GERMAIN HENRI HESS

Germain Henri Hess was born in Geneva in1802. Three years later he was brought to Russia. He was a Swiss chemist and doctor. He died in 1850 at the age of 48 but managed to make a significant contribution to the world of chemistry before doing so.

Hess’s most famous paper was published in St. Petersburg in 1840 when Hess was 38 years old. To see some of Hess’s original papers visit:

http://dbhs.wvusd.k12.ca.us/Chem-History/Hess-1840.html

This paper was and is still very useful to chemists especially those who work in the field of thermal chemistry.

Hess is most famous for his work on heat summation. He developed his own law which was named after him entitled ‘Hess’s Law’. There are many different wordings of this law but the one that I have found most understandable is:

If a change can be brought about by more than one route,then the overall enthalpy change for each route must be the same-provided that the starting and finishing conditions are the same for each route.

The AQA AS syllabus states that you must know Hess’s Law and be able to use it to perform simple calculations.

To be able to do any calculation using Hess’s Law we must understand it first.

Hess’s Law is used to calculate enthalpy changes in reactions. Hess’s Law can be backed up by the law of conservation of energy as energy cannot be created nor destroyed but simply changed from one form into another. In simpler terms the same reactions have the same energy changes no matter what way they are carried out or how many steps they are carried out in.

D H₁ = D H₂ + D H₃

For example:

•  H₃PO₄ + NaOH ® NaH₂PO₄ + H₂O = STEP 1 (X)

• NaH₂OH + NaOH ® Na₂HPO₄ + H₂O = STEP 2 (Y)

• Na₂HPO₄ + NaOH ® Na₃PO₄ + H₂O = STEP 3 (Z)

• H₃PO₄ + 3NaOH ® Na₃PO₄ + 3H₂O = (A)

Therefore A = X + Y + Z (where A is the total enthalpy change during the reaction.

Return to Home Page.