3.1–3.6The pH scale and neutralisation
Acids produce hydrogen ions (H⁺) in aqueous solutions. Aqueous solutions of alkalis contain hydroxide ions (OH⁻).
The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using universal indicator or a pH probe.
A solution with pH 7 is neutral. Aqueous solutions of acids have pH values of less than 7, and aqueous solutions of alkalis have pH values greater than 7.
In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water. This reaction can be represented by the equation:
H⁺(aq) + OH⁻(aq) → H₂O(l)
That one equation is the whole of neutralisation. Whichever acid and whichever alkali are used, the reaction that removes the acidity is always the same — the rest of the ions are spectators and end up as the salt.
You should be able to describe the use of universal indicator, or a wide range indicator, to measure the approximate pH of a solution, and use the pH scale to identify acidic or alkaline solutions.
The method is to add a few drops of universal indicator to the solution — or dip a strip of indicator paper into it — and match the colour against the chart. The word to notice is approximate: universal indicator gives a pH to the nearest whole number at best, which is why a pH probe is used when an accurate value is needed.
Common mistake
Universal indicator is the wrong choice for a titration. It changes colour gradually through a range of pH values, so there is no sharp end point — a single indicator such as phenolphthalein is used instead.
- pH scale
- a scale from 0 to 14 measuring the acidity or alkalinity of a solution; pH 7 is neutral, below 7 is acidic and above 7 is alkaline
3.4–3.5, 3.7–3.8Strong and weak acidsHigher tier
A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and sulfuric acids.
A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and carbonic acids.
For a given concentration of aqueous solutions, the stronger an acid, the lower the pH.
You should be able to use and explain the terms dilute and concentrated, in terms of amount of substance, and weak and strong, in terms of the degree of ionisation, in relation to acids.
These are two independent ideas, and confusing them is the classic error here. Concentrated and dilute describe how much acid was dissolved in a given volume — the amount of substance. Strong and weak describe what fraction of that acid has ionised to release hydrogen ions. A dilute solution of a strong acid and a concentrated solution of a weak acid are both perfectly ordinary things.
As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.
You should be able to describe neutrality and relative acidity in terms of the effect on hydrogen ion concentration and the numerical value of pH, using whole numbers only. This is an order of magnitude calculation: each whole step down the pH scale is a tenfold increase in hydrogen ion concentration.
Worked example
Solution A has pH 5 and solution B has pH 2. How many times greater is the hydrogen ion concentration in B?
- Find the difference in pH: 5 − 2 = 3 units.
- Each unit decrease multiplies the hydrogen ion concentration by 10.
- Three units means 10 × 10 × 10.
- So the factor is 10³.
Answer: 1000 times greater in solution B.
Common mistake
A lower pH means more hydrogen ions, not fewer. The scale runs the opposite way to the concentration, so pH 2 is a thousand times more acidic than pH 5, not three times.
- Strong acid
- an acid that is completely ionised in aqueous solution, such as hydrochloric, nitric and sulfuric acids
- Weak acid
- an acid that is only partially ionised in aqueous solution, such as ethanoic, citric and carbonic acids
- Concentrated
- describes a solution containing a large amount of solute in a given volume; a separate idea from strong, which is about the degree of ionisation
Before you practise
- An acid in solution is a source of hydrogen ions, H⁺; an alkali is a source of hydroxide ions, OH⁻.
- pH 7 is neutral, below 7 acidic, above 7 alkaline.
- Learn the indicator colours: litmus red in acid and blue in alkali, methyl orange red in acid and yellow in alkali, phenolphthalein colourless in acid and pink in alkali.
- The more hydrogen ions, the lower the pH; the more hydroxide ions, the higher the pH.
- Each factor of ten in hydrogen ion concentration changes the pH by exactly 1.
- Strong acids ionise completely in water; weak acids only partially.
- Strong is about how far it ionises; concentrated is about how much is dissolved. A concentrated weak acid is perfectly possible.
- A base reacts with an acid to give a salt and water only; an alkali is a base that dissolves.