CHE 141 1st Edition Lecture 17Outline- Mixture of Acidic and Basic Ions- The Common Ion Effect- Buffer Solutions- Calculating the pH of a Buffer Solution/ Henderson-Hasselbalch EquationMixture of Acidic and Basic Ions- Salts in which the cation acts as an acid and the anion acts as a base- Cation=conjugate acid of weak base or small, highly charged metal ion- Anion=conjugate base of a weak acid- We can determine the overall pH of a solution containing a salt where the cation acts as a weak acid and the anion acts as a weak base by comparing the Ka of the acid to the Kb of the base- The ion with the higher value of K dominates and determines whether the solutionwill be acidic or basic- Ka>Kbacidic- Kb>Kabasic- Ka=KbneutralThe Common Ion EffectThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- So far we have only considered reaction system with a single acidic or basic reactant- Natural system are often more complicated than that, for instance containing a soluble salt that can influence pH (acidic/basic/neutral salts) or resist pH change - Common ion effect: the shift in the position of an equilibrium caused by the addition of an ion taking part in the reaction- The addition of the common ion is adding more products to the equilibrium- Increasing the concentration of a product would shift the equilibrium to the left (concentration stress rules)- Addition of a common ion suppresses the ionization of the acid, decreasing [H3O+], which in turn increases pH (less acidic)- In other cases the addition of a common ion suppresses the protonation of the base, decreasing [OH-], which in turn decreases pH (less basic)- Most solutions undergo a significant change in pH when an acid or base is added to them- However, when a common ion is present it suppresses the ionization of the weak acid or weak base, so that it does not ionize as much as it would have without the common ion- Buffering effect the ionization of an acid or base is limited by the presence of its conjugate base or conjugate acid- The presence of a common ion also affect solubility equilibriumBuffer Solutions- A pH buffer is a solution that resists changes in pH when acids or bases are added to it by neutralizing the added acid or base- A buffer contains significant amounts of eithero A weak acid and its conjugate baseo A weak base and its conjugate acid- The weak acid neutralizes added base- The weak base neutralizes added acid- Buffers are often prepared by mixing a weak acid or base with a salt of that acid or baseCalculating the pH of a Buffer Solution/ Henderson-Hasselbalch Equation- Consider a buffer solution containing a weak acid HA and its conjugate base A- The acid ionizes : HA(aq)+H2O(l) reversible A-(aq)+H3O+(aq)- For which the acid dissociation equilibrium expression is=Ka=[A-][H3O+]/[HA]- We can derive an equation to quickly calculate the pH a buffer solution without anICE table- Rearrange in terms of [H3O+]: [H3O+]=Ka[HA]/[A-], take –log of both sides andremember AB=logA+logB so –logAB=-logA-logB(-log[H3O+]=-logKa-log[HA]/[A-], and (A/B)= –log(B/A) so –log(A/B)=log(B/A) (-log[H3O+]=-logKa+log[A-]/[HA]- Since pH=-log[H3O+] ndpKa=-logKa (pH=pKa+log[A-]/[HA])- Since A- is a weak base and HA is a weak acid, we can generalize the equation pH=pKa+log[base]/[acid]- This is the Henderson-Hasselbalch equation, which is used to calculate the pH of a buffer solution in which the concentrations of acid and conjugate base are known- You can use either the equilibrium approach, or the Henderson-Hasselbalch equation to calculate the pH of a
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