Lecture 27. Acid-base equilibria IV

Thursday 2 May 2024

Of course... Absolute zero!! The ideal gas law and applications. Mixtures of gases and partial pressures. Dalton's law of partial pressures. Collecting gases over water. Stoichiometry of gas-phase reactions.

Reading: Tro NJ. Chemistry: Structure and Properties (3rd ed.) - Ch.16, §16.1 - §16.7 (pp.713-736t)

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Summary

The ideal gas law

As we have seen, all of the simple gas laws can be captured in a single equation involving the state variables called the ideal gas law:

PV = nRT

where R is an experimentally determined constant of nature. Its value of course depends as well on the units we choose for the choose for the state variables. In particular, we have seen a number of different units are employed in measuring pressure. We will frequently use the value of 0.082057 L atm mol^–1 K^–1, but it is best to bear in mind that alternative values of R are used in other applications.

We also treat the case of gas mixtures, introducing and applying the quantity partial pressure, which is defined for each chemically distinct component of a mixture of gases, such as the earth's atmosphere.

The base hydrolysis equation and the definition of K_b

K_b is known as the base hydrolysis constant, or simply "base constant". It is defined by the chemical equation for abstraction by a base of a proton from water, called the base hydrolysis equation.

Note that the base hydrolysis equation is a complete Brønsted-Lowry acid-base reaction, with water as the reactant acid. The expression for the base constant is derived by following the usual rules for writing an equilibrium constant for a chemical reaction, with the additional feature that the concentration of water is dropped from the expression. This is because in dilute aqueous solution, the concentration of water remains practically constant.

Since the for any acidic or basic substance in water, the equilibrium for the autoionization of water must still be satisfied, we ought to be able to show that for any Brønsted-Lowry conjugate pair, K_b· K_b = K_w. In other words, the equilibrium relations are not independent!

The quantitative treatment of weak acid and weak base equilibria can be found here.