Acids as proton donors, strong and weak acids, bases, alkalis and neutralisation, the reactions of acids with metals, carbonates and bases, salt preparation, and the techniques of standard solutions and acid-base titration.

What this topic is asking

OCR specification point 2.1.4 wants you to describe acids as proton donors, distinguish strong and weak acids, recall the reactions of acids with metals, carbonates, metal oxides and alkalis, prepare salts, and carry out the titration techniques of making a standard solution and finding an unknown concentration.

Acids, bases and alkalis

Common acids are hydrochloric ($\text{HCl}$), sulfuric ($\text{H}_2\text{SO}_4$) and nitric ($\text{HNO}_3$); common alkalis are sodium hydroxide and aqueous ammonia.

Strong and weak acids

Reactions of acids

Acids undergo three reaction types you must be able to write:

• With reactive metals: acid + metal $\rightarrow$ salt + hydrogen. Example: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$.
• With carbonates (or hydrogencarbonates): acid + carbonate $\rightarrow$ salt + water + carbon dioxide. Example: $\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2$.
• With bases (metal oxides and alkalis): acid + base $\rightarrow$ salt + water (neutralisation). Example: $\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$.

The ionic equation for neutralisation by an alkali is always $\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$.

Preparing a standard solution

A standard solution has an accurately known concentration. It is the foundation of every titration.

Titration technique

In an acid-base titration, a pipette delivers a fixed volume of one solution into a conical flask; the other is added from a burette until an indicator shows the end point. Repeat until concordant titres (within $0.10\ \text{cm}^3$) are obtained, then average only the concordant results.

Examples in context

Example 1. Standardising sodium hydroxide. Sodium hydroxide absorbs water and carbon dioxide from the air, so its exact concentration drifts. Chemists standardise it by titrating against a primary standard such as potassium hydrogenphthalate, applying the moles toolkit to fix the true concentration, exactly the technique tested in Paper 3.

Example 2. Antacid tablets. Indigestion remedies neutralise excess stomach acid using bases such as calcium carbonate or magnesium hydroxide. A back-titration of a dissolved tablet against standard acid measures how much base it really contains, linking the acid reactions of this topic to a real consumer product.

Try this

Q1. Write the equation for the reaction of nitric acid with potassium hydroxide. [1 mark]

• Cue. $\text{HNO}_3 + \text{KOH} \rightarrow \text{KNO}_3 + \text{H}_2\text{O}$.

Q2. Explain why a $0.1\ \text{mol dm}^{-3}$ solution of ethanoic acid has a higher pH than $0.1\ \text{mol dm}^{-3}$ hydrochloric acid. [2 marks]

• Cue. Ethanoic acid is weak and only partially dissociates, so $[\text{H}^+]$ is lower than for the fully dissociated strong acid.