Chapter 20 - Acids, Bases and pH

Define Bronstead-Lowry acid

Proton donor

Define Bronstead-Lowry base

Proton acceptor

What is a conjugate acid-base pair

Two species which can be interconverted by transfer of H+

How can you identify the acid and the base in a conjugate acid-base pair

Acid - has the H, it accepts the proton

Base - doesn’t have the H, it releases the proton

What do monobasic, dibasic and tribasic refer to

Refer to the total number of hydrogen ions in the acid that can be replaced per molecule in an acid-base reaction. E.g. in monobasic acid, one mole of acid dissociates to form one mole of H+

E.g. Monobasic - HCl

Dibasic - H2CO3

Tribasic - H3BO3

Reactions of acids with:

• Metal

• Carbonate

• Metal oxide (base)

• Alkali

• Metal + Acid → Salt + Hydrogen

• Acid + Carbonate → Salt + Water + Carbon dioxide

• Acid + Base → Salt + Water

• Acid + Alkali → Salt + Water

What do the values of the pH scale (1-14) mean

pH < 7 → Acidic

pH = 7 → Neutral

pH > 7 → Alkaline

Equations for pH and [H+] concentration calculations

How to calculate pH of strong acids

Strong acids completely dissociates therefore the [H+] is equal to the concentration of the acid

Define strong and weak acids and give examples

Strong acid - Completely dissociates in aqueous solution e.g. HCl

Weak acids - Partially dissociates in aqueous solution e.g. ethanoic acid

What is Ka and give the equation for Ka

Ka is the acid dissociation constant

[H+] → H+ concentration

[A-] → Concentration of conjugate base

[HA] → Concentration of weak acid

What does larger Ka mean

Larger Ka means greater dissociation and therefore stronger acid

How do you convert between Ka and pKa

What are the two approximations for finding pH of weak acids

Approximation 1:

[H+] = [A-] , therefore in equation the top part of the equation can simply be written as [H+] squared

Approximation 2:

Dissociation of weak acids is small so you can neglect any decrease in concentration, therefore [HA] at start = [HA] equilibrium

What is the simplified Ka expression

How can Ka for a weak acid be found experimentally

• Prepare standard solution of the weak acid of known concentration

• Measure pH using a pH meter

What is Kw and give the equation

Kw - the ionic product of water, value of (1.00 × 10^-14 mol² dm^-6)

Kw = [H+] [OH-]

What is a strong base and weak base and give examples

Strong base - An alkali which completely dissociates in aqueous solution releasing OH- ions e.g. NaOH

Weak base - An alkali which partially dissociates in aqueous solution releasing OH- ions e.g. NH3