  Remember: Whatever you do to your equation, do to your ∆H!

  7. Sources • http://www.science.uwaterloo.ca/~cchieh/cact/c120/hess.html • http://chemwiki.ucdavis.edu/Physical_Chemistry/Thermodynamics/Thermodynamisc_Cycles/Hess'_Law • http://www.chemguide.co.uk/physical/energetics/sums.html • Our Chemistry Book 

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