Acids - Bases - Salts

Acids

• An acid is a substance that provides H+ ions in water.

• Here are some examples of substances that are acids;

• HCl: hydrochloric acid

• HNO3: nitric acid

• H2SO4 : sulphuric acid

• H3PO4: phosphoric acid.

Bases

• A base is a substance that provides OH- ions in water.

• Here are some examples of bases:

• NaOH: Sodium hydroxide

• KOH: Potassium hydroxide

• Ba(OH)2: Barium hydroxide

Ionic equations- Acids

• When acids dissolve in water they produce hydrogen (H+) ions. We say that the ions are in aqueous solution and write (aq) in the equation to tell the state of the reaction. Different acids can produce different numbers of H+ ions.

• Let us look at the representation for each of the following acids.

Ionic equations- acids

• HCl (g) H+ (aq) + Cl- (aq)• Hydrochloric acid dissolves in water to produce hydrogen

ions and chloride ions.• Another example; Sulphuric acid dissolves in water

to produce in the first instance hydrogen ions and hydrogen sulphate ions then further dissociates in water to form hydrogen ions and sulphate ions. Therefore, we can say that when sulphuric acid dissolves in water it produces 2 moles of H+ ions. Sulphuric acid is a dibasic acid.

• H2 SO4(l) H+(aq) + HSO4- (aq) H+(aq) + SO4 2 –(aq).

Ionic equations - bases

• When bases dissolve in water they produce OH- ions.

• The following are examples of what happens when bases dissolve in water.

• Sodium hydroxide : NaOH(s) Na+ (aq) + OH- (aq)

• ( lye or caustic soda)• Potassium hydroxide: KOH (s) K+

(aq) + OH- (aq)

Ionic equations- bases

• Similar to the number of ions that can be produced by a dibasic acid a similar thing can occur with a dibasic acid a similar thing can occur with a diacidic base.

• Barium hydroxide :

Ba (OH )2 (s) Ba 2+ (aq) + 2 OH-(aq)

• Barium hydroxide is a diacidic base !

Acid-base neutralization process

Acid-base neutralization reactions in which an acid reacts with a base to yield water plus an ionic compound called a salt.

• Let’s look at the neutralization that occurs between hydrochloric acid and aqueous sodium hydroxide to yield water and aqueous sodium chloride.

HCl (aq) + NaOH (aq) H2O (l) + NaCl (aq)

Salts

• Salts can also be formed by precipitation reactions.

• When precipitation reactions occur, soluble reactants yield an insoluble solid product that drops out of the solution (precipitates!). As this precipitate forms it drives the force of the reaction. When positive ions and negative ions change positions in two reacting compounds precipitation occurs. The product that precipitates is called an insoluble salt.

Salts

• For example:

• Lead nitrate is formed when an aqueous solution of lead II nitrate reacts with an aqueous solution of potassium iodide. This can be represented in an equation;

Pb(NO3)2 (aq) + 2 KI (aq) PbI2 + 2 KNO3 (aq)

A neutralization reaction

• In a neutralization reaction Anions and cations react in proportion to produce water and a salt.

• The anion of the salt ( A- ) comes from the acid, and the cation of the salt ( M+) comes from the base:

Neutralization cont’d

• This can be represented as follows:

• HA (aq) + MOH (aq) H2O (l) + MA (aq)

• Acid Base Water A salt.

Strong acids / strong bases

• Let us look at the reaction between a strong acid and a strong base in water.

• We will now write the ionic equation for these reactants and products.

H+ (aq) + A-

(aq) + M+ (aq) + OH-

(aq) H2O (l) + M+ (aq) + A- (aq)

Ionic equations-neutralization

Cancel the ions on both sides that are similar

H+ (aq) + A-

(aq) + M+ (aq) + OH-

(aq) H2O (l) + M+ (aq) + A- (aq)

Equation:

H+ (aq) + OH-

(aq) H2O (l)

Or we could also write the equations as follows:

H3O + (aq) + OH- (aq) 2 H2O (l)

Ionic equations-neutralization

• Or we could also write the equations as follows:

• H3O + (aq) + OH- (aq) 2 H2O (l)

Summary of acid – bases – salts

Acids liberate hydrogen ions (protons) in aqueous solution.

• The basicity of an acid is the number of ionisable hydrogen atoms it contains in its molecule.

• Acids react with: (i) metals to give hydrogen (ii) bases to give a salt and water only (iii) carbonates to give a salt, water and carbon dioxide.

• Acidity and Alkalinity are measured on the pH scale• The pH scale runs from 1 to 14. Ranges between 1 and

7 are acid. Ranges between 7 and 14 are alkaline or basic. A range of 7 is neither acidic nor basic.

Summary of acid – bases – salts

• Normal salts contain no ionisable hydrogen atoms• Acid salts contain one or more ionisable hydrogen

atoms.• Bases are substances which accept protons ( they are

often oxides or hydroxides)• A solution of a salt is an electrolyte. An electrolyte can

conduct electricity when molten or in solution.• Strong acids and strong bases are completely ionised

in solution.• Weak acids and weak bases are incompletely ionised

in solution.