Titrations of Acids and Bases, Part 2
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Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach Part 2 about Titrations of Acids and Bases. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
We’ve discussed the titration curves of monoprotic acids, or acids that dissociate to only form one hydrogen ion per molecule. For instance, HCl has only one hydrogen ion to donate per molecule of HCl. Here we discuss the titration of polyprotic acids, such as H2SO4, which contain multiple hydrogen ions per molecule. An acid with two hydrogen ions to donate is diprotic, one with three hydrogen ions is triprotic, and so on.
Polyprotic Titrations
The graph of a polyprotic titration is very different from the graph of a monoprotic titration. Recall that the graph of a monoprotic titration contained one steep region, containing the equivalence point, which eventually leveled off. A polyprotic titration contains multiple steep regions, each one containing the equivalence point for the dissociation of each one of the polyprotic acid’s hydrogen ions. As you might guess, each one of equivalence points has a corresponding half equivalence point. A half equivalence point on the graph of a polyprotic acid corresponds to the pKa of the hydrogen ions that is being deprotonated. For instance, the graph of the titration of H2SO4 has two equivalence points, one representing the deprotonation of H2SO4 to form HSO4-, and another representing the deprotonation of HSO4- to form SO42-. The first half equivalence point corresponds to the pKa of H2SO4, and the second half equivalence point corresponds to the pKa of HSO4-.
pH Indicators
pH indicators are weakly acidic or basic molecules that are colored differently when they're protonated versus deprotonated. For example, the molecule bromocresol green is yellow in its protonated state, but blue in its deprotonated state. Remember that the molecule is predominantly protonated below its pKa, and predominantly deprotonated above its pKa. Therefore, if we know the pKa of bromocresol green, and we observe a change in its color, we know that the pH of the solution changed from less than the pKa of bromocresol green to greater than the pKa. Put simply, if we know the pKa of a color indicator, a change in color means that the pH has reached the pKa of the indicator. This principle is extremely useful because it means that if we are interested in a solution at a particular pH, we can pick an indicator that has a pKa near the pH of interest and wait for a change in color to occur. As we add acid or base to the solution, if we see the color of the solution change, we know that the solution has reached our desired pH.
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4 سال پیش
در تاریخ 1399/06/25 منتشر شده
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