Acids, Bases and Buffers

Acids, Bases and Buffers

History and Definitions

• Your task is to research the history of acids and bases.

• Doing this you will need to find out about Arrhenius, Bronsted & Lowry and Lewis.

• By the end of the lesson you will also need to definitions of acids and bases- with examples as equations.

Bronsted-Lowry

• Bronsted-Lowry acids and bases– A Bronsted-Lowry acid is any substance from

which a proton can be removed– A Bronsted-Lowry base is any substance that can

remove a proton from an acid

• A single proton doesn’t really exist in a solution. Acids only release protons if a base can accept it.

Conjugate Acid-Base Pairs

• Instead of a floating proton in solution, water molecules accept protons to form hydronium ions, H3O+

(aq)

• This is sometimes called an oxonium ion.

This is an acid

This is a thing that can accept a proton, it’s the acids conjugate baseWhich one is the base and conjugate

acid between these two?

An acid-base pair is a set of two species that transform into each other by gain or loss of a proton

Calculations

• Practice questions on page 139.

pH

pH

• Don’t ask what it means. Noone knows.

• pH is all about the concentration of hydrogen ions in solution.

• It is a logarithmic scale of concentration of hydrogen ions.

pH calculations

pH = -log[H+(aq)]

[H+] = 10–pH

Your Calculator

Calculations

• Attempt calculations on page 141 of text book.

Strong and Weak Acids

Strong Acids

• Strong acids completely dissociate in aqueous solution.

• Only a few exist, the rest are weak.– HCl -HI– HNO3 -HClO4

– H2SO4

– HBr

Weak Acids

• Weak acids only partially dissociate in aqueous solution, the equilibrium lies well to the left.

Ka The Acid Dissociation Constant

• A weak acid has the following equilbrium:HA H+ + A-

• The expression for the acid dissociation constant is:

Ka =

• Units are always:

Ka Context

• A strong acid has a high Ka value.• A weak acid has a small Ka value.

• Can also convert these into logs, which makes the numbers more manageable.

pKa = -log10Ka

Ka = 10-pKa

• Taking logs inverts the values. High pKa is a weak acid and vice versa.

pH of Strong Acids

• For a strong acid:HA(aq) H+

(aq) + A-(aq)

–HA totally dissociates: [HA] = [H+]–Use pH = -log[H+]

• A bottle of HCl has a concentration of 1.22 x10-3 mol dm-3. What is the pH?

pH of Weak acids

• For a weak acid:HA(aq) H+

(aq) + A-(aq)

–HA only partially dissociates.–H+ and A- are formed equally. [H+]=[A-] –In our equation for Ka: [H+][A-] = [H+]2

–Due to the small partial dissociation we can assume that the equilibrium concentration of HA is the same as the start concentration. This gives us the equation:

pH of Weak Acids

Ka = [H+]2

[HA]

Or

[H+] = Ka x [HA]

Weak Acid Practice

• A sample of nitric acid, HNO2, has the concentration 0.055 mol dm-3. Ka = 4.70 x10-4

mol dm-3 at 25oC. Calculate the pH.

• Ka =• [H]+ =• pH =

Kw

Ionisation of Water

H2O H+ + OH-

equilibrium

Kc = [H+][OH-]

[H2O]

Kw

• Rearranging this gives usKc x [H2O] = [H+] [OH-]

• [H2O] is always 55.6 mol dm-3

• Kc is a constant too. We can create a new constant Kw

• Kw = [H+] [OH-]

Significance of Kw

• At 25oC Kw = 1.00 x 10-14 mol2 dm-6

– This number is found on your data sheet.• This is due to the pH of water being 7.• So [H+] = [OH-] = 10-7

• This value of Kw lets us calculate [OH-] if we know [H+].

pH of Bases

pH of Bases

• Using the Kw equation we can calculate the pH of a strong base.

• We wont have to calculate pH for a weak base.

• The strength of a base is a measure of its dissociation in solution to produce OH- ions.– Exactly like acids.

pH Calculations

• To work out the pH of a base we need to know two things

• [H+]• Kw

• For a strong base it totally dissociates. [OH-] = [base]

Example

• A solution of KOH has a concentration of 0.050 mol dm-3. What is it’s pH?

• Kw = • [H+] = • pH =