Topic 18.3 - IB Chemistry HL

When you add an acid to a base?

The graph looks different. Use this to calculate Kb of a weak base.

What happens when you titrate a weak acid with a strong base?

The pH at equivalence is > 7. This is because the anion hydrolysis releases OH-. 1.) the initial pH is fairly high because it is a weak acid 2.) the pH stays relatively constant until equivalence, and this is labelled as the buffer region 3.) there is a jump in pH at equivalence from about pH ... to pH ..., which is not as much as a jump as for a strong acid-strong base titration 4.) after equivalence the curve flattens out at a high value (pH of strong base) 5.) the pH at equivalence is > 7 Of particular interest is the reaction mixture after the addition of half of the base (25cm^3 in this case). This point is labelled as half the equivalence point.

What happens when you titrate a weak acid with a weak base?

The pH at equivalence is difficult to define. 1.) the initial pH is fairly high (pH of the weak acid) 2.) the addition of the base causes the pH to rise steadily 3.) the change in pH at the equivalence point is much less sharp than in the other titrations 4.) after equivalence the curve flattens out at a fairly low pH (pH of weak base) There is no clear equivalence point as there is no significant jump to identify, because several equilibria are involved. It is better to use conductimetric measurements to determine equivalence point for this one.

What happens when you titrate a strong acid with a weak base?

The pH at equivalence is less than 7 because the cation hydrolysis releases H+. 1.) the initial pH is low because it is a strong acid 2.) the pH stays relatively constant through the buffer region to equivalence 3.) jump in pH at equivalence from about pH... to pH ... 4.) after equivalence the curve flattens out at a fairly low pH (pH of weak base) 5.) pH at equivalence is < 7

What happens when you titrate a strong acid with a strong base?

You assume full dissociation because they are strong acids and bases. The base is added to the acid, and as this happens some of the acid is neutralized while excess acid remains. When equivalence is reached, the amount of acid and base have fully reacted. After equivalence, the mixture contains excess base. As the volume changes during the addition, this must be taken into account when determining the concentration. 1.) the initial pH of a strong acid is low. 2.) pH changes only gradually until equivalence is reached. 3.) there is a very sharp jump in pH at equivalence from pH ... to pH .... 4.) after equivalence the curve flattens out at a high value pH of strong base) 5.) pH at equivalence is 7

What is a buffer solution?

A buffer is generally something that acts to reduce the impact of one thing on another. The official definition of a buffer is that "a buffer solution is resistant to changes in pH on the addition of small amounts of acid or alkali". Buffers do not have infinite ability to maintain the pH. The pH of a non-buffered solution changes when there is added acid of base. This is because water is very vulnerable to significant fluctuations in its pH and this can impact the chemical reactions in aqueous solutions. There are two types of buffer solutions: acidic buffers - they maintain pH at values below 7, and basic buffers - they maintain pH at values greater than 7. Buffers are a mixture of 2 solutions: each solution contains two species of a conjugate acid-base pair. How can you understand their buffering action? Focus on the equilibria in the solutions and pick out the species that respond to added H+ or OH-. In summary: Buffer solutions are a mixture containing both an acid and a base of a weak conjugate pair. The buffer's acid neutralizes the added alkali, and the buffer's base neutralizes the added acid. Like so, pH is resisted.

What is the volume of one drop of a burette?

About 0.05cm^3

Outline the composition of an acidic buffer solution.

An acidic buffer is made by mixing an aqueous solution of a weak acid with a solution of its salt of a strong alkali. An example would be CH3COOH as the weak acid and NaCH3COO as the salt of the weak acid with the strong alkali. There are two equilibria that exist in a solution of this mixture: CH3COOH <> CH3COO- + H+ and NACH3COO <> Na+ + CH3COO- CH3COOH is a weak acid, so the eqm lies to the left. CH3COOH and CH3COO- is also the species that responds to the added H or H+. So this mixture contains relatively high concentratio ns of both CH3COOH and CH3COO-, that is an acid and its conjugate base. They are ready to react with added OH- or H+ in neutralization reactions. pH is thus unchanged. H+ combines with the base CH3COO- OH- combines with the acid CH3COOH.

How to choose the appropriate indicator?

An indicator signals the equivalence point of a titration when its endpoint coincides with the pH at equivalence point. Different indicators must be used for different titrations depending on the pH at the equivalence point. Determine what combination of weak and strong acid and base are reacting together. Deduce the pH of the salt solution at equivalence from the nature of the parent acid and base. Choose an indicator with an endpoint in the range of the equivalence point by consulting data tables. Equivalence points: Strong A + strong B: 3-11 Weak A + strong B: 7-11 Strong A + Weak B: 3-7

Outline the composition of a basic buffer solution.

Basic buffer solutions are made by mixing an aqueous solution of a weak base with its salt of a strong acid. NH3 (weak base) & NH4Cl (salt of weak base with strong acid). There are two equilibria that exist in this solution: (i) NH3 (aq) + H2O(l) <-> NH4+ (aq) + OH- (aq) NH3 is a weak base, so its eqm lies to the left (ii) NH4Cl (aq) -> NH4+ + Cl- (aq) NH4+ is a soluble salt, so is fully dissociated in solution. So the mixture contains high concentrations of both NH3 and NH4+. They will act as reservoirs to react with H+ and OH- in neutralization reactions. If you add an acid, then H+ will combine with NH3 base to form NH4+ and thus removing most of the H+. If you add a base, OH- will combine with the acid NH4+ to form NH3 and H2), thus removing most of the OH-. So, the removal of the added H+ and OH- by reactions with the components of the buffer solution means that they do not persist in the solution and do not alter the pH.

What is the equivalence point in an acid-base titration?

Equivalence point is where an acid and base exactly neutralize each other. Occurs when stoichiometrically equivalent amounts of acid and base have been reacted together. At this point the solution contains salt and water only.

Name and outline the factors that can influence buffers.

Factors that can influence buffers are: dilution, temperature, and salt hydrolysis. Dilution - Ka and Kb are equilibrium constants and are not changed by dilution. Dilution doesn't change the ratio of acid base to salt concentration, as both components will be decreased by the same amount. Diluting a buffer does not change its pH. Diluting a buffer does change the amount of acid or base it can absorb without significant changes in the pH - which is called the buffering capacity. The amount of acid or base that a buffer can absorb depends on molar concentrations of its components, so it decreases as they are lowered by dilution. Temperature - the values of Ka and Kb are affected by temperature and thus the pH of the buffer is affected too. This is why a constant temperature should be maintained in all work involving buffers (such as the calibration of pH meters). Temperature fluctuations must be minimized. Salt hydrolysis - A neutralization reaction between an acid and a base creates a salt. A salt is an ionic compound that contains a cation from the parent base and an anion from the parent acid. The cation is the conjugate acid of the parent base and the anion is the conjugate base of the parent acid. Although salts are the products of a neutralization reaction, they do not all form neutral aqueous solutions. Their pH in solution depends on whether and to what extent their ions, which are conjugate acids/bases, react with water and hydrolyse it, releasing H+ or OH- ions. The weaker the acid/base, the stronger its conjugates. The relative strengths of the conjugate acids and bases in the salts determines the extent of hydrolysis reactions and so the pH of the salt solution.

How do indicators work?

Indicators are substances that change color reversibly according to the pH of a solution. Indicators undergo a distinct color change. They are weak acids/bases in which the dissociated and undissociated forms have different colors. How does the equilibrium respond to a change in pH in the medium? increasing [H+]= eqm shifts to the left favoring HIn decreasing [H+]=eqm shifts to the right in favor of In-. At low pH color, color A will dominate. At higher pH, color B will dominate.

When do indicators change color?

Indicators change color when the pH is equal to their pKa. A change in color is called an end point or change point. What determines this? Consider the equilibrium expression for the above reaction. At the point where the equilibrium is balanced between the acid and its conjugate base, [In-]=[HIn] This is when the indicator is exactly in the middle of its color change. Ka=[H+] or pKa=pH. Different indicators with different pK values will have different end points and will change color at different pH values.

Outline the situations for salt hydrolysis.

Salt of strong acid and strong base - no hydrolysis: - both conjugate acid and base are weak so there is basically no hydrolysis of ions and the pH is close to neutral. - solutions of a strong base and acid have a pH around 7 at 298K. Salt of a weak acid and a strong base - anion hydrolysis: - the anion is the conjugate base of the parent acid, and when the acid is weak then this conjugate base is strong enough to cause hydrolysis. - the release of OH- causes the pH of the solution to increase. - solutions of salts of weak acids with strong bases have a pH > 7 at 298K. Salt of a strong acid and weak base - cation hydrolysis: - The cation is a conjugate acid of the parent base. When the base is weak and this conjugate is a non-metal, it is able to hydrolyze water. The release of H+ ions causes the pH of the solution to decrease. Salt of weak acid and weak base: - both conjugates are relatively strong and carry out hydrolysis - the pH of the solution depends on Ka and Kb values of the acids and bases involved. So, in summary: the pH of a salt solution depends on the relative hydrolysis of its anions and cations, which can be deduced from the relative strengths of the parent acids and bases.

How to you make a buffer solution?

The pH of a buffer is determined by the interactions of its components. It depends on: 1) the pKa or pKb of its acid or base, and 2) the ratio of the initial concentrations of acid and salt, or base and salt, used in its preparation. Buffer solutions can be prepared by starting with an acid or base that has a pKa or pKb value as close as possible to the required pH or pOH of the buffer. This is then either: 1.) mixed with a solution of salt containing its conjugate or 2.) partially neutralized by a strong base or acid. The neutralization reaction should ensure that approximately one half of the starting acid or base is converted into salt. After the reaction, the mixture will contain the unreacted acid or base and its salt in equimolar amounts. (Check notes for sample equation and sample exercise.) If the full solution (of those two mixtures) contains a conjugate pair, then it should be a buffer.

What happens during acid-base titrations?

When a base is added to acid in neutralization reaction, there is a change in pH. This change does not show a linear relationship with the volume of base added, and this is partly due to the logarithmic nature of the pH scale. In most titrations, a big jump occurs at equivalence, which is known as the point of inflection. The equivalence point is halfway up this jump.

What is half equivalence point?

When exactly half of the acid has been neutralized by the base and converted into salt. This mixture has equal quantities of its weak acid and its salt. (i.e. it is a buffer). This explains why the pH in this region is shown to be relatively resistant to change in pH on the addition of small amounts of base. the pH at half equivalence point let's us easily calculate pKa. [acid]=[salt] Ka= [H+][A-]/[HA] assume that [HA] = acid and that [A-]= salt. Then [HA]=[A-]. Ka=[H+] and pKa=pH, so just read the pH directly from the graph.