Brønsted–Lowry Acids, pH & Kw (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching A-Level Chemistry, and this video is about Brønsted-Lowry acids and bases, pH, and the ionic product of water.

Brønsted-Lowry defines what an acid or base actually is, pH is the scale used to measure them, and the ionic product of water, Kw, is what lets that same pH approach be extended to bases, which is why these three topics are covered together.

A Brønsted acid is a species that can donate a proton, and a Brønsted base is one that can accept a proton. The acidity of a solution depends on how many of those protons, known as H plus ions, are actually in it. pH is the logarithmic scale used to measure that: pH equals minus log of the hydrogen ion concentration. Water itself dissociates slightly into H plus and OH minus ions, and the equilibrium constant for that, Kw, links the two concentrations together — which means the same pH approach used for acids can be extended to find the pH of bases too.

This video starts with what Brønsted-Lowry acids and bases actually are, then covers the pH scale and how to calculate it from the concentration of H plus ions, before finishing with the ionic product of water and how it's used to find the pH of bases.

A Brønsted-Lowry acid is a species that can donate a proton — hydrogen chloride, for example, donates a proton to form H plus and Cl minus ions. A Brønsted-Lowry base is a species that can accept a proton, as a hydroxide ion does when it accepts H plus to form water. In an equilibrium reaction like a weak acid dissociating, both reactants and products are present at the same time, and the acid and the species it becomes are linked by the transfer of one proton — these are called conjugate acid-base pairs. This is the definition that everything else in this video builds on: pH measures how much of this proton transfer has happened, and Kw is what makes the same idea work for bases.

The acidity of a solution depends on how many H plus ions it contains, and because that concentration can range so widely, a logarithmic scale is used to measure it. pH equals minus log of the hydrogen ion concentration. Rearranging that equation gives the hydrogen ion concentration as 10 to the power of minus pH. Because it's a logarithmic scale, each pH value is ten times the one below it — pH 5 is ten times more acidic than pH 6 — and pH values are normally given to two decimal places. For a strong acid, which is completely ionised in solution, the hydrogen ion concentration is simply equal to the acid concentration.

Water dissociates slightly into hydrogen ions and hydroxide ions, and the equilibrium constant for that reaction is called the ionic product of water, Kw. The product of the two ion concentrations is always 1 times 10 to the power of minus 14 at 25 degrees celsius.

Because the dissociation of water is endothermic, increasing the temperature shifts the equilibrium to produce more H plus and OH minus ions, so Kw increases and the pH of pure water decreases as temperature rises. For a strong base, which is completely ionised, the hydroxide ion concentration is equal to the base concentration. Because Kw links the concentrations of H plus and OH minus ions, it can be used to find the pH of that base in exactly the same way pH is found for an acid.

Always give pH to two decimal places. And make sure you know how to use the antilog function on your calculator to reverse a pH calculation — on most calculators it's the 10 to the x button, but on others it's labelled LOG to the minus one, ALOG, or a two-button sequence like INV plus LOG.

An acid is a proton donor and a base is a proton acceptor — that's the definition everything else here is built on. pH measures the resulting hydrogen ion concentration on a logarithmic scale, as minus log of the concentration of H plus ions. The ionic product of water, Kw, is always 1 times 10 to the power of minus 14 at 25 degrees celsius, linking the concentrations of H plus and OH minus ions. And because of that link, the same pH approach used for acids extends to bases too.