Welcome to Acid-Base Equilibria!

In this chapter of the Elements of Life module, we are exploring the chemical "yin and yang" of the world: acids and bases. These substances are vital for life, from the hydrochloric acid in your stomach that helps digest food to the salts found in our blood and the sea. Don't worry if these terms sound a bit scary—we’re going to break them down into simple steps that anyone can follow!

1. The Language of Acids and Bases

Before we can make anything in the lab, we need to know what we are talking about. Chemistry has its own "dictionary" for these substances.

Key Terms to Know

  • Acid: A substance that releases hydrogen ions \( (H^+) \) in solution. Think of acids as proton donors.
  • Base: A substance that can neutralise an acid to form a salt and water.
  • Alkali: A base that dissolves in water, forming hydroxide ions \( (OH^-) \). Not all bases are alkalis, but all alkalis are bases!
  • Neutralisation: The chemical reaction where an acid and a base react together to form a salt and water.

The Three Mineral Acids

In your exams, you are expected to recognize and write the formulae for three specific mineral acids:

  1. Hydrochloric acid: \( HCl \)
  2. Nitric acid: \( HNO_3 \)
  3. Sulfuric acid: \( H_2SO_4 \)

Quick Review Box:
Acid + Base \(\rightarrow\) Salt + Water
Example: \( HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l) \)

2. Making Soluble Salts

A salt is formed when the hydrogen ions of an acid are replaced by metal ions (or ammonium ions). If a salt dissolves in water, it is a soluble salt. Here is how to prepare one in the laboratory from an acid and an insoluble base (such as a metal oxide or hydroxide).

Step-by-Step Guide:

  1. React: Warm the acid and add the solid base in excess until no more dissolves. This ensures all the acid has completely reacted.
  2. Filter: Filter the mixture to remove the unreacted excess solid base. The clear filtrate is the salt solution.
  3. Evaporate: Gently heat the filtrate in an evaporating basin to evaporate water until the crystallisation point is reached.
  4. Crystallise: Leave the concentrated solution to cool and crystallise slowly. Filter the crystals and pat them dry.

Memory Aid: Use RFEC to remember the steps: React, Filter, Evaporate, Crystallise!

3. Making Insoluble Salts (Precipitation)

If the salt to be prepared does not dissolve in water, it is an insoluble salt. These are made using a precipitation reaction.

What is a precipitate?
A precipitate is an insoluble solid that emerges from a liquid solution upon mixing two soluble solutions.

How to do it:

Mix two soluble solutions containing the required ions. The insoluble salt precipitates immediately, which you then filter, wash with distilled water to remove soluble impurities, and dry.

Example: Making lead(II) chloride by mixing lead(II) nitrate and sodium chloride solutions:
\( Pb(NO_3)_2(aq) + 2NaCl(aq) \rightarrow PbCl_2(s) + 2NaNO_3(aq) \)
Ionic equation: \( Pb^{2+}(aq) + 2Cl^-(aq) \rightarrow PbCl_2(s) \)

4. Percentage Yield in Salt Preparations

Whenever you prepare a salt, practical losses mean you rarely obtain the full theoretical amount calculated from the balanced equation.

Calculating Percentage Yield:

\( \text{Percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100\% \)

The theoretical yield is the maximum mass of product calculated using reacting quantities (stoichiometry), assuming \( 100\% \) conversion. The actual yield is the mass of pure, dry product collected.

Why is the yield less than \( 100\% \)?

  • Some product remains dissolved in solution during crystallisation.
  • Losses occur when transferring liquids between glassware or on filter paper.
  • The reaction may not go to complete completion, or side reactions may occur.

5. Group 2 Oxides and Hydroxides

Group 2 elements (magnesium to barium) form oxides and hydroxides that are basic in nature.

Reaction with Water

Group 2 oxides react with water to form hydroxides:

\( MgO(s) + H_2O(l) \rightarrow Mg(OH)_2(s) \)
\( CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq) \)

Reaction with Acids

Because they are bases, Group 2 oxides and hydroxides react with acids in neutralisation reactions to produce salts and water.

\( MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l) \)
\( Mg(OH)_2(s) + H_2SO_4(aq) \rightarrow MgSO_4(aq) + 2H_2O(l) \)

Real-World Application:
Magnesium hydroxide \( Mg(OH)_2 \) is the active ingredient in "Milk of Magnesia." It acts as an antacid, safely neutralising excess hydrochloric acid in the stomach.

Summary Checklist

  • Can you define acid, base, alkali, and neutralisation?
  • Do you know the formulae for hydrochloric, nitric, and sulfuric acids?
  • Could you describe the steps to make a soluble salt and an insoluble salt?
  • Can you calculate the percentage yield of a salt preparation?
  • Can you describe the basic nature of Group 2 oxides and hydroxides in reactions with acids?