Heterogeneous reactions are called, in which the components are in different phases. Natural processes in most cases heterogeneous. For heterogeneous processes, the condition of equilibrium (2.70):

saved. Chemical potentials of substances are expressed by the same equations [(2.77) - (2.81)]. Chemical potentials of pure substances are equal to the standard chemical potentials. The activity of pure solids and liquids is unity.

In the expression of law of mass action in this case includes the activity of substances in condensed phases and volatility (or pressure) components in the gas phase.

Let's consider some examples.

Example 1. Chemical equilibrium heterogeneous reactions involving two monooksidom lead - red and yellow varieties in 298 K and P = 1 ATM:

PbO_top "PbO_W (3.37)

The equilibrium constant of this reaction by mass action law:

(3.38)

As and_TV = 1, the two pure solids will be in equilibrium, when

DG_{T,response, which will.} = DG⁰_T = 0.

For this reaction the change in Gibbs energy can be calculated on the basis of data on the Gibbs energies for the processes of formation of substances, ΔG_f :

DG⁰_T = DG_{f, PbOж} -DG_{f, PbOкр} = -188,5 -(-189,3) = +0,8 kJ/mol

A positive value of the Gibbs energy of reaction suggests, it turns out that more sustainable phase PbO_red.

This method is useful for determining the relative sustainability of any polymorphic substances under standard conditions.

Example 2. If the reaction involves gaseous substances, the equilibrium can be achieved by changing the partial pressure of this gas. As an example, consider the equilibrium in the earth's surface of iron oxide FeO in relation to the siderite FeCO₃:

FeO_(t) + WITH_2(g) FeCO_3(t) (3.39)

The equilibrium constant of this reaction:

(3.40)

Since the activity of solids in the total pressure 1 atmospheres equal to one, then this expression simplifies:

(3.41)

Thus, the reaction is an equilibrium only if a certain partial pressure P_WITH2. On the reference data, standard Isobaric capacity reactions

Then from the equation of standard isotherms (3.15) equilibrium ATM. Full isotherms of the equation of this reaction:

(3.42)

Since the partial pressure₂ in the atmosphere, , approximately 10^-3,5 ATM, from the analysis of the equation (3.42) should, in terms of the earth's surface, the compound FeO is unstable relative to FeCO₃ and the balance of the reaction (3.39) offset to the right.

Example 3. Contact solid precipitate VASO₃ natural waters, containing ions SO₄^2-and WITH₃^2- , a transition according to the equation:

VASO_3(t)+ SO₄^2-_(aq) BaSO_4(t) + CO₃^2-_(aq) (3.43)

Standard Isobaric capacity reactions DG⁰₂₉₈=-6,12 kJ/mol. The equilibrium constant, expressed through the activity of substances:

(3.44)

Unnecessarily. the activity of pure solids and_ВаЅО4(t)=1 and and_ВаСО3(t)=1, it

(3.45)

According to the isotherm equation of Van ' t Hoff:

DG=DG⁰+ RT ln , (3.46)

where

DG⁰= - RT lnK_a = - 6120 J/mol (3.47)

From (3.47) it is possible to calculate the equilibrium constant of a chemical reaction under standard conditions To_a =11.8 and substitute the value found into the equation isotherms:

DG=RT ln — RT ln11,8 (3.48)

where — non-equilibrium ion activity in water.

If their attitude in the water will be less than the equilibrium:

<11,8 , the DG of the reaction, in accordance with the equation (3.48), will be negative, ie. the balance of the reaction (3.43) moves to the right, there is the precipitation of barium sulfate. This simple analysis explains the reason of the crystallization of barium sulphate in the natural environment, even from carbonate solutions, having a significant ion concentration and the low maintenance .