What are Strong Acids, Weak Acids and pH?

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What Exactly is an Acid?

We talk about acids but you may not have had a proper definition of acids, what it is that makes a substance an acid? When do we get to include the word “acid” in it’s name?

You may be surprised to find that there are several ways of defining acids (and bases). The most commonly used definition is the Brønsted-Lowry definition, and that’s the definition you are expected to know and use for your A-Level chemistry.

Brønsted-Lowry Acids and Bases

A Brønsted-Lowry acid is a substance that is able to donate an H⁺ ion.

A Brønsted-Lowry base is a substance that is able to accept an H⁺ ion.

Why are Acids So Reactive?

We already mentioned that acids are substances that are able to donate an H⁺ ion, and it’s the H⁺ ion’ nature that makes acids so reactive.

Let’s give some consideration to what an H⁺ ion is:

An H⁺ ion is a hydrogen atom that has lost one electron. And, of course, a hydrogen atom only had one electron to start with, so it hasn’t just lost an electron, it has lost its only electron.

And that makes all the difference. Why?

An H⁺ ion is very different from any other cation because of its’ size. Having lost its only electron just the nucleus of the atom remains. Just one proton, which is very much smaller than the atomic radius of even the smallest atoms.

That means that the proton’s positive charge is focused on that much smaller surface area, resulting in a much more intense electric field than other ions. This causes more intense attraction to negatively charged entities such as anions and lone electron pairs.

In case you’re wondering about it, it’s true that a very small proportion of H⁺ ions also have a neutron because they derived from a ²H atom. These also have a full 1+ charge and are also very much smaller than ions of other elements. Hence they also have a very intense electric field and are reactive like their neutron-free cousins.

Strong Acids and Weak Acids

Now we need to define what we mean by strong acids and weak acids, and to be clear about what it doesn’t mean.

A strong acid is an acid that can completely dissociate in water, so that every molecule dissociates into H⁺ ions and the remaining anion. The dissociation is complete, meaning there isn’t a reversible reaction and there isn’t an equilibrium to consider. Examples include sulfuric acid, nitric acid and hydrochloric acid.

A weak acid is an acid that only partially dissociates in water. In fact, only a small proportion of the molecules dissociate into H⁺ ions and the anions. The dissociation is a reversible reaction and a dynamic equilibrium will establish. And because there is an equilibrium, there is an equilibrium constant to consider – we will look at that in more detail in the next article and the next podcast episode.

Examples of weak acids include methanoic acid and ethanoic acid.

Your syllabus and your exam pay a lot of attention to weak acids, and to the calculations that come with this study. Weak acids are also a significant part of acidic buffers, which are also a key part of your syllabus. We will work through example calculations in future articles.

A Common Mistake

Please do not confuse the concentration of acids with their strength. Remember that strong acids and weak acids are defined by whether they dissociate completely in water or not.

So a strong acid may be dilute or concentrated, or somewhere in between.

And a weak acid may also be concentrated, dilute or somewhere in between.

If there is only one thing you take away from this article, let it be this: An acids concentration has nothing to do with whether it is a weak or a strong acid.

What is pH?

pH is a scale that allows us to easily show the acidity of a solution, and easily compare the acidity of two or more solutions. When we want to measure or compare acidity, we are actually measuring the H⁺ ion concentration of the solution.

How To Calculate pH

To calculate pH we need to know the H⁺ ion concentration of the solution. We apply a logarithmic function to get the pH value like this:

pH = – log [H⁺]

Note that [H⁺] means the concentration H⁺ ions in mol dm^-3

pH values don’t have units – just quote the numerical value.

Because strong acids dissociate completely it is usually easy to calculate their pH. But weak acids don’t behave like that – their dissociation is an equilibrium that varies with temperature. We will go into more details about the necessary calculations for weak acids in our next article.