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Acids and Bases


What are acids and bases?

Properties of Acids and Bases

We can find so many substances in our daily life that are acids and bases. For example lemon juice and vinegar are acidic. Soap, baking soda, and bleach are basic.

Acids and Bases

Physical properties of acids and bases

You are already familiar with some of the physical properties of acids and bases. For example, carbonic and phosphoric acids give carbonated beverages a sharp taste; citric and ascorbic acids give lemons their tartness, and acetic acid makes vinegar taste sour.

Basic solutions taste bitter and feel slippery. A bar of soap tastes bitter, and it becomes slippery when it gets wet. You should never attempt to identify an acid or a base of any substance in the laboratory, by its taste.

Chemical properties of acids and bases

Acids and bases can be identified by their reactions with litmus paper. Acidic solutions cause blue litmus paper to turn red but, basic solutions cause red litmus paper to turn blue.

In addition acids can reacts with metal, for example magnesium and zinc react with acids to produce hydrogen gas.

Metal carbonates and hydrogen carbonates also react with acids to produce carbon dioxide (CO2) gas. For example when vinegar is added to baking soda, a foaming reaction occurs between acetic acid in the vinegar, and sodium hydrogen carbonate.

Acids and Bases

Hydrogen and hydroxide ions

In all water solutions we can find hydrogen ions (H+) and hydroxide ions (OH). The amounts of these two ions determine if the solution is acidic, basic, or neutral. Neutral solutions are neither acidic nor basic. An acidic solution contains more hydrogen ions. A basic solution contains more hydroxide. A neutral solution contains equal concentration of the two ions.

Acids and Bases

The Arrhenius Model of acids and bases

If pure water is neutral, how does an aqueous solution become acidic or basic? The Swedish chemist Arrhenius was the first person that answer this question.

The Arrhenius model explains that an acid is a substance containing hydrogen and ionizes to produce hydrogen ions in aqueous solution. A base is a substance containing a hydroxide group and dissociates to produce a hydroxide ion in aqueous solution.

As an example of the Arrhenius model, imagine what occurs when hydrogen chloride gas dissolves in water. HCI molecules form H+ ions, which give an acidic solution.

HCl(g) —> H+(aq) + Cl(aq)

The other example is when the ionic compound sodium hydroxide (NaOH) dissolves in water, it produces OH ions, which give a basic solution.

NaOH(s) —> Na+ (aq) + OH(aq)

The Arrhenius model has some problems because for example, ammonia (NH3) and sodium carbonate (Na2CO3) do not contain a hydroxide group, but both of them produce hydroxide ions and they are well-known as bases.

The Bronsted-Lowry Model of acids and bases

The danish chemist Johannes Bronsted and the english chemist Tomas Lowry proposed a new model of acids and bases, a model that focuses on the hydrogen ion (H+). In this model, an acid can give hydrogen ion, it is a hydrogen ion donor, but the base accept the hydrogen ion.

The symbols X and Y represent non-metallic elements. So, the general formula for an acid can be HX or HY. When a molecule of acid HX dissolves in water, it donates a H+ ion. The water molecule acts as a base and accepts the H+ ion.

HX(aq) + H₂O <==> H3O+(aq) + X(aq)

After accepting the H+ ion, the water molecule becomes an acid. The hydronium ion (H3O+) is an acid because it has an extra H+ ion that it can donate. After giving its H+ ion, the acid HX becomes a base. X is a base because it has a negative charge and can accept a positive hydrogen ion.

An acid-base reaction in the reverse direction can occur.

Conjugate acid and base

In the equilibrium already shown, the reaction is the reaction of an acid and a base. The reverse reaction is also the reaction of an acid and a base. The acid and base that react in the reverse reaction are the conjugate acid and a conjugate base. A conjugate acid is produced when a base accepts a H+ ion. A conjugate base is the result when an acid donates a hydrogen ion. The acid HX donates its hydrogen ion and becomes the conjugate base X .

In the equilibrium, the hydronium ion (HO) is the conjugate acid of the base H2O. The X ion is the conjugate base of the acid HX. Bronsted-Lowry interactions involve conjugate acid-base pairs. A conjugate acid-base pair consists of two substances related to each other by the donating and accepting of a single hydrogen ion.

Acids and Bases

Monoprotic and Polyprotic acids

From the chemical formulas of HCl and HF, you know that each acid has one hydrogen ion per molecule. An acid that can donate only one hydrogen ion is called a monoprotic acid. Examples of monoprotic acid: perchloric acid (HCOOH), nitric acid (HNO3), hydrobromic acid (HBr), and acetic acid (CH3COOH).

Some acids donate more than one hydrogen ion For example, sulfuric acid (H2SO4) and carbonic acid (H2CO3) can donate two hydrogen ions. In each compound, both hydrogen atoms are attached to oxygen atoms by polar bonds. Acids that contain two ionizable hydrogen atoms per molecule are called diprotic acids. Phosphoric and (H3PO4) and boric acid (H3BO3) contain three ionizable hydrogen atoms per molecule and they are called triprotic acids. The term polyprotic acid can be used for any acid that has more than one ionizable hydrogen atom.