SCH4U - Chemistry 12 (2024-25) - A

You will recall that an acid is a compound that dissolves in water to produce hydrogen ions (H+) in solution. Acids have a sour taste and in solution, they can conduct electricity because they contain ions. An acidic solution is corrosive and can destroy metal, clothing, and even skin. For example, solutions of HCl, H₂SO₄, and HNO₃ are all acidic in nature.

Whereas, bases are compounds that dissolve in water to produce hydroxide ions (OH-) in solution. Bases have a bitter taste and have a slimy or soapy feel on fingers. Basic solutions conduct electricity. Concentrated or strong bases are caustic (corrosive) on organic matter and react violently with acidic substances. For example, solutions of NaOH, Mg(OH)₂, and NH₄OH are all bases.

Bases burn skin faster than acids. In addition, the majority of our household cleaning products are bases; hand and body soap, dish soap, laundry detergent, bleach, and oven cleaner are just some examples.

Now the question is what happens when an acid and a base are combined?

They neutralize each other! This kind of reaction is known as Neutralization reaction. It is a special kind of double displacement reaction in which an acid and a base react to form a salt and water. The resulting pH is close to 7. In other words, the H⁺ ion in the acid reacts with the OH^- ion in the base, causing the formation of water.

Generally, the product of this reaction is some ionic salt and water:

Acid + Base → Water + Salt

HA + BOH → H₂O + BA

(A represents a negative ion. B represents a positive ion).

Neutralization Reactions Illustrated with Examples:

Example 1:

Sulfuric acid is the most widely used industrial chemical in the world. It is frequently used for neutralizaiton in industry because it is easier to use. The reaction of sulfuric acid and lime to produce calcium sulfate and water is given as:

H₂SO₄ + Ca(OH)₂ --> CaSO₄ +2H₂O

Example 2:

Phosphoric acid is a main ingrediant in rust remover solutions. Rust is a mixture of iron compounds one of which is iron(II) oxide, which reacts with phosphoric acid to produce iron (II) phosphate and water. The reaction can be represented as:

2H₃PO₄ + 3Fe(OH)₂ --> Fe₃(PO₄)₂ + 6H₂O

Example 3: 

Potassium, a metal, burns with oxygen to produce potassium oxide which is a metal oxide.

4K + O₂ --> K₂O

Potassium oxide then reacts with water to form potassium hydroxide which is a bse.

K₂O + H₂O --> 2KOH

Example 4:

Nitrogen can react with oxygen to form nitrogen dioxide.

N₂ + 2O₂ --> 2NO₂

Nitrogen dioxide reacts with water to form nitric acid.

3NO₂ + H₂O --> 2HNO₃ + NO

pH and the Environment

How does the environment make acids and bases naturally?

Metals react with oxygen to form metal oxides. Metal oxides are always solids. Metal oxides react in water to form metal hydroxides, which are bases.

Non-metals react with oxygen to form non-metal oxides. Non-metal oxides are often gases or liquids. Non-metal oxides react in water to form acids.

Many non-metal oxides are released into the atmosphere as a result of industrial processes and fuel burning which can produce the following compoudns: nitrogen oxides, sulphur oxides, and carbon dioxides.

These oxides combine with water vapour in the air to form nitric acid, sulphuric acid, and carbonic acid. When this water falls to Earth, and has a pH of less than 5.6, it is considered acid rain.

Negative Effects of Acid Rain

• Damaging aquatic life (decreased reproduction, interfere with nutrient absorption)

• Helping non-desirable species such as certain plants, masses and insects thrive.

• Washing away nutrients from forest areas.

• Damaging property.

Your Turn to Practise

Complete and balance the following neutralization reactions.

a. HBr + NaOH -->

b. H₃PO₄ + Mg(OH)₂ -->

c. HCl + Pb(OH)₂ -->

d. Al(OH)₃ + HClO₄ --> 

Answers:

a. HBr + NaOH --> H2O + NaBr

b. H₃PO₄ + Mg(OH)₂ --> 6H₂O + Mg₃(PO₄)₂

c. HCl + Pb(OH)₂ --> 2H₂O + PbCl₂

d. Al(OH)₃ + HClO₄ --> 3H₂O + Al(ClO₄)₃