Water ion-product constant for

When the molar concentrations of all species in an equilibrium reaction are known, Units are generally not the Keq can be calculated by substituting the concentrations into the equilibrium con stant expression. 

Calculate i q for the following reaction based on concentrations of PCI5 = 0.030 mol/L,PCl3 = 0.97mol/L, and CI2 = 0.97mol/L at300°C. 

First write the expression then substitute the respective concentrations into this equation and solve  

This Keq is considered to be a fairly large value, indicating that at 300°C Remember Units are not the decomposition of PCI5 proceeds far to the right. included for 

Calculate the i q for this reaction. Is the forward or the reverse reaction favored  

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These equations say that the Kh of an amine multiplied by the of the corresponding ammonium ion is equal to Kw, the ion-product constant for water (1.00 x 10 14). Thus, if we know Ka for an ammonium ion, we also know for the corresponding amine base because /stronger base has an ammonium ion with a larger p... 

So few ions form that the concentration of water is essentially constant. The product fclHaO] is equal to the product of the concentrations of hydronium ions and hydroxide ions. The equilibrium value of the concentration ion product [HaO HOH"] at 25°C is called the ion product constant for water. It is given the symbol K . 

Similarly, the concentration of hydroxide ions can he determined from the concentration of the dissolved base. If the solution is a strong base, you can ignore the dissociation of water molecules when determining [OH ], unless the solution is very dilute. When either [HsO ] or [OH ] is known, you can use the ion product constant for water,, to determine the concentration of the other ion. Although the value of i w for water is... 

Note H+ exists in water as H3O+.) The product of these two concentrations is the ion-product constant for water, K ... 

For any conjugate acid-base pair, the product of the acid-dissociation constant for the acid and the base-dissociation constant for the base always equals the ion-product constant for water ... 

The ion-product constant for water, Kw is the equilibrium constant for the dissociation of H20. Water molecules may donate protons to other water molecules in a process known as autoionization ... 

Application of the mass law to the dissociation of water leads to Kw = [H30" ][0H ], where Kw is called the ion-product constant for water. At 25°C the ion-product constant has the numerical value of 1.0 X 10 " mole /L. In pure water, the concentrations of hydronium and hydroxide ions are identical, and pure water is neutral. 

The result is a special equilibrium constant expression that applies only to the self-ionization of water. The constant, K, is called the ion product constant for water. The ion product constant for water is the value of the equilibrium constant expression for the self-ionization of water. Experiments show that in pure water at 298 K, [H+] and [OH ] are both equal to 1.0 X 10 M. Therefore, at 298 K, the value of K is 1.0 X 10 ... 

How does the ion product constant for water relate to the concentrations of H+ and OH in aqueous solutions ... 

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 result of adding the acid to the water will be an increase in hydronium ions and a corresponding decrease in hydroxide ion. The product of the concentrations of hydronium ions and hydroxide ions is a constant, called the ion-product constant for water, which is symbolized as kw. The formula for this constant is shown here ... 

At 25°C, the ion-product constant for water is 1.008 X 10 For convenience, we use the approximation that at room temperature = 1.00 X 10 . Table 9-3 shows the dependence of this constant on temperature. The ion-product constant for water permits the ready calculation of the hydronium and hydroxide ion concentrations of aqueous solutions. 

Note that [H2O] does not appear in the denominator of either equation because the concentration of water is so large relative to the concentration of the weak acid or base that the dissociation does not alter [H2O] appreciably (see Feature 9-2). Just as in the derivation of the ion-product constant for water, [H2O] is incorporated into the equilibrium constants and Dissociation constants for weak acids are found in Appendix 3. 

Figure 10-1 further illustrates the effect of electrolytes. Curve A is a plot of the product of the molar hydronium and hydroxide ion concentrations (XIO " ) as a function of the concentration of sodium chloride. This concentration-hasedi ion product is designated K. At low sodium chloride concentrations, K becomes independent of the electrolyte concentration and is equal to 1.00 X 10 , which is the thermodynamic ion-product constant for water, K . A relationship that approaches a constant value as some parameter (here, the electrolyte concentration) approaches zero is called a limiting law the constant numerical value observed at this limit is referred to as a limiting value. 

At the equivalence point, neither HCl nor NaOH is in excess, and so the concentrations of hydronium and hydroxide ions must be equal. Substituting this equality into the ion-product constant for water yields... 

We now have four algebraic equations (Equations 15-10 and 15-11 and the two dissociation constant expressions for H2A) and need one additional expression to solve for the five unknowns. The ion-product constant for water serves this purpose ... 

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