18.3 ph Curves
Buffer solutions Strong acids and bases dissociate completely and change the ph of a solution drastically. Buffers are solutions that resist changes in ph even when acids and bases are added to a solution. Buffers are made from weak acids/bases because they can follow Le Chatelier s principles because they are reversible reactions in equilibrium Buffer capacity is how well a buffer can resist a change in ph. The buffer capacity increases as concentration of the acid/base increase.
Buffer solutions You can see the relatively flat parts of the titration and notice that the ph does not change as fast during the buffer ranges. Buffers are important for industry and biological systems When an acid and its conjugate base are in solution its is a buffer Buffer can be made by the partial neutralization of a weak acid/base by a strong acid/base
Salt hydrolysis After acids and bases in neutralization reactions are mixed a salt is formed. The salt dissolves in water to form cations and anions. Hydrolysis is the ionization of water that occurs when the salts dissolve. Hydrolysis cause a change in the ph of the water solution.
Salt hydrolysis Weak acid and strong base will create a basic salt because the strong base will dissociate completely and not reform leaving OH - ions in solution. Weak base and strong acid will create a acidic salt because the strong acid will dissociate completely and not reform leaving H + ions in solution. Strong acid and base will create a neutral salt. Weak acids and bases will create salts that must be anaylzed using pk a s and pk b s. Whichever is lower will decide if the salt is an acid or a base. Positive metals salts- Metals with large charges (+3) will polarize water molecules more than metals with smaller charges (+2) and create hydrogen ions and acid solutions.
ph (titration) Curves ph curves are usually created by putting an indicator and 25 cm 3 of 0.1 mol dm -3 acid into a Erlenmeyer flask and a 0.1 mol dm -3 base into a burette. The base is added to the acid an the ph is recorded and qualitative observations are recorded. We will look at four different titrations in this section.
Titration of strong acid and strong base HCl + NaOH NaCl + H 2 O The starting part of the curve starts out very low because its is a strong acid that has a ph of around 1 There is a slow gradual rise in the ph until the equivalence point is reached. The equivalence point (theoretical endpoint) is when all the acid has been reacted and there is a sharp increase in the ph After the equivalence point there is no remaining acid left so the graph reaches the ph of the base.
Titration of a weak acid with a strong base CH 3 COOH + NaOH CH 3 COONa + H 2 O The starting point is higher than when using a strong acid. The ph will be around 3 at the start The curve will go up a lot at the start. Neutralization is happening quickly The curve starts to even out and become flat. This is when the weak acid reaches its buffer zone where the ph doesn t change very quickly. The buffer resists changes to ph. Continual addition of the base continues to use up the acid and the ph will continue to rise
Titration of a weak acid with a strong base CH 3 COOH + NaOH CH 3 COONa + H 2 O The half equivalence point is when half the weak acid is neutralized. This is equal to the pk a The equivalence point is when the graph makes a sharp rise and this will be at a ph higher than 7. No more acid is present and the solution reaches the ph of the strong base. This is another example of a weak acid with a strong base titration
Titration of a strong base with a weak acid HCl + NH 3 NH 4 Cl The weak base has a high ph of about 11. The graph reaches the buffer zone after decrease quickly. The buffer resists changes to the ph. pk b = poh at the half equivalence point. This is when half the base has been used. The graph will continual to gradually decrease after the half equivalence point.
Titration of a strong base with a weak acid HCl + NH 3 NH 4 Cl The graph s ph will drastically fall until it reaches the equivalence point, which all the base is used up. This will be at a ph of less than 7 because of salt hydrolysis. The ph will continue to fall until it reaches the ph of the strong acid.
Titrations of weak base and weak acid The ph starts out at around 11 The change in ph throughout is very gradual. The equivalence point is not as steep and hard to determine using this type of graph. This titration has little use in real life. The equivalence point will be around 7 Once the base is neutralized the graph evens out at the end to the ph of the weak acid.
Indicators Indicators are weak acids and bases that change colors when they change from weak acids to conjugate bases or weak bases to conjugate acids.
Picking an indicator The ph range of an indicatorthis is is a transitional period between the colors of an indicator. When bromothymol blue is in its acidic form it will be yellow and in its basic form it will be blue. Between 6.0-7.6 (near its pka) there will be both forms present and thus the solution will be green
Picking an indicator Choose an indicator that has a pk a that is close to the equivalency point of the titration.
Titrations (Nature of Science) Titrations Titrations are a quantitative tool used by scientist for over 150 They allow scientist to see patterns and unlock a wealth of chemical knowledge Indicators and endpoints Even though we have advance technologies like ph meters to track the equivalence points we still use indicators. Indicators change colors during a narrow range that help the scientist to see when the equivalence point has been reached.