What thermochemistry? It is part of the chemistry which studies the thermal phenomena which accompany the chemical reactions. Certain reactions can release from the Chaleur, they are Exothermique S and others absorb some, they are Endothermique S. Marcellin Berthelot is regarded as the founder of thermochemistry. He created the calorimetric Bombe and took part in the improvement of the Calorimètre. The calorimetric bomb, which is a very great invention, makes it possible to measure the calorific value of a fuel; capacity of a fuel to produce energy. Developed in 1881, it is still of use today.

During the 20th century, thermochemistry developed much. The application of the Second principle of thermodynamics to the chemical systems made it possible to envisage the direction of the reactions, the positioning of the chemical balances and thus to define the output and the composition of the system after reaction.

Definitions

Heat of reaction

During a chemical reaction, the system energy exchange with the external medium in the form of Heat.

This energy exchanged in the form of heat Q depends on the experimental conditions under which the reaction occurs:

• With constant volume (isochoric transformation), thermodynamics shows that Q is equal to the variation of internal energy of the system: Q_v = ΔU (case of the calorimetric bomb).
• With constant pressure , heat is equal to the variation of Enthalpie: Q_p = ΔH (very frequent case of the reactions carried out with the free air).

Free enthalpy

The free Enthalpy, G is the Fonction of essential state for the study of the chemical balances. It indeed makes it possible to determine whether a chemical reaction carried out with T and p constant, is thermodynamically possible and in which direction it will take place.

Operator of Lewis

One calls operator of Lewis , noted $\backslash \; Delta\_\; \{R\}\; ~$, the derivative of a size X , compared to the progress report of the reaction, $\backslash \; xi\; ~$, at temperature T and pressure p constant:

$\backslash \; Delta\_\; \{R\}\; X\_\; \{(T,\; p)\}\; =\; \backslash \; left\; (\backslash \; frac\; \{\backslash \; partial\; X\}\; \{\backslash \; partial\; \backslash \; xi\}\; \backslash \; right)\; \_\; \{(T,\; p)\}~$

Enthalpy of reaction

One calls enthalpy of reaction to T and p constants, the size $\backslash \; Delta\_\; \{R\}\; H\_\; \{(T,\; p)\}\; =\; \backslash \; left\; (\backslash \; frac\; \{\backslash \; partial\; H\}\; \{\backslash \; partial\; \backslash \; xi\}\; \backslash \; right)\; \_\; \{(T,\; p)\}\; =\; \backslash \; sum\_\; \{I\}\; \backslash \; nu\_i.\; h\_\; \{I\; (T,\; p)\}~$.

The variation of enthalpy of the reactional system, for a state of advance given, corresponds to concerned heat and is equal to:

$\backslash \; Delta\; H\; =\; \backslash \; int\_\; \{0\}\; ^\; \{\backslash \; xi\}\; \backslash \; Delta\_\; \{R\}\; H\_\; \{(T,\; p)\}.\; D\; \backslash \; xi\; =\; Q\_\; \{p\}\; ~$

In the case of an ideal system, $\backslash \; Delta\_\; \{R\}\; H\_\; \{(T,\; p)\}~$ is independent of $\backslash \; xi~$.

One from of deduced:

$\backslash \; Delta\; H\; =\; \backslash \; Delta\_\; \{R\}\; H\_\; \{(T,\; p)\}.\; \backslash \; xi\; =\; Q\_\; \{p\}\; ~$

One could consider that enthalpy of reaction $\backslash \; Delta\_\; \{R\}\; H\_\; \{(T,\; p)\}~$ is the " to be able calorifique" reaction, concerned partially or completely according to whether the reaction is partial or total.

If the reaction is carried out in such a way that each component is under the standard pressure, p⁰ , one obtains the standard enthalpy of reaction , $\backslash \; Delta\_\; \{R\}\; H^\; \{0\}\; \_\; \{(T)\}~$. The standard state is defined:

• for a gas: Perfect gas under the pressure of 1 bar,
• for a liquid or solid: 1 bar,
• aqueous solution: concentration being worth a concentration of reference $C^\; \{0\}\; =\; 1\; mol.\; L^\; \{-\; 1\}$.

$\backslash \; Delta\_\; \{R\}\; H^\; \{0\}\; \_\; \{(T)\}~$ is function only of T and can be obtained starting from the thermodynamic tables drawn up at the temperature of reference of 298 K.

One has according to the law of Kirchoff:

$\backslash \; Delta\_\; \{R\}\; H^\; \{0\}\; \_\; \{(T)\}\; =\; \backslash \; Delta\_\; \{R\}\; H^\; \{0\}\; \_\; \{(298)\}\; +\; \backslash \; int\_\; \{298\}\; ^\; \{T\}\; \backslash \; sum\_\; \{I\}\; \backslash \; nu\_\; \{I\}.\; C\_\; \{p,\; I\}.\; dT\; ~$

$\backslash \; sum\_\; \{I\}\; \backslash \; nu\_i.\; C\_\; \{p,\; I\}\; =\; \backslash \; Delta\_r\; C\_p~$

$C\_\; \{p,\; I\}\; ~$, is the molar heat-storage capacity with constant pressure of the component $i~$. It is function of T but one can regard it as constant if the interval of T is not too large. In this case one obtains the approximate relation:

$\backslash \; Delta\_\; \{R\}\; H^\; \{0\}\; \_\; \{(T)\}\; =\; \backslash \; Delta\_\; \{R\}\; H^\; \{0\}\; \_\; \{(298)\}\; +\; \backslash \; Delta\_r\; C\_\; \{p\}.\; (T\; -\; 298)\; ~$

Attention: This relation is valid only if there is no Changement of state or of phase in the interval of temperature considered. In the contrary case, it is necessary to carry out a cycle thanks to the Loi of Hess to take them into account.

The calculation of $\backslash \; Delta\_\; \{R\}\; H^\; \{0\}\; \_\; \{298\}\; ~$ is carried out using the standard Enthalpie of formation, $\backslash \; Delta\; H^\; \{0\}\; \_\; \{F,\; 298\}\; ~$ whose values are tabulées for the majority of the compounds.

Sizes standards of reaction

They are sizes associated with the writing with the equation-assessment. They relate to primarily the Enthalpie, the Entropie and the free Enthalpie.

In conditions standards, the sizes of reactions obey the law of Hess which stipulates that:

$\backslash \; Delta\_r\; X^0\; =\; \backslash \; sum\_i\; \backslash \; nu\_i\; \backslash \; Delta\; X^0\_\; \{F,\; I\}\; ~$

where $X\_i~$ is an extensive size, $\backslash \; Delta\; X^0\_\; \{F,\; I\}\; ~$ is a standard size of formation.

by convention,

for the reagents and $\backslash \; nu\_i\; >\; 0~$ for the products (this convention is imposed by the definition even Avancement of reaction $\backslash \; xi\; =\; \backslash \; frac\; \{\backslash \; Delta\; n\_i\}\; \{\backslash \; nu\_i\}$).

Standard entropy of reaction: $\backslash \; Delta\_r\; S^0\_\; \{(T)\}~$

The third principle of thermodynamics stipulates that the entropy of a pure substance is null to 0 K. Thus one can calculate the molar entropy of a pure substance absolutely (see Calculs of the entropy of a pure substance. It is thus not necessary to define a standard entropy of formation. One drew up molar tables of entropies standards to 298 K for the majority of the pure substances, which make it possible to calculate the entropies standards of reaction:

$\backslash \; Delta\_r\; S^0\_\; \{298\}\; =\; \backslash \; sum\_i\; \backslash \; nu\_i.\; S^0\_\; \{I\; (298)\}~$

It is then possible to calculate $\backslash \; Delta\_r\; S^0\_\; \{(T)\}~$, thanks to the relation of Kirchoff applied to the entropy.

Remarque

the sign of the entropy of reaction is often foreseeable, since the entropy perhaps considered as a measurement of the order (or disorder). The more important the disorder is, the more the entropy is large, from where the classification .

Standard free enthalpy of reaction: $\backslash \; Delta\_r\; G^0\_\; \{(T)\}~$

It can be calculated thanks to the law of Hess, starting from the free enthali standards of formation, $\backslash \; Delta\; G^0\_\; \{F\; (T)\}~$ whose numerical values are tabulées to 298 K.

$\backslash \; Delta\_r\; G^0\_\; \{298\}\; =\; \backslash \; sum\_i\; \backslash \; nu\_i\; \backslash \; Delta\; G^0\_\; \{F,\; I\; (298)\}~$.

It is also possible to calculate the free enthalpy of standard reaction by the relation resulting directly from the definition of the free function enthalpy G = H - TS :

$\backslash \; Delta\_r\; G^0\_\; \{(T)\}\; =\; \backslash \; Delta\_r\; H^0\_\; \{(T)\}\; -\; T\; \backslash \; Delta\_r\; S^0\_\; \{(T)\}~$

The calculation of the standard free enthalpy of reaction revêt an major importance for the study of the chemical balances since the knowledge of this size makes it possible to have access to the constant of balance.

$\backslash \; Delta\_r\; G^0\_\; \{(T)\}\; =\; -\; RT\; \backslash \; ln\; K\_\; \{(T)\}~$

From where

$K\_\; \{(T)\}\; =\; exp\; \backslash \; left\; (\backslash \; frac\; \{-\; \backslash \; Delta\_r\; G^0\_\; \{(T)\}\}\; \{R.T\}\; \backslash \; right)\; ~$

See

Law of action of masses

chemical Balance

Random links:

Antonello Gagini | Felix de Mérode | Simon Grynaeus | Zsuzsa Körmöczy | Acts of Pierre