13: Acid-Base Equilibria
Acid-base chemistry can be extremely confusing, particularly when dealing with weak acids and bases. This set of lessons presents an updated view of the Brønsted-Lowry theory that makes it easy to understand answers to common questions: What's the fundamental difference between a strong acid and a weak acid? Can acid A neutralize base B? Why are some salts acidic and others alkaline? How do buffers work? What governs the shapes of titration curves?
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- 13.1: Introduction to Acid/Base Equilibria
- Acid-base reactions, in which protons are exchanged between donor molecules (acids) and acceptors (bases), form the basis of the most common kinds of equilibrium problems which you will encounter in almost any application of chemistry. Acid-base reactions, in which protons are exchanged between donor molecules (acids) and acceptors (bases), form the basis of the most common kinds of equilibrium problems which you will encounter in almost any application of chemistry.
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- 13.2: Strong Monoprotic Acids and Bases
- To a good approximation, strong acids, in the forms we encounter in the laboratory and in much of the industrial world, have no real existence; they are all really solutions of H3O+. So if you think about it, the labels on those reagent bottles you see in the lab are not strictly true! However, if the strong acid is highly diluted, the amount of H3O+ it contributes to the solution becomes comparable to that which derives from the autoprotolysis of water.
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- 13.3: Finding the pH of weak Acids, Bases, and Salts
- Most acids are weak; there are hundreds of thousands of them, whereas there are fewer than a dozen strong acids. We can treat weak acid solutions in much the same general way as we did for strong acids. The only difference is that we must now take into account the incomplete "dissociation"of the acid. We will start with the simple case of the pure acid in water, and then go from there to the more general one in which salts of the acid are present - these are known as buffer solutions.
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- 13.4: Conjugate Pairs and Buffers
- We often tend to regard the pH as a quantity that is dependent on other variables such as the concentration and strength of an acid, base or salt. But in much of chemistry (and especially in biochemistry), we find it more useful to treat pH as the "master" variable that controls the relative concentrations of the acid- and base-forms of one or more sets of conjugate acid-base systems. In this Module, we explore this approach in some detail, showing its application to buffer solutions.
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- 13.5: Acid/Base Titration
- The objective of an acid-base titration is to determine Ca, the nominal concentration of acid in the solution. In its simplest form, titration is carried out by measuring the volume of the solution of strong base required to complete the neutralization reaction . The point at which this reaction is just complete is known as the equivalence point, which is distinguished from the end point, which is the value we observe experimentally.
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- 13.7: Exact Calculations and Approximations
- The methods for dealing with acid-base equilibria that we developed in the earlier units of this series are widely used in ordinary practice. Although many of these involve approximations of various kinds, the results are usually good enough for most purposes. Sometimes, however — for example, in problems involving very dilute solutions, the approximations break down, often because they ignore the small quantities of H+ and OH– ions always present in pure water.