Advanced Chemistry Projects - Acids and Bases

To make your Science Project significant choose topics from your curriculum that you have studied in theory. A science project at the K-11 or K-12 grade or for your A levels, requires demonstration of your understanding the concept taught in class supported with meaningful experimental work. It is important that your experiments are simple and non-hazardous. Irrespective of the boards for which you appear, you all learn about acids and bases.

Theoretically, acids and bases are learnt according to different theories put forth by Arrhenius, Bronsted and Lowry as well as G.N.Lewis. In contrast, we also study the classical concept of acids and bases based on qualitative tests. If it tastes sour –it is an acid, but if it tastes slippery and bitter on the tongue, then it is surely a base. An acid reacts with a base to give salt and water and vice versa a base reacts with an acid also to give salt and water. According to the Arrhenius concept, an acid releases a proton in solution and a base releases hydroxyl ions in solution.

The Bronsted and Lowry concept defines an acid as a proton donor but a base as a proton acceptor. The Lewis concept of an acid and base is that of an electron pair acceptor and donor with the formation of a coordinate covalent bond. The project at hand is to explain these concepts through simple experimentation.

Strength versus Concentration
The Lewis concept of acids and bases is incapable of explaining strengths of acids and bases. The strength of an acid or a base is directly proportional to the extent to which it produces hydronium or hydroxide ions in solution. Strong species produce stoichiometric equivalents of hydroxide ions while weak species produce less than a stoichiometric equivalent of hydroxide ions. Concentration on the other hand refers to the molarity or molar concentration in moles per litre of the solute in solution. The terms concentration and strength are not interchangeable.

A 0.001molar solution of HCl is 100 % dissociated in solution and so it is a strong acid as it furnishes almost all its protons in solution. However a 2 M solution of acetic acid is a more concentrated solution than 0.001M HCl but is not stronger than HCl as the number of protons dissociated are very few. Here a meaningful experiment would be to demonstrate the difference between a weak strength acid and a strong strength acid.

The simplest determination is carrying out a pH measurement. From the pH measurement the H3O+ ion concentration can be determined. pH= - log10 H3O+ from this relationship the H3O+ ion concentration for a weak as well as a strong acid can be determined. The stronger acid will have a lower pH value and thus a higher hydronium ion concentration.

Ka, Kb and Acid/Base Strength
Most acids, like acetic acid, are weak. They do not completely ionize in water to produce a stoichiometric equivalent of hydronium ions. This means aqueous acetic acid molecules are in equilibrium with hydronium and acetate ions.

For any system at equilibrium, the law of chemical equilibrium results in an expression with the equilibrium constant K. Including the equilibrium concentrations of aqueous species in equilibrium constant expressions, the K expression for acetic acid in water is:

where the subscript "a" indicates that an acid ionizes in water to form hydronium. Ka is the acid ionization constant or the equilibrium constant for the ionization of a weak acid in water to produce hydronium ions and the conjugate base of the acid.

An equilibrium constant expression may also be written for a weak base like ammonia in water.

The subscript "b" indicates that a base hydrolyses water to form hydroxide. Kb is the base ionization constant or the equilibrium constant for the ionization of a weak base in water to produce hydroxide and the conjugate acid of the base.

Ka and Kb values indicate acid strength and base strength respectively. For example, a higher Ka value indicates higher hydronium ion concentrations or greater ionization. The equilibrium position lies further to the right when Ka is higher. A higher Kb value means a higher hydroxide ion concentration.

The strength of acetic acid is known to you. From the pH measurement the H3O+ ion concentration is known. The number of H3O+ ions in the solution will equal the number of acetate ions and so
= [H3O+]2/[CH3COOH]

Thus an experimental determination of the Ka values for different acids and Kb values for different bases can be undertaken.

This project conveys through an experimental determination
-the understanding of the concepts of acids and bases
-the difference between strengths of acids and bases
-pH+ pOH=14
-the calculation of Ka or Kb and its significance, as well as,
-pH and its use to determine the hydronium ion concentration.