Hessian heat law

Born-Haber cycle for carbon dioxide.

The Hessian heat set (also set by Hess ) is used for calculation of enthalpy changes in chemical reactions . It was erected in 1840 by Germain Henri Hess .

In many cases, the products of a chemical reaction can arise in several ways. According to the law of conservation of energy , the reaction path has no influence on the energy that is absorbed or given off during a reaction. According to Hess's law of heat, the same applies to the enthalpy of a chemical reaction.

statement

The enthalpy change of an overall process is the sum of the enthalpy changes of the individual process steps. Assuming standard conditions, the standard enthalpy of reaction of a substance is the difference between the standard enthalpy of formation of the products minus the standard enthalpy of formation of the educts: ${\displaystyle \Delta H}$

${\displaystyle \Delta _{r}H^{0}=\underbrace {\sum \Delta _{f}H^{0}} _{\text{Produkte}}-\underbrace {\sum \Delta _{f}H^{0}} _{\text{Edukte (Reaktanten)}}}$

It follows from this that the enthalpy of reaction does not depend on the reaction path, but only on the initial and final state of the system.

Extensions

One can extend Hess's theorem to include the Gibbs energy :

${\displaystyle \Delta _{r}G^{0}=\underbrace {\sum \Delta _{f}G^{0}} _{\text{Produkte}}-\underbrace {\sum \Delta _{f}G^{0}} _{\text{Edukte (Reaktanten)}}}$

Furthermore, the change in entropy can also be calculated using Hess's theorem. Note that entropy has the unit Joule per Kelvin and is an absolute quantity (no delta in the sum!):

${\displaystyle \Delta _{r}S^{0}=\underbrace {\sum S_{r}^{0}} _{\text{Produkte}}-\underbrace {\sum S_{r}^{0}} _{\text{Edukte (Reaktanten)}}}$

example

Graphite can be burned directly to carbon dioxide (1) or indirectly via the intermediate carbon monoxide (2), (3):

${\displaystyle {\begin{matrix}(1)\;&\mathrm {C(Graphit)} &+&\mathrm {O_{2}(g)} &\rightarrow &\mathrm {CO_{2}(g)} &\;\;\Delta _{r}H_{1}=-393\,\mathrm {kJ\cdot mol^{-1}} \;\;\\\\(2)\;&\mathrm {C(Graphit)} &+&{\frac {1}{2}}\mathrm {O_{2}(g)} &\rightarrow &\mathrm {CO(g)} &\;\;\Delta _{r}H_{2}=-111\,\mathrm {kJ\cdot mol^{-1}} \\\\(3)&\mathrm {CO(g)} &+&{\frac {1}{2}}\mathrm {O_{2}(g)} &\rightarrow &\mathrm {CO_{2}(g)} &\;\;\Delta _{r}H_{3}=-282\,\mathrm {kJ\cdot mol^{-1}} \end{matrix}}}$

The total enthalpy of reaction is the same in both cases: ${\displaystyle \Delta _{r}H}$

${\displaystyle {\begin{aligned}\Delta _{r}H_{1}&=\Delta _{r}H_{2}+\Delta _{r}H_{3}\\&=-111\,\mathrm {kJ\cdot mol^{-1}} +(-282\,\mathrm {kJ\cdot mol^{-1}} )=-393\,\mathrm {kJ\cdot mol^{-1}} \end{aligned}}}$

literature

• Chemistry today, upper secondary level, Schroedel Verlag, 1998, ISBN 3-507-10630-2 .
• Silberberg: Chemistry, The Molecular Nature of Matter and Change. Fourth Edition. Mc Graw Hill, 2006, ISBN 0-07-111658-3 .
• Focus on chemistry, secondary level II. Cornelsen Verlag, 2018, ISBN 978-3-06-015656-6 .

Web links

Commons : Hess cycles  - collection of images, videos and audio files

Video: HESS theorem and thermochemistry - How much heat is released during chemical reactions? . Jakob Günter Lauth (SciFox) 2013, made available by the Technical Information Library (TIB), doi : 10.5446 / 15661 .