Acids and Bases

Topic 3.4.3

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  • Created by: Mary
  • Created on: 06-03-13 17:34

Definitions

acid = proton donor

base = proton acceptor

Therefore acid-base equilibria involves the transfer of protons. 

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pH

pH = -log10[H+]

where [ ] represents the concentration in mol dm-3

[H+] = 10 ^ -pH

A strong acid is almost completely ionised in water, therefore to find out its concentration you can assume the concentration of the acid is equal to the concentration of H+ and put it into the above formula. 

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Ionic Product of Water

This is given the abbreviation, Kw

Water can be thought of as a product of its ions, H+ and OH-. Therefore the ionic product of water is:

Kw = [H+][OH-] = 1.0 x 10-14 mol2 dm-6  <<< You must learn this value!

Therefore if you know [OH-] you can work out the [H+] and the pH.

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Weak Acids and Bases

Only dissociate slightly in water, therefore when writing the equation for Ka you do not include [H2O]!

We also have another term, pKa that is related to Ka:

pKa = -log10Ka

For a weak acid you can work out the concentration of [H+] using the expression for Ka and put it into the equation for pH. 

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pH Curves

Througout this I will use the abbreviations (S = strong, W = weak, A = acid, B = base)

SASB: the pH curve will start off at low pH (ie: pH 1) and end at high pH (ie: pH 13)

SAWB: will start off at low pH and end about pH 10

WASB: the pH curve will start off about pH 4 and end at pH 13

WAWB: the pH curve will start off about pH 4 and end about pH 10

Be careful if the question states that acid is added to the alkali, in this case the curve will start off at high pH and end at a lower pH

Halfway up the vertical portion of the curve is the equivalence point - the point at which the solution has been neutralised. 

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Choosing an Indicator

The indicator used depends on its pH range. The indicator's pH range must fall between that of the vertical section.

You do not need to remember the pH range of common indicators, just be able to use the information given to choose the appropriate one for a given titration. 

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Calculations

Monoprotic acids: contain one H+ ion per acid molecule

Diprotic acids: yield two H+ ions per molecule (eg: H2SO4). There will be two vertical sections in the pH curve and the volume of alkali required to neutralise the acid will be twice that required to neutralise the first equivalence point. 

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Buffers

A buffer solution is one which resists changes in pH when small amounts of acid or base are added to it. LEARN!

Acid buffers: consists of a weak acid and a salt of that acid. 

CH3COOH = CH3COO- + H+

Basic buffers: consist of weak bases and a salt of its base. It produces OH- ions, therefore the pH is above 7. 

NH3 + H2O = NH4+ + OH-

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Uses of Buffers

  • in the body, a buffer of carbonic acid and hydrogen carbonate ions maintains the pH of blood at the correct level
  • buffers are added to shampoo and other cosmetics to balance the alkalinity so they do not irritate your scalp or skin
  • used to maintain the correct pH for dying fabrics in the textile industry
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pH of an Acidic Buffer Solution

In order to calculate the pH of an acidic buffer solution, you need to write the expression for Ka. 

If you know the Ka, the concentration of the acid and the salt, you can work out the concentration of [H+] and therefore the pH. 

1. Ka = [salt][H+] / [acid]

2. Find [H+]

3. Find pH using pH = -log10[H+]

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