5.1.3 Acids, Bases and Buffers

Bronsted-Lowry acid

Proton donor

Bronsted-Lowry base

Proton acceptor

Lewis acid

Electron pair acceptor

Lewis base

Electron pair donor

H+ (hydrogen ion) or H3O+ (oxonium ion)

Ion that causes a solution to become acidic.

-OH (hydroxide ion)

Ion that causes a solution to become alkaline.

Ionisation of water equation

2H2O (l) ⇌ H3O+ (aq) + -OH (aq) OR H2O (l) ⇌ H+ (aq) + -OH (aq)

Monobasic acid example

HCl

Dibasic acid example

H2SO4

Tribasic acid example

H3PO4

Acid base pairs for CH3COOH + H2O ⇌ CH3COO- + H3O+

Acid 1: CH3COOH, Base 1: H2O, Base 2: CH3COO-, Acid 2: H3O+

Strong acid

Acids dissociate completely.

Examples of strong acids

Hydrochloric acid, Sulfuric acid, Nitric acid.

Difference between concentrated and strong acids

Concentrated means many mol per dm3, strong refers to amount of dissociation.

Weak acids

Acids that only partially dissociate.

Examples of weak acids

Methanoic acid, any organic acid.

Constant used to measure the extent of acid dissociation

Acid dissociation constant.

Symbol of acid dissociation constant

Ka.

Acid dissociation constant expression

Answer.

Larger Ka value meaning

Larger the Ka - greater the extent of dissociation.

Equation to convert Ka into pKa

pKa = -log10Ka.

Equation to convert pKa into Ka

Ka = 10-pKa.

Relationship between pKa and strength of the acid

Smaller the pKa stronger the acid.

Equation to convert concentration of H+ into pH

pH = -log[H+].

Equation to convert pH into concentration of H+

[H+] = 10-pH.

pKa

Smaller the pKa stronger the acid

pH equation from H+ concentration

pH = -log[H+]

H+ concentration from pH

[H+] = 10^-pH

Usefulness of pH scale

pH scale allows a wide range of H+ concentration to be expressed as simple positive values

Relationship between pH and [H+]

High pH value means a small [H+]

pH difference and [H+] difference

A factor of 10

[H+] of a strong acid

[H+] = [HA]

pH calculation assumption for weak acids

It is assumed that the concentration of acid at equilibrium is equal to the concentration of acid after dissociation. This is because only very little of the acid dissociates.

Ionic product of water (Kw)

Kw = [H+][OH-]

Units for Kw

mol^2dm^-6

Value of Kw at 298 K

1.0 x 10^-14

Factors affecting Kw

Temperature only - if temperature is increased, the equilibrium moves to the right so Kw increases and the pH of pure water decreases.

Sum of indices of [H+] and [OH-]

-14

Strong base definition

Base that dissociates 100% in water.

Examples of strong bases

NaOH, KOH, Ca(OH)2

Example of a weak base

Ammonia

[H+] calculation for strong bases

[H+] = Kw / [OH-]

Buffer solution definition

A mixture that minimises pH change on addition of small amounts of an acid or a base.

Ways to make buffers

Weak acid and its conjugate base, Weak acid and a strong alkali.

Equilibrium shift when acid is added to buffer

Equilibrium shifts to the left because [H+] increases and the conjugate base reacts with the H+ to remove most of the H+.

Equilibrium shift when alkali is added to buffer

The equilibrium shifts to the right.

Equilibrium shift

Equilibrium shifts to the right, because [OH-] increases and the small concentration of H+ reacts with OH-. To restore the H+ ions HA dissociates shifting the equilibrium.

Buffer solution [H+]

Write the equation used to calculate [H+] of buffer solution.

Blood pH buffer system

H+ + HCO3-- ⇌ CO2 + H2O. Add OH- → reacts with H+ to form H2O, then shifts equilibrium left to restore H+ lost. Add H+ → equilibrium shifts to the right, removing excess H+.

Titration

The addition of an acid/base of known concentration to a base/acid to determine the concentration. An indicator is used to show that neutralization has occurred, as is a pH meter.

Equivalence point

The point at which the exact volume of base has been added to just neutralise the acid, or vice-versa.

End point

The point at which pH changes rapidly.

Properties of a good indicator

Sharp colour change (not gradual) - no more than one drop of acid/alkali needed for colour change. End point must be the same as the equivalence point otherwise titration gives wrong answer. Distinct colour change so it is obvious when the end point has been reached.

Indicator for strong acid-strong base titration

Phenolphthalein or methyl orange, but phenolphthalein is usually used as clearer colour change.

Indicator for strong acid-weak base titration

Methyl orange.

Indicator for strong base-weak acid titration

Phenolphthalein.

Indicator for weak acid-weak base titration

Neither methyl orange nor phenolphthalein is suitable, as neither give a sharp change at the end point.

Methyl orange in acid and alkali

Red in acid; yellow in alkali.

Phenolphthalein in acid and alkali

Colourless in acid; red in alkali.

Bromothymol blue in acid and alkali

Yellow in acid and blue in alkali.

Using a pH meter

Remove the pH probe from storage solution and rinse with distilled water. Dry the probe and place it into the solution with unknown pH. Let the probe stay in the solution until it gives a settled reading.