chem 3.9 - acid-base equilibria

what is an acid?

a proton donor

what is an acid? (lowry-bronsted theory)

a substance that dissolves to give hydrogen ions (H+) when they are dissolved in water

what is a monobasic acid?

an acid that donates one proton (H+) per molecule

HCl(aq) → H+(aq) + Cl-(aq)

what is a dibasic acid?

an acid that can donate two protons per molecule

H₂SO₄(aq) → H+(aq) + HSO4-(aq)

what is a tribasic acid?

an acid that can donate three protons per molecule

H₃PO₄(aq) → H+(aq) + H₂PO₄-(aq)

what is a base?

a proton acceptor

acid-base pair equartion

HA(aq) + H2O ⇌ H3O+(aq) + A-(aq)

HA - acid 1 (conjugate acid of base 1)

H2O - base 2 (conjugate base of acid 2)

H3O+ - acid 2 (conjugate acid of base 2)

A- - base 1 (conjugate base of acid 1)

what does the pH scale measure?

the concentration of H+(aq) ions

how do you calculate pH?

pH = -log₁₀[H+]

How do you calculate [H+]?

[H+] = 10^-pH

describe the pH scale?

logarithmic scale

low pH means high [H+], high pH means low [H+]

what does a logarithmic scale for pH mean?

a change of 1 in pH means a change of 10 in the [H+']

what is a strong acid?

an acid that completely dissolves in aqueous solutions to produce H+ ions and equilibria lie completely to the right

HA(aq) ⇌ H+(aq) + A-(aq)

examples of strong acids

HCl

HNO3

H2SO4

what can we assume about [H+] in strong acids?

[H+] is the same as the concentration of the acid

[H+] = [acid]

how do you calculate pH of strong acid?

find [H+] - use concentration of acid

use -log10 to find pH

what is the expression for the equilibrium constant of the self ionisation of water?

not Ka its Kc btw

what is Kw?

the ionic product of water

1.01 × 10^-14 mol²dm^-6 at 25 degrees

what is the expression for Kw?

Kw = [H+][OH-]

• can rearrange to find OH- or H+

relationship between [H+] and [OH-] in pure water and neutral solutions?

[H+] = [OH-]

relationship between [H+] and [OH-] in acidic solutions?

[H+] > [OH-]

relationship between [H+] and [OH-] in alkaline solutions?

[H+] < [OH-]

How do you calculate the pH of a strong base?

find H+ using [H+] = Kw/[OH-]

use -log10 to find pH

what are weak acids?

acids that only partially dissociate in aqueous solutions and the equilibria lie to the left

CH3COOH ⇌ CH3COO- + H+

what can we assume about [H+] in weak acids?

[H+] = [A-]

what can we assume about [HA] in weak acids?

[HA] = [acid]

what are the units of Ka?

moldm-3

what does Ka indicate?

the strength of an acid

how can you calculate Ka?

Ka = 10^-pKa

how can you calculate pKa?

pKa = -log₁₀Ka

what does a higher Ka mean?

lower pKa

stronger acid

what does a lower Ka mean?

higher pKa

weaker acid

what is the expression for Ka of weak acids?

how do you calculate the pH of a weak acid?

use Ka = [H+]²/[acid]

rearrange to find H+ using [H+]²=√Ka x [acid]

use -log10 to find pH

what is a buffer?

a solution that resists change in pH on addition of small amounts of an acid or a base

cannot prevent pH from changing completely but pH changes are minimised

what is a buffer a mixture of?

a weak acid (HA) and its conjugate base (A)

how can you make a buffer? (method 1)

mix together a weak acid and soluble salt of the weak acid

e.g. ethanoic acid and sodium ethanoate

how can you make a buffer? (method 2)

add a strong base to double the quantity of a weak acid

base will neutralise half of the acid -`. forms its conjugate base

conjugate base forms mixture with the other half of the acid which remains in the solution

how does a buffer work if an acid is added?

H+ conc increases

reaction is reversible

so equilibrium moves to left

most of additional H+ is removed

because A- and HA are so big, when equilibrium moves these changes are negligible so H+ remains constant

how does a buffer work if an alkali is added?

H+ conc decreases

reaction is reversible

added OH- ions removes H+ ions

so equilibrium moves to right

because A- and HA are so big, when equilibrium moves these changes are negligible so H+ remains constant

what are some examples where buffer solutions are used?

fermentation using yeast

dyeing cloth

using or storing enzymes of pharmaceuticals

human blood

label the titration curve

what is the equivalence point of a titration curve?

vertical section

region where an indicator works if the pH range of that indicator falls in the vertical region

how do you calculate the pH of a solution of acid-base reactions?

1. work out moles of acid

2. work out moles of base

3. find difference between moles to get excess mol of acid or base

if H+ (acid) is in excess

4. conc of H+ = mol/vol

5. pH = -log[H+]

If OH- (base) is in excess

4. conc of OH- = mol/vol

5. H+ = Kw/OH- = 10^-14/OH-

6. pH = -log[H+]

how do you calculate the pH of a buffer? ( equations)

or

[H+] = Ka x [HA]/[A-]

or

[H+] = Ka x [ACID]/[SALT]

how do you step by step calculate pH of a buffer?

1. calculate mol of acid → use mol = conc x vol

2. calculate mol of salt → use mol = conc x vol

3. calculate conc of acid AND salt → mol/vol (use total volume)

4. sub into [H+] = Ka x [ACID]/[SALT]

5. pH = -log[H+]

if a salt is formed from a strong base and a strong acid, what will its pH be?

neither of the ions they contain interact with water

neutral salt

pH = 7

e.g. NaCl, KBr

if a salt is formed from a strong base and a weak acid, what will its pH be?

salt will dissociate to produce conjugate base of weak acid

conjugate base will accept proton from surrounding water molecules

there will be excess of OH- in solution

so pH > 7

if a salt is formed from a weak base and a strong acid, what will its pH be?

salt will dissociate to produce conjugate acid of weak base

conjugate acid will donate protons

pH < 7