a) define the terms strong and weak for acids and bases using Bronsted-Lowry theoryb) define the terms Ka, Kb, pH, pKa, pKb, Kw, pKw

c) perform calculations involving Ka, Kb, pH, pKa, pKb, Kw, pKw

d) sketch pH changes during various types of acid-base titrationse) describe and explain pH changes of an indicator (use methyl orange and phenolphthalein as examples)f) state basis of choice of indicator for use in a titration

ACID-BASE EQUILBRIA

Bronsted-Lowry theory for strong and weak acids

A strong acid dissociates completely donating hydrogen ions when dissolved in water e.g. HCl, H2SO4, HNO3

A weak acid partially dissociates donating hydrogen ions when dissolved in water e.g. CH3COOH, citric acid, lactic acid

pH of an acid represents the hydrogen ion concentration and can be shown via a formula such as: pH = -log [H+] NB [H+] represents the MOLAR concentration of hydrogen ions

Example #1 What is the pH of a 0.2 mol dm-3 solution of HCl HCl H+ + Cl-

Therefore [H+] = 0.2Then pH = -log 0.2 = 0.69

Example # 2 What is the pH of a 0.26 mol dm-3 solution of H2SO4

H2SO4 2H+ + SO42-

Therefore [H+] = 0.26 x 2 = 0.52Then pH = -log 0.52 = 0.28

Checkpoint A1. Calculate the pH of the following acid solutionsa) 0.5 mol dm-3 solution of HCl b) 0.01 mol dm-3 solution of H2SO4

c) 0.25 mol dm-3 solution of HNO3

pOH of an alkali represents the hydroxide ion concentration and can be shown via a formula such as: pOH = -log[OH-] NB [OH-] represents the MOLAR concentration of hydroxide ions

Relationship of pH and pOH pH + pOH = 14

Example #1 What is the pH of a 0.5 mol dm-3 solution of NaOH?

NaOH is a strong alkali and would dissociate completely. Hence 1 mol of NaOH would form 1 mol of OH- ionsTherefore 0.5 mol of NaOH would form 0.5 mol of OH- ions

pOH = -log 0.5 = 0.22

therefore pH = 14 – 0.22 = 13.78

Checkpoint B

What is the pH of the following strong alkali solution?

0.01 mol dm-3 NaOH

Pure water self-ionises according to the equation below

H2O (l) ↔ H+ (aq) + OH- (aq)The equilibrium expression would be [H + ] [OH - ] H2O

However the concentration of water would be a constant so it is removed from the expression and a new expression is derived:- [H+] [OH-]

This expression is called Kw the ionic product of water [H+] [OH-]

Note: In pure water, the molar concentrations of hydrogen ions and hydroxide ions ARE ALWAYS THE SAME!!!!At 25°C Kw = 1 x 10-14 mol2 dm-6 therefore [H+] = [OH-] = √1 x 10-14 = 1 x 10-7 pH = -log 1x10-7 = 7

At different temperatures, the value of Kw would be change and the pH OF PURE WATER WOULD NO LONGER BE 7, however the water would still be considered neutral as the molar concentrations of hydrogen ions and hydroxide ions are equal.

Example: What is the pH of pure water at 100 °C with a Kw value of 1.5 x 10-13

[H+] = √{1.5 x 10-13) = 3.87 x 10-7

pH = -log 3.87 x 10-7 = 6.4

Conjugate acid-base pairs

Remember acids donate hydrogen ions and bases accept them according the Bronsted-Lowry theory.

Hence when HCl reacts with water, the following equation can be written

HCl + H2O ↔ H3O+ + Cl-

On the left hand side, HCl is the acid and H2O is the baseHowever on the RIGHT HAND SIDE, H3O+ is now the acid and Cl- is now the base.A conjugate base is the anion remaining after the acid donates its hydrogen ions e.g. Cl- is the conjugate base of the acid HCl

A conjugate acid is the cation remaining after it has accepted one or more hydrogen ions e.g. H3O+ is the conjugate acid of the base H2O

Checkpoint CLook at the equations below and state the acid-conjugate base pairs and the base-conjugate acid pairs, by writing A, B, CA and CB above the respective substance

a) CH3CO2H + H2O CH3CO2- + H3O+

b) H2O + NH3 OH- + NH4+

The pH of weak acids and weak bases CANNOT be done using the calculations performed previously. A new method must be used

Consider the weak acid HA

HA + H2O A- + H3O+

The equilibrium expression would be [A - ][H 3O + ] [HA][H2O]

However the concentration of water molecules is a constant, thus it is removed from the expression to give [A - ][H 3O + ] [HA]

This new expression is called Ka (the acid dissociation constant) [A - ][H 3O + ] [HA]

Similarly consider the weak base BB + H2O BH+ + OH−

(base dissociation constant) Kb = [ BH + ][OH − ] B pKa = -log Ka AND pKb = -log Kb pKa + pKb = pKw

Note The LARGER the Ka and Kb value the stronger the acid or baseOR The SMALLER the pKa or pKb value, the stronger the acid or base.

Example What is the pH of 1.2 mol dm-3 ethanoic acid with Ka 1.7 x 10-5?

Ka = [A - ][H 3O + ] [HA]

1.7 x 10-5 = [H3O + ] 2 1.2 0.0000204 = [H3O+]2

Note the acid dissociates in a 1: 1 molar ratio of hydrogen ions and ethanoate ions, therefore the concentration of hydrogen ions and ethanoate ions would be equal.

0.0045 = [H3O+] pH = -log 0.0045 = 2.3

Titration curves for acid-base reactions

NB The equivalence point is the stiochiometric ratio of reagents for complete reaction. This can be determined only via calculations.These curves are based on the fact that alkali is in the conical flask. If acid was in the flask and alkali was added, the mirror image of these titration curves would be obtained.

pH range of an indicatorAll indicators change colour as pH of the mixture changes. The range of pH values in which an indicator changes colour is called the pH range of the indicator.

Indicator pH rangeMethyl orange 3.1 – 4.4

Strong acid vs strong base Strong acid vs weak base

Weak acid vs strong baseWeak acid vs weak base

In all cases, indicators become protonated in acidic conditions and their configuration changes which causes a change in the absorption of light, resulting in a different colour being seen than when neutral or basic.

Phenolphthalein 8.3 – 10.0

Suitability of an indicator for an acid-base reaction

If the pH range of an indicator occurs within the inflexion point (the vertical portion of the titration curve), it means that it is suitable. If the pH range occurs outside the inflexion point, it is unsuitable for the titration.

weak acid vs strong base weak acid vs weak base

In

the strong acid vs strong base, both indicators shown are suitable as their pH ranges are in the

inflexion point. In the strong acid vs weak base, only the methyl orange is suitable, while phenolphthalein is unsuitable.

Strong acid vs strong base Strong acid vs weak base

In the weak acid vs strong base curve, only phenolphthalein is suitable.In the weak acid vs weak base, none of the two indicators are suitable.

Worksheet