buffers. buffers are solutions that resist changes in ph when small amounts of acid or base are...
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Buffers
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Buffers are solutions that resist changes in pH when small amounts of acid or base are added, or when they are diluted.
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There are two types of buffer, acidic and basic.
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Let’s look at the reactions that can take place in an acidic buffer made from ethanoic acid and sodium ethanoate.
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This reaction happens when hydroxide ions from an alkali are added to the buffer.
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Let’s look at the reactions that can take place in a basic buffer made from ammonia and ammonium chloride.
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The standard entropy change for the decomposition of calcium carbonate is positive.
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This reaction happens when hydrogen ions from an acid are added to the buffer.
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This reaction happens when hydroxide ions from an alkali are added to the buffer.
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This expression shows the relationship between the acid dissociation constant, and the concentrations of the hydrogen ions, the acid and its salt.
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The acid dissociation constant gives a rough guide to the pH of the buffer.
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The ratio of the concentrations of acid and its salt provides a 'fine tuning' of a buffer's pH. For an effective buffer, this ratio must not be too large or too small.
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We can use this expression to calculate the pH of a buffer if the acid dissociation constant and concentrations of acid and salt are known.
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The expression can be rearranged to find the hydrogen ion concentration.
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Let's use this expression to work out the pH of an acidic buffer containing ethanoic acid and sodium ethanoate.
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The Ka value for ethanoic acid is 1.7 x 10–5 mol dm–3 at 298 K.
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In our example, the concentration of ethanoic acid is 0.1 mol dm–3.
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In our example, the concentration of sodium ethanoate is 0.2 mol dm–3.
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This means that the hydrogen ion concentration is 8.5 x 10–6 mol dm–3.
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Using pH = –log10[H+(aq)], we find that the pH of this buffer would be 5.07 at 298 K.