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Titrations and Indicators IB Chemistry Power Points Topic 18 Acids and Bases www.pedagogics.ca
Acid Base Titrations If a strong acid is titrated using a strong base the pH will start off as 0-1 and gradually increase. Once enough base is added that it is now in excess the pH will change very suddenly to about 12. The point at which this change is seen is when the amount of acid = amount of base. This is called the equivalence point. With this combination it occurs at pH 7 as the acid and base combine to make a neutral solution.
Although use of phenolphthalein is common, most indicators will work for this combination. the sharp change in pH (vertical line on plot) is called the inflection point.
Weak Acid + Strong Base A weak acid will have a pH of 3-5. When a strong base is added the pH will increase gradually as HA is converted to A -. This is called the buffering region as the solution is acting like a buffer. When half the amount of base required for neutralization has been added it is called the half-neutralization point and [HA] = [A - ] at this point, K a = [H + ] and pK a = pH
K a = [H + ] and pK a = pH This is the best way to determine the K a for a weak acid. The equivalence point occurs when pH >7. Why? Most suitable indicator is phenolphthalein
Consider a titration of 50 cm 3 of 0.05 mol dm -3 of acetic acid with 0.10 mol dm -3 NaOH Use your graph to determine the pH of the half equivalence point and the K a for a acetic acid.
A suitable indicator here is methyl orange. Weak Base + Strong Acid
When a strong acid is added to a weak base B(aq) + H + (aq) ∏ BH + (aq) At half equivalence point [B] = [BH + ] so K b = [OH - ] And pK b = pOH = 14 – pH At the equivalence point pH is <7 and exact pH can be found from pK b of base (or pK a of the conjugate acid). Why is this?
When equivalence point is reached moles of B = moles of H + B(aq) + H + (aq) ∏ BH + (aq) BH + is the conjugate acid and will react with water to form the equilibrium BH + (aq) + H 2 O ∏ B(aq) + H 3 O + (aq) pK b of the base + pK a of conjugate acid = 14 Knowing pK a and [BH + ] (or pK b and [A - ]) means you can calculate equivalence point pH – see next two slides if you are brave
50.0 cm 3 of 0.1 M NH 3 are titrated with 0.20 M HCl. What is the pH at the equivalence point? NH 3 (aq) + H + (aq) ∏ NH 4 + (aq) At equivalence point, 0.1 x 0.050 = 0.0050 mol of NH 4 + is produced by the neutralization (moles NH 3 = moles HCl) NH 4 + is the conjugate acid and will react with water to form the equilibrium NH 4 + (aq) + H 2 O ∏ NH 3 (aq) + H 3 O + (aq) pK b of the NH 3 + pK a of NH 4 + = 14 4.75 + 9.25 = 14
NH 4 + (aq) + H 2 O ∏ NH 3 (aq) + H 3 O + (aq) pK b of the NH 3 + pK a of NH 4 + = 14 4.75 + 9.25 = 14
Weak Acid + Weak Base When a weak acid is added to a weak base the change in pH is gradual from acidic to basic so it is hard to detect the equivalence point. It is hard to find a suitable indicator.
How Indicators Work An indicator is a substance (often an organic dye) that has a different color in acidic and alkaline solutions. The color change is seen because the indicator is a weak acid/base in which the two forms have different colors. HIn(aq) H + (aq) + In - (aq) litmus: red blue HIn stands for the indicator and In - is the other form when it dissociates.
What happens with the addition of acid? the eq m is driven to the left to form HIn as the H + ions combine with In - to reduce [H + ]. What happens with the addition of base? the equilibrium shifts to the right to form more In - as the OH - of the base combines with H + reducing [H + ] so more HIn will dissociate. HIn(aq) H + (aq) + In - (aq)
The color present depends on the pH and [H + ] and also on K a so different indicators change color over different pH ranges. When pH = pK a the two colored forms will have equal concentrations and the indicator will be in the middle of its color change. Rule of thumb: If one form is in excess by 10 fold then the color will be of that form.
Consider Bromophenol blue pK a = 4.0 yellow in acidblue in alkali at pH = 4.0, pH = pK a and the [H + ] = 10 -4
Consider Bromophenol blue pK a = 4.0 yellow in acidblue in alkali at pH = 5.0, pH > pK a and the [H + ] = 10 -5
Consider Bromophenol blue pK a = 4.0 yellow in acidblue in alkali at pH = 3.0, pH < pK a and the [H + ] = 10 -3
Choosing an Indicator – data book Phenolphthalein pK a = 9.6 pH range: 8.3 -10.0 Color in acid: colorless Color in alkali: pink Good for titrations with strong bases
Choosing an Indicator – data book Methyl Orange pK a = 3.7 pH range: 3.1 - 4.4 Color in acid: red Color in alkali: yellow Good for titrations with strong acids