Equilibrium constant expressions homogeneous equilibria

You have been given the equation for the reaction, which provides the information needed to write the equilibrium constant expression. The equilibrium is homogeneous because the reactants and product are in the same physical state. The form of the equilibrium constant... 

When excess solid is present in a saturated solution, you can write the equilibrium constant expression for the dissolution of the solid in the same way that you wrote the equilibrium constant expression for a homogeneous equilibrium in Chapter 7. For example, the equilibrium constant expression for barium sulfate is written as follows ... 

What may be the equilibrium constant expression for a homogeneous gas reaction whose mole number-time relationship is given in the graph ... 

The mass action law assumes that the reaction medium is homogeneous. In heterogeneous reactions (involving different substances in multiple phases), the densities and effective concentrations of pure condensed phases (liquids or solids) are constant. The concentrations of such species are set to unity in the equilibrium constant expression for such reactions. For example, given the following decomposition,... 

Write equilibrium constant expressions for these homogeneous equilibria. 

The equilibrium constant expression below is the one derived from the guidelines used for homogeneous equilibria. 

Equilibrium constant expression An expression showing the ratio of the concentrations of products to the concentrations of reactants for a reversible reaction at equilibrium. For homogeneous equilibria, the concentrations of all reactants and products are described in moles per liter, and the concentration of each is raised to a power equal to its coefficient in a balanced equation for the reaction. 

The concentrations of pure solids, pure liquids (in heterogeneous equilibria), and solvents (in homogeneous equilibria) do not appear in the equilibrium constant expressions. 

A chemical equilibrium process in which all reactants and products are in the same phase is homogeneous. If the reactants and products are not all in the same phase, the equilibrium is heterogeneous. The concentrations of pure solids, pure liquids, and solvents are constant and do not appear in the equilibrium constant expression of a reaction. 

Expressions for homogeneous equilibria Gaseous hydrogen iodide is produced by the equilibrium reaction of hydrogen gas with iodine. Iodine and some of its compounds have important uses in medicine, as illustrated in Figure 17.7. How would you write the equilibrium constant expression for this reaction in which hydrogen and iodine react to form hydrogen iodide ... 

Compare homogeneous and heterogeneous equilibria. Give a balanced chemical equation and write the corresponding equilibrium constant expression as an example of each of these cases. How does the fact that an equilibrium is heterogeneous influence the expression we write for the equilibrium constant for the reaction ... 

Example 10.1 shows how to write equilibrium constant expressions for homogeneous reactions. 

Explain the important distinctions between each pair of terms (a) reaction that goes to completion and reversible reaction (b) and (c) reaction quotient (Q) and equilibrium constant expression (K) (d) homogeneous and heterogeneous reaction. 

Given measured species concentrations for a homogeneous reaction in a rock, cooling rate at Tae can be found as follows if the equilibrium constant K and the forward reaction rate coefficient k as a function of temperature are known. First, the apparent equilibrium temperature is calculated from the species concentrations. Then kf and kb at Tae are calculated. Then the mean reaction time Xr at Tae is calculated using expressions in Table 2-1. From x, the cooling rate q at Tae can be obtained using Equation 5-125. Two examples are given below. 

Ion-exchange equilibrium can be considered to be analogous to chemical equilibrium. From that point of view, the mass-action law can be used to express the state of equilibrium despite the fact that this law is defined exclusively for homogeneous systems. Derived this way, the so-called pseudo-equilibrium constant Ke is not really a constant, since it depends on the total concentration ... 

Expressions for equilibrium constants for homogenous reactions taking place in solution are readily derived by use of the appropriate expressions of the chemical potentials. When mole fractions are used to express the compositions, no difficulty occurs, and the expression for the equilibrium constant for the generalized change of state vkBk = 0 is... 

The condition of equilibrium is also applicable to changes of state that involve heterogenous reactions, and the same methods used for homogenous reactions to obtain expressions of the equilibrium constant are used for heterogenous reactions. One difference is that in many heterogenous reactions one or more of the substances taking part in the change of state is a pure phase at equilibrium. In such cases the standard state of the substance is chosen as the pure phase at the experimental temperature and pressure. The chemical potential of the pure substance in its standard state still appears in Y.k vkPk but the activity of the substance is unity and its activity does not appear in the expression for the equilibrium constant. 

For practical heterogeneous catalyst kinetics this principle has the following consequence. Usually, the assumption of a homogeneous surface is not valid. It would be more realistic to assume the existence of a certain distribution in the activity of the sites. From the above, certain sites will, however, contribute most to the reaction, since these sites activate the reactants most optimally. This might result in an apparently uniform reaction behaviour, and can explain why Langmuir adsorption often provides a good basis for the reaction rate description. This also implies that adsorption equilibrium constants determined from adsorption experiments can only be used in kinetic expressions when coverage dependence is explicitly included otherwise they have to be extracted from the rate data. 

Both Equations 1.87 and 1.88 describe the selectivity coefficients by polynomial expressions. Equation 1.87 can be tested on ion exchangers mixed in well-defined ratios. In practice, however, we meet the reverse case the selectivity coefficients are known at different surface concentrations, and the ratio of the homogeneous parts and their equilibrium constants should be estimated, the reality of which cannot be tested experimentally. This is the case for the Hogfeldt model. 

Homogeneous Equilibrium. For the change in state in aqueous solution given in (1), the thermodynamic equilibrium constant K, which must be expressed in terms of the activities of the chemical species involved, is given to a good approximation by the analogous expression involving concentrations ... 

By purely thermodynamical methods we can now deduce the following theorem For every chemical reaction in a homogeneous system under constant external conditions e.g. under constant temperature and pressure, or constant temperature and volume) there is a certain function of the concentrations of the components (no matter how these are expressed) which has a definite constant value when equilibrium is established. We may therefore write... 

Constants for homogeneous equilibria How would you write the equilibrium constant expression for this reaction in which hydrogen and iodine... 

This reaction is a homogeneous equilibrium, which means that all the reactants and products are in the same physical state. All participants are gases. To begin writing the equilibrium constant expression, place the product concentration in the numerator and the reactant concentrations in the denominator. 

A eq for this homogeneous equilibrium at 731 K is 49.7. Note that 49.7 has no units. In writing equilibrium constant expressions, it s customary to omit units. Considering the size of K, are there more products than reactants present at equilibrium ... 

For more practice writing homogeneous equilibrium constant expressions, go to Supplemental Practice Problems in Appendix A. 

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