Aqueous Ionic Equilibrium

Buffers: solution that resist Ph changeSolubility equilibria: the extent to which slightly soluble ionic compounds dissolve in water16.2 Buffers: Solutions that resist ph changeA buffer contains significant amounts of both a weak acid and its conjugate base (or a weak base and its conjugate acid) A buffer must contain significant amounts of both a weak acid and its conjugate base. ex.) Consider the simple buffer made by dissolving acetic acid and sodium acetate and suppose that we add a strong base NaOH, As long as the amount of added NaOH is less than the amount of CH3COOH in solution, the buffer neutralizes the added NaOH and the resulting pH change is small.HOW DO WE CALCULATE A SOLUTION CONTAINING BOTH ACID AND ITS CONJUGATE BASE?- consider a solution that initially contains CH3COOH and CH3COONa each at a concentration of 0.100 mol L-1. The acetic acid ionizes, however the ionization of acetic acid in the solution is suppressed compared to its ionization in a solution that does not initially contain any CH3COO- bc the presence of that shifts the equilibrium to the left. The presence of that ion causes the acid to ionize even less than it normal would resulting in a less acidic solution. This effect is known as the common ion effect bc the solution contains two substances that share a common ion (CH3COO-)CALCULATING pH CHANGES IN A BUFFER SOLUTION.- Calculating the ph change requires breaking up the problem into two parts. 1.) a stoichiometry calculation (in which we calculate how the addition changes the relative amounts of acid and conjugate base); 2.) an equilibrium calculation in which we calculate the ph based on the new amounts of acid and conjugate base.ex.) neutralizing 0.025 mol of the strong acid H3O+ requires 0.025 mol of the weak base (A-). consequently, the amount of A- decreases b/c it is being used to neutralize the acid and the amount of HA increases by 0.025 mol because it is the product of H3O+ and A-.

Henderson-Hasselbach Equation: allows us to quickly calculate the pH of a buffer solution from the initial concentrations of the buffer components as long as the x is small approximation is valid. The x is small approx applies to problems in which both of the following are true a.) the initial concentrations of acids and or bases are not too dilute, and b.) the equilibrium constant is fairly small.

16.3 Buffer Effectiveness: Buffer Range and Buffer Capacity- What factors influence the effectiveness of a buffer? The relative amounts of the acid and conjugate base and the absolute concentrations of the acid and conjugate base.Capacity of a Buffer: How much added acid or base it can effectively neutralizeRange of a buffer: the ph range over which a particular acid and its conjugate base can be effective.- a buffer is most effective when the concentrations of acid and conjugate base are equal. a buffer becomes less effective as the difference in the relative amounts of acid and conjugate base increases. we can say that an effective buffer must have a base/acid ratio in the range of 0.10 to 10. in order for a buffer to be reasonably effective, the relative concentrations of acid and conjugate base should not differ by more than a factor of 10.- a buffer is most effective when the concentrations of acid and conjugate base are high. The more dilute the buffer components, the less effective the buffer- we can calculate the ph range over which a particular acid and its conjugate base can be used to make an effective buffer.- Lowest oh for effective buffer occurs when the base is one tenth as concentrated as the acid (ph=pka-1)- highest ph for the effective buffer occurs when the base is 10 times as concentrated as the acid (ph=pka+1) - for example, the buffering system with a weak acid pka of 5.0 can be used to prepare a buffer in the range of 4.0-6.0Buffer Capacity: is the amount of acid or base that you can add to a buffer without causing a large change in ph. Buffer capacity increases with increasing absolute concentrations of the buffer components. The more concentrated the weak acid and conjugate base that compose the buffer, the higher the buffer capacity. Overall, buffer capacity increases as the relative concentrations of the buffer components become more similar to each other. As the ratio of the buffer components gets closer to 1, the overall capacity of the buffer becomes greater.

16.4 Titrations and ph Curves- In an acid-base titration, a basic or acidic solution of unknown concentration is reacted with an acidic or basic solution of known concentration.- equivalence point: the point in the titration when the number of moles of base is stoichiometrically equal to the number of moles of acid -- the titration is complete. When the equivalence point is reach, neither reactant is in excess and the number of moles of the reactants are related by the reaction stoichiometry.-the point of inflection in the middle of the curve is the equivalence point.

The titration of a strong acid with a strong base:- the ph at the equivalence point of a strong acid-strong base titration will always be 7.00 at 25C. at the equivalence point, the strong base has completely neutralized the strong acid. The only source of hydronium ions then is the ionization of water.- as base is added beyond the equivalence point, it becomes the excess reagent.

The titration of a weak acid with a strong base:- the volume at the equivalence point in an acid-base titiration does not depend on whether the acid being titrated is a strong acid or a weak acid; it depends on the on the amount of (in moles) of acid present in solution before the titration begins and on the concentration of the added base.- when titrating a weak acid with a strong base, the added base converts stoichiometric amount of the acid into its conjugate base- added half the volume needed for equivalence point results in equal amounts of weak acid and conjugate base therefore ph=pka- ph after added the volume needed for equivalence point no longer results in a buffer b/c it no longer contains significant amounts of both a weak acid and its conjugate base- the titration of a weak acid by a strong base will always have a basic equivalence point because at the equivalence point all the acid has been converted into its conjugate base resulting in weakly basic solution

The titration of a Weak base with a strong acid:- curve starts basic and has an acidic equivalence point

The titration of a polyprotic acid:- when a diprotic acid is titrated, with a strong base, and if Ka1 and ka2 are sufficiently different, the ph curve will have two equivalence points.- notice that the volume required to reach the first equivalence point is identical to the volume required to reach the second one because the number of moles of the acid in the first step determines the number of moles of acid in the second step.

Indicators: pH dependent colours- with an indicator, we rely on the point where the indicator changes colour -- called the endpoint -- to determine the equivalence point.- an indicator is a weak organic acid that has a different colour in solution than its conjugate base- when the ph of the solution equals the pka of the indicator, the solution will have an intermediate colour

16.5 Solubility Equilibria and Solubility Product Constant- we can better understand the solubility of an ionic compound by applying the concept of equilibrium to the process of dissolution.- the equilibrium expression for a chemical equation representing the dissolution of an ionic compound is the solubility product constant (Ksp) the value of ksp is a measure of the solubility of a compoundKsp and Molar Solubility- solubility of a compound is the quantity of the compound that dissolves in a certain amount of liquid. The molar solubility is the solubility in units of moles per liter (mol L-1). - we can calculate the molar solubility of a compound directly from Ksp- the solubility product constant has only one value at a given temperature. The solubility however can have different values in different kinds of solutionsKsp and relative solubility- you cannot generally use the ksp values of two different compounds to directly compare their relative solubilities.- the relationship between ksp and molar solubility depends on the stoichiometry of the dissociation reaction. any direct comparison of ksp values for different compounds can only be made if the compounds have the same dissociation stoichiometry- *solubility of an ionic compound is lower in a solution containing a common ion than in pure water- *The effect of adding high concentrations of uncommon ions is to slightly increase the solubility of an insoluble salt an dis called the uncommon ion effect or salt effect- *the solubility of an ionic compound with a strongly basic or weakly basic anion increases with increasing acidity

16.6 Precipitation- precipitation reaction can occur upon the mixing of two solution contation ionic compounds when one of the possible cross products is insoluble- many compounds are slightly soluble and even those that we categorized as insoluble have some limited degree of solubility- we can better understand precipitation reactions by looking at reaction quotient (Q). The reaction quotient for the reaction by which an ionic compound dissolves is the product of the concentrations of the ionic components raised to their stoichiometric coefficients.- the difference between Q and Ksp is that ksp is the value of this product at equilibrium only whereas Q is the value of the product under any conditions-if Q is less than ksp the reaction will proceed to the right- if all solid has already dissolved the solution will simply remain as it is containing less than the equilibrium amount of the dissolved ions. such a solution is called an unsaturated solution.- when Q=ksp it is at equilibrium solution is called saturated solution.- when Q>ksp, reaction will proceed to left and solids will form.- under certain circumstances, Q can remain greater than Ksp for an unlimited period of time such a solution is called a supersaturated solution and it is unstable and it will form a precipitate when disturbed.- A solution may contain several different dissolved metal cations which can often be separated by selective precipitation, a process involving the addition of a reagent that forms a precipitate with one of the dissolved cations but not the other

16.8 Complex Ion Equilibria- a complex ion contains a central metal ion bound to one or more ligands.- a ligand is a neutral molecule or ion that acts as a lewis base with the central metal ion. - the equilibrium constant associated with the reaction for the formation of a complex ion is called the formation constant (Kf). The expression of kf is determined by the law of mass action- in general, kf values are very large indicating that the formation of complex ions is highly favourable in each caseThe effect of complex ion equilibria on solubility- solubility of an ionic compound with a basic anion increases with increasing acidity because the acid reacts with the anion and drives the reaction to the right. - the solubility of an ionic compound containing a metal cation that forms complex ions increases in the presence of lewis bases that complex with the cation. The most common lewis bases that increase the solubility of metal cations are NH3, CN- and OH-