Equilibrium expression liquids

Water is considered like any other pure liquid its concentration does not alter significantly in dilute solutions, so it does not figure in the equilibrium expression ... 

The reactant quotient can be written at any point during the reaction, but the most useful point is when the reaction has reached equilibrium. At equilibrium, the reaction quotient becomes the equilibrium constant, IQ (or Kp if gas pressures are being used). Usually this equilibrium constant is expressed simply as a number without units, since it is a ratio of concentrations or pressures. In addition, the concentrations of solids or pure liquids (not in solution) that appear in the equilibrium expression are assumed to be 1, since their concentrations do not change. 

Equation 27 is similar to the solid-liquid equilibrium relation used for non-electrolytes. As in the case of the vapor-liquid equilibrium relation for HC1, the solid-liquid equilibrium expression for NaCl is simple since the electrolyte is treated thermodynamically the same in both phases. 

This is also true for pure liquids their equilibrium concentration is also given the value 1 in the equilibrium expression. It is not true for aqueous solutions. 

The solubility of the gas is needed so that CAI the concentration of the reactant A at the interface, can be calculated. The solubility is often expressed through the Henry Law constant M, this is defined by PAe = 3KCA, where PAe is the partial pressure of the gas A at equilibrium with liquid in which the concentration of the dissolved gas is CA. This immediately raises a problem for many systems how can the solubility, which requires gas and liquid to be at equilibrium, be determined when the gas reacts with the liquid The answer is by one of several methods. 

The LCM is a semi-theoretical model with a minimum number of adjustable parameters and is based on the Non-Random Two Liquid (NRTL) model for nonelectrolytes (20). The LCM does not have the inherent drawbacks of virial-expansion type equations as the modified Pitzer, and it proved to be more accurate than the Bromley method. Some advantages of the LCM are that the binary parameters are well defined, have weak temperature dependence, and can be regressed from various thermodynamic data sources. Additionally, the LCM does not require ion-pair equilibria to correct for activity coefficient prediction at higher ionic strengths. Thus, the LCM avoids defining, and ultimately solving, ion-pair activity coefficients and equilibrium expressions necessary in the Davies technique. Overall, the LCM appears to be the most suitable activity coefficient technique for aqueous solutions used in FGD hence, a data base and methods to use the LCM were developed. 

For a heterogeneous equilibrium (a chemical equilibrium with components in different phases), reactants or products may be pure liquids or solids. The concentration of a pure liquid or solid in moles/liter cannot change. It is a constant property of the material, and these constants are incorporated into the equilibrium constant. Therefore the concentrations of pure liquids and solids are absent from equilibrium expressions for heterogeneous equilibria. 

There are two main ways to describe the equilibrium of a reaction. The first is in terms of the concentrations of reactions and products. The expression that describes the equilibrium of a reaction where the concentrations of the materials are known is Kc. When the reactants and products are in the gaseous state, we can also use the equilibrium constant expression, Kp, where partial pressures are used instead of concentration units. Solids and pure liquids (like water) are omitted from equilibrium expressions because their concentrations do not change during chemical reactions. 

However, since the concentration of liquid water is constant and so large relative to the concentration of ions, it can be omitted from the equilibrium expression. At 25°C, the equilibrium expression can be rewritten as Equation 14.1 ... 

This equilibrium is known as a heterogeneous equilibrium, which is to say that it consists of substances that are in different states (phases). If you recall from Chapter 13, the equilibrium constant expressions for such equilibria do not contain the concentrations of liquids or solids. The equilibrium expression for this reaction will describe the degree to which the solid dissolves in solution, which is another way to say the degree to which it is soluble. And since the solid is not shown in the equilibrium expression, the equilibrium constant will express the product of the concentrations of the dissolved solute ions. For this reason, the equilibrium constant is referred to as the solubility-product constant. For this equilibrium, the solubility-product constant, Ksp is ... 

Remember that the equilibrium expression calls for products over reactants, coefficients become powers. Solids and liquids are not included in the expressions. This means that the correct expression is... 

Foam instability is expressed mainly by the processes of excess (referring to equilibrium quantity) liquid outflow, diffusion gas transfer from smaller to larger bubbles and coalescence. If the vapour pressure of the solvent in the surrounding medium of foam is lower than its saturated vapour pressure, then the process of evaporation influences significantly foam collapse. Finally, if a foam is produced from a gas phase, the main component of which is the solvent vapour, condensation of these vapours appears to be the determining process of bubble expansion. 

What about solids and liquids Pure liquid and solid concentrations do not vary significantly in chemical processes, and their concentrations are always equal to their standard concentrations (usually one). So pure liquids and solids are omitted from equilibrium expressions. Of course, aqueous species are always included in the equilibrium expression. 

However, experimental results show that the position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present (see Fig. 6.6). This result makes sense when the meaning of an activity for a pure liquid or solid is understood. For a pure liquid or solid the reference state is the pure liquid or solid. Thus, for the composition of CaC03 considered above, we do not insert [CaC03] or [CaO] into the equilibrium expression but rather into the activity of each ... 

Note that the net effect of inserting an activity of 1 into the equilibrium expression for each pure solid or liquid in the reaction has the same effect as simply disregarding them. If pure solids or pure liquids are involved in a chemical reaction, their concentrations are not included in the equilibrium expression for the reaction. This simplification occurs only with pure solids or liquids, not with solutions or gases, because in these last two cases the activity cannot be assumed to be 1. 

In Chapter 6 we saw that pure solids and liquids are always omitted from the equilibrium expression because they have unit activities. In a dilute solution containing an acid we can assume that the activity of water is 1. Thus the term [H2O] is not included in Equation (7.2), and the equilibrium expression for Ka has the same form as that for the simple dissociation... 

Pure liquids and pure solids are never included in an equilibrium expression because they have an activity of 1. 

With these relations, the combined films and interface is regarded as an effective interface . There is no need to assume phase equilibrium between liquid and vapour. The entropy production rate can alternatively be expressed by the measurable heat flux in the vapour and fluxes of mass. - This set of flux equations was used to explain the entropy production in tray distillation columns. However, it has not yet been used for predictive purposes. Much work remains to be done to include these equations in a software that is useful for industrial purposes. 

For example, let us consider the monovariant equilibrium between liquid water and its vapour. Suppose the total mass of the system (given initially) is 1,000 g. We wish to see how the state of the system is completely determined if we know one intensive variable (T), and the total volume let us take 7 ==199 °C and V = 2 litres. The pressure exerted in the system is now determined—it is the vapour pressure of water at 199 °C i.e. 15 atm. Under these conditions the specific volumes of steam and water are 129 3 cm. /g. and 115 cm. /g. respectively. The total volume of the system can therefore be expressed in terms of the mass m of the liquid phase ... 

Experiments show that as long as some liquid water is in the container, the pressure of water vapor at 25°C is 0.03126 atm. The position of this equilibrium is not affected by the amount of liquid water present, and therefore liquid water should not appear in the mass action law. Recall that for a gas or solute, a ratio of pressures or concentrations appears in the law of mass action. This ratio is equal to 1 when the gas or solute is in its reference state (1 atm or 1 M). For a pure liquid appearing in an equilibrium chemical equation, the convention is to take that pure liquid as the reference state, so the liquid water contributes only a factor of 1 to the equilibrium expression and can thus be entirely omitted. We postpone justification of this rule to Section 14.3. 

Pure solids and pure liquids do not appear in equilibrium expressions neither does a solvent taking part in a chemical reaction, provided the solution is dilute. 

The equilibrium constant for this particular reaction has a special symbol K, and a special name, the ion product constant for water its value is 1.0 X 10 at 25°C. Because the liquid water appears in this equilibrium reaction equation as a pure substance, it is considered already to be in its reference state, and therefore contributes only the factor 1 to the mass action law equilibrium expression. The reasons for this are discussed more fully in Sections 14.2 and 14.3. The temperature dependence of is given in Table 15.1 all problems in this chapter are assumed to refer to 25°C unless otherwise stated. 

The concentration of a pure liquid or a pure solid is usually given a value of one for the equilibrium expression. Concentrations are only approximations for activities. Activities for pure solids and pure liquids are like effective mole fractions. The mole fraction of a pure solid or liquid is one. Although solvents are not actually pure, they are usually considered ideally dilute on the MCAT, which means that their moLe fraction is one. The activity then of a pure solid or liquid is approximately one. Be aware that pure solids or liquids can still participate in the equilibrium. When they do, they must be present in order for equilibrium to exist. 

The equilibrium of a solvation reaction has its own equilibrium constant called the solubility product Ksp. Use Ksp the same way you would use any other equilibrium constant. Remember that solids and pure liquids have an approximate mole fraction of one and can be excluded from the equilibrium expression. Thus, solids are left out of the solubility product expression as in the example of the Ksp for barium hydroxide shown below. 

Use Kj, like any other equilibrium constant to create an equilibrium expression. Set the Ksp equal to products over reactants raised to the power o( their coefficients in the balanced equation, ks always, leave out pure solids and liquids. 

C is correct. The equilibrium constant is products over reactants with the coefficients as exponents. However, reactants and products in pure liquid and solid phases generally have an exponent of zero, so they are not included in the equilibrium expression. 

The applicability of these equations could be expanded by keeping more terms inside the integral and carrying out the integration numerically. Another technique for the unimolar diffusion case is to express liquid and vapor flow rates on a solute-free basis, so they could be assumed constant over broader operating conditions. Compositions in this case would be expressed as mole ratios instead of mole fractions, and the phase equilibrium data would be converted accordingly. 

When a non-ideal binary liquid solution is equilibrated with a solid whose adsorption sites are all identical, a mass-action equilibrium expression can be written which contains the activity coefficients (yA and 7b) of the liquid components ... 

For example, if at equilibrium the liquid contains 0.1 mol fraction isobutane (iC4), and if the equilibrium ratio of iC4 under the system conditions is 2.5, then 0.25 mol fraction iC4 is in the vapor. The equilibrium ratio is some function of temperature, pressure, and the composition of the liquid, which may be expressed as... 

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