Chemical Systems and Equilibriums - Titrations and Buffers

titration set up

base in the burette, acid and indicator in erlenmeyer flask

titration

an analytical lab technique used to determine the concentration of a given solution (acid or base)

• neutralization reaction is carried out quantitatively

titrant

solution of known concentration (solution in burette)

sample

solution of unknown concentration (solution in Erlenmeyer flask)

equivalence pt (or stoichiometric pt)

the point at which the reaction is complete (=same number of moles of hydronium and hydroxide)

indicator

substance that changes colour at (or near) the equivalence pt

endpoint

point in a titration where a sharp change in a measurable and characteristic property occurs (ie. colour change of acid-base indicator – record final volume and compare to the initial volume)

equipment in titration (6)

• Burette

• Burette Clamp

• Erlenmeyer Flask

• Volumetric Pipette

• Beakers

• Phenolphthalein

neutralization rxn

double displacement reaction between an acid and a base to produce a salt and water (solvent)

• acidic and basic properties are destroyed

• acid + base → salt + water

titration curve of a strong acid with a strong base

pH increases slowly at first and rapidly as it approaches the equivalence pt

titration curve

a graph of the pH of an acid (or base) against the volume of an added base (or acid)

• equivalence point is equal to the midpoint on the titration curve

titrations involding WEAK acids and STRONG bases

to calc concen of H+, we must consider WEAK ACID IONIZATION EQ

indicators are used to

pinpoint the neutralization pt of the rxn (ENDPT)

• indicators are molecules that have a diff colour in acid than in does in base (basically according to the pH)

phenolphthalein

colourless in acid, pink in base (pH 8.2 to 10)

bromothymol blue

yellow in acid, blue in base (pH 6 to 7.6)

acid base indicators

• a weak acid or base that has diff colours in the non-ionized and ionized forms

• HIn(aq) ⇌ H+ (aq) + In- (aq)

• Red Yellow

• if acidic, the rxn shifts left to oppose it, therefore is red

• if basic, rxn shifts right and therefore is yellow

buffer

a solution that contains a weak acid/conjugate base mixture or a weak base/conjugate acid mixture

what do buffers do

resists changes in pH when a moderate amount of an acid or a base is added to it

• able to neutralize acids and bases without allowing the pH of the solution to change greatly

2 ways to make buffers

• by using a weak acid and one of its salts; for example, by mixing acetic acid and sodium acetate

• by using a weak base and one of its salts; for example, by mixing ammonia and ammonium chloride

buffers usually have ____ in them that act as a reservoir and help mainatin a relatively const pH

common ions; and therefore should be treated as a common ion problems

importance of buffers

• Living organisms are very sensitive to pH changes as enzymes carry out their function optimally over a small pH range.

• Any change in pH of more than 0.2 induced by poisoning or disease is life-threatening.

buffer capacity

amount of acid or base that can be added before considerable change occurs to the pH of the solution

what is buffer capacity dependent on

• the concentration of the buffer components

• concentrated buffer resists to changes more than a diluted buffer

if there was no buffer, the pH would

change by a significant factor instead of by a slight factor