Acids and bases

Acids

Acids

Proton donors (H+)

Bases

Proton acceptors

Alkalis

A type of base that dissolves in water to form hydroxide ions

Acid + alkali →

Salt + Water

Acid + carbonate →

Salt + water + carbon dioxide

Acid + metal →

Salt + hydrogen gas

Monoproctic acids

Each molecule can release 1 ptoron

Strong acids dissociate…

Fully in water

General acid dissociation equation

HA + H2O → H3O+ + A-

Weak acids

Partially dissociate in water

What type of reaction are weak acid dissociations

Reversible

General equation for weak acid dissociation

HA + H2O → ← H3O+ + A-

Where does equilibrium lie in weka acid dissociations

Well over to the left

Ka=

Acid dissociation constant

What are the Ka values for weak acids

Less than 1

Ka equation

Ka= ([H+(aq)] [A-(aq)]) / [HA]

General acid dissociation equation

HA→ H+ + A-

What does a larger Ka indicate about PoE

Favours the right hand side- is a stronger acid

What does a lower pKa value indicate

A stronger acid

pKa=

-log10(Ka)

Ka using pKa =

Ka= 10^-(pKa)

[H+ (aq) ] =

10^-(pH)

pH=

-log10([H+(aq)])

What type of scale is the pH scale and what does this mean about each value

Is logarithmic- each value differs by a factor of 10

What does a high [H+] indicate about pH

A low pH

What would a pH change of 1 mean for the [H+]

It would change x10

What equations calculate the pH of strong acids

[H+(aq)] = [HA(aq)] then pH= -log10^[H+(aq)]

How t roughly check pH of strong acid calculations

Should be less than 4

Equations too work out pH of weak acids

[H+(aq)] = sq rt ( Ka x [HA(aq)] ) then pH= -log10([H+(aq)]

What assumptions do we make when calculating pH of weak acids

1) equilibrium constant of HA= conc. of undissociated HA

2) conc. of H+ ions are = A- ions

Amphoteric

Substance can act as both an acid and a base

Example of water acting as a base

H2O + HCl → H3O + + Cl-

Water acting as an acid

H2O + NH3 → NH4+ + OH-

Kc equation for water

Kc= [OH-][H+] / [H2O] → will be less than 1

Extent of ionisation of water

Very small- approx 1 water molecule. In every 500,000 dissociates

Dissociation reaction of water

H2O (l) → H+(aq) + OH-(aq)

Where does PoE lie on water dissociation reaction

Well to the left

Kw definition

The ionic product of water

What is Kw made up from

Kc and [H2O]

Kw=

Kc x [H2O(l)] = [H+(aq)][OH-(aq)]

At 25 deg c, pH of H2O is 7 what is [H+(aq)]

10^-7

At 25 deg c, pH of H2O is 7, what does [OH-(aq)] =

10^7 as [H+]=[OH-}

At 25 deg c, pH of H2O is 7, what is Kw

[H+]x[OH-]= 10^7 × 10^7 = 10^14, Kw= 1 × 10^-14 mol² dm^-6

In water and neutral solutions [H+(aq)] =

[OH-(aq)]

In acidic solution s [H+(aq)]

>[OH-(aq)]

In alkaline solution [H+(aq)]

< [OH-(aq)]

At 25 deg c, what must Kw=

1×10^14 mol² dm^-6

Dissociation of NaOH

NaOH→ Na+ + OH-

[NaOH]=

[OH-]

How to find the pH of NaOH

Kw= [H+][OH] therefore [H+]= Kw/[OH-] , pH= -log10([H+])

What is the pH of KOH with a conc. of 0.05 mol dm^-3

[KOH]=[OH-] = 0.05, Kw= 1×10^14, [H+]= Kw/[OH-} = 1×10^-14/ 0.05= 2×10^-13, pH= -log10(2×10^-13)= 12.7

Buffer solution

A mixture that minimises pH changes on addition of small amounts of acid or base

What are buffers made from

Weak acid (HA) and its conjugate base (A-)

When an acid (H+) is added to a buffer solution, what happens

[H+] increases, conjugate base A- reacts with the excess H+ ions, PoE shifts to the left removing most of the excess H+ ions

When an alkali (OH-) is added to a buffer solution what happens

[OH-} increases, the small conc. of H+ ions react with the OH- ions (H+ + OH- → H2O), HA dissociates shifting the equilibrium right to restore most of the H+ ions that have reacted

What is pH dependent on

Acid dissociation constan, Ka, of buffer solution, conc. ration of weak acid: conjugate base

Find pH at 25deg c of a buffer containing 0.06 mol dm^-³ CH3COOH and 0.1 mole dm^-³ CH3COO-Na+, fro CH3COOH, Ka= 1.7 × 10^- 5

[H+] = (1.7 × 10^-5) x (0.05/ 0.1) = 8.5 × 10^6, pH= -log10(8.5 × 10^6) = 5.08

Step by step method to work out pH of Buffers

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

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

pH=-log([H+])

Example o

Carbonic acid, hydrogen carbonate buffer in blood

How does the Carbonic acid, hydrogen carbonate buffer work

Blood pH has to be between 7.35-7.45, carbonic acid is the wea acid, hydrogen carbonate is the conjugate base, most materials released into blood are acidic so HC3O- ions are removed by being converted into H2CO3 which then gets converted to dissolved CO2 and removed by the lungs

Equation for Carbonic acid, hydrogen carbonate buffer

H2CO3 → ← HCO3- + H+

What happens when

an acidic/ alkaline substance is added