pH curves (titration curves)

Point then there is a really steep plunge if you

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Unformatted text preview: l amount until quite near the equivalence point. Then there is a really steep plunge. If you calculate the values, the pH falls all the way from 11.3 when you have added 24.9 cm3 to 2.7 when you have added 25.1 cm3. Note: If y ou need to k now how to c alc ulate pH c hanges during a titration, y ou may be interes ted in my c hemis try c alc ulations book . Running alkali into the acid This is very similar to the previous curve except, of course, that the pH starts off low and increases as you add more sodium hydroxide solution. Again, the pH doesn't change very much until you get close to the equivalence point. Then it surges upwards very steeply. Titration curves for strong acid v weak base This time we are going to use hydrochloric acid as the strong acid and ammonia solution as the weak base. Running acid into the alkali Because you have got a weak base, the beginning of the curve is obviously going to be different. However, once you have got an excess of acid, the curve is essentially the same as before. At the very beginning of the curve, the pH starts by falling quite quickly as the acid is added, but the curve very soon gets less steep. This is because a buffer solution is being set up composed of the excess ammonia and the ammonium chloride being formed. Note: You c an find out more about buffer s olutions by following this link . Howev er, this is a v ery minor point in the pres ent c ontex t, and y ou would probably do better to read the whole of the c urrent page before y ou follow this up. Notice that the equivalence point is now somewhat acidic ( a bit less than pH 5), because pure ammonium chloride isn't neutral. However, the equivalence point still falls on the steepest bit of the curve. That will turn out to be important in choosing a suitable indicator for the titration. Running alkali into the acid At the beginning of this titration, you have an excess of hydrochloric acid. The shape of the curve will be the same as when you had an excess of acid at the start of a titration running sodium hydroxide solution into the acid. It is only after the equivalence point that things become different. A buffer solution is formed containing excess ammonia and ammonium chloride. This resists any large increase in pH - not that you would expect a very large increase anyway, because ammonia is only a weak base. Titration curves for weak acid v strong base We'll take ethanoic acid and sodium hydroxide as typical of a weak acid and a strong base. Running acid into the alkali For the first part of the graph, you have an excess of sodium hydroxide. The curve will be exactly the same as when you add hydrochloric acid to sodium hydroxide. Once the acid is in excess, there will be a difference. Past the equivalence point you have a buffer solution containing sodium ethanoate and ethanoic acid. This resists any large fall in pH . Running alkali into the acid The start of the graph shows a relatively rapid rise in pH but this slows down as a buffer solution...
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This note was uploaded on 09/29/2013 for the course CHEM 14CL taught by Professor Henary during the Fall '08 term at UCLA.

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