Buffers. A buffer is a solution whose pH is resistant to change on the addition of relatively small quantities of an acid or base. Buffers have the ability.

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Presentation transcript:

Buffers

A buffer is a solution whose pH is resistant to change on the addition of relatively small quantities of an acid or base. Buffers have the ability to absorb added H + or OH - ions.

Acidic Buffers Contain a weak acid and its salt formed with a strong base Ethanoic acid & Sodium ethanoate CH 3 COOH & CH 3 COO - Na +

Buffer Action The important species present in the buffer solution are the undissociated weak acid CH 3 COOH and its conjugate base CH 3 COO -

Undissociated ethanoic acid can remove any added OH - ions. CH 3 COOH + OH - → CH 3 COO - + H 2 O The conjugate base can remove any added H + ions CH 3 COO - + H + → CH 3 COOH

For a buffer to work effectively it must contain a large reservoir of the weak acid and its conjugate base. CH 3 COOH + H 2 O CH 3 COO - + H 3 O + CH 3 COOH CH 3 COO - + H + Write an expression for K a state units

K a = [CH 3 COO - ] [H + ] [CH 3 COOH] CH 3 COOH CH 3 COO - + H + [H + ] = K a x [CH 3 COOH] [CH 3 COO - ] If both [CH 3 COOH] and [CH 3 COO - ] are large………..

[H + ] = K a x [CH 3 COOH] [CH 3 COO - ] If both [CH 3 COOH] and [CH 3 COO - ] are large small changes in their concentrations will not affect the overall ratio significantly so [H + ] remains almost constant So very small change in pH

Basic Buffers Contain a weak base and its salt formed with a strong acid Ammonia & Ammonium chloride NH 3 & NH 4 + Cl -

Buffer Action The important species present in the buffer solution are the undissociated weak base NH 3 and its conjugate acid NH 4 +

Undissociated ammonia can remove any added H + ions. NH 3 + H + → NH 4 + The conjugate acid can remove any added OH - ions NH OH - → NH 3 + H 2 O

Calculating the pH of a Buffer When performing acidic buffer calculations it is assumed that The acid is completely undissociated The A - ions are formed solely from the salt

K a = [H + ] [A - ] [HA] HA H + + A - [H + ] = K a x [HA] [A - ] From salt Weak acid completely undissociated

Calculate the pH of a buffer solution produced by adding 3.28g of sodium ethanoate to 1dm 3 of 0.01M ethanoic acid. The K a of ethanoic acid is 1.84 x at 300K moles = mass/Mr 1. Find the number of moles of sodium ethanoate Moles = 3.28/82 = Find the [CH 3 COOH] and [CH 3 COO - ] [CH 3 COOH] = 0.01M[CH 3 COO - ] = 0.04M

3. Find [H + ] [H + ] = K a x [CH 3 COOH] [CH 3 COO - ] [H + ] = 1.84 x x = 4.6 x Find pH pH = -log[H + ] pH = -log 4.6 x = 5.34

Calculate the pH of this buffer if 10cm 3 of 0.1M HCl are now added 1. Find the number of moles of acid added moles = conc x volume moles of acid = 0.1 x 10/1000= 1 x CH 3 COO - + H + → CH 3 COOH 1 x x → 1 x 10 -3