Law of conservation of mass states that matter can neither be created nor destroyed in a chemical reaction.

The bonds between atoms in the reactants are rearranged to form new compounds in chemical reactions, but none of the atoms disappear and no new ones are formed.

Chemical equations must be balanced equations, meaning that the numbers and kinds of atoms must be the same on both sides of the reaction arrow.

Most chemical equations can be balanced by following the four step approach:

• STEP 1: Write an unbalanced equation, using the correct formulas for all given reactants and products.

• STEP 2: Add appropriate coefficients to balance the numbers of atoms of each element.

• STEP 3: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are the same.

• STEP 4: Make sure the coefficients are reduced to their lowest whole-number values.

Ionic reactions can be classified into three types:

1)precipitation reactions

2)acid-base neutralization reactions

3)oxidation-reduction reactions.

Precipitation reactions are processes in which an insoluble solid called a precipitate forms when reactants are combined in aqueous solution. Most precipitations take place when the anions and cations of two ionic compounds change partners.

To predict whether a precipitation reaction will occur upon mixing aqueous solutions of two ionic compounds, you must know the solubilities of the potential products—how much of each compound will dissolve in a given amount of solvent at a given temperature.

• If a substance has a low solubility in water, then it is likely to precipitate from an aqueous solution. If a substance has a high solubility in water, then no precipitate will form.

General Rules on Solubility

RULE 1: A compound is probably soluble if it contains one of the following cations:

• Group 1A cation: Li +, Na +, K+, Rb+, Cs+

• Ammonium ion: NH4+

RULE 2: A compound is probably soluble if it contains one of the following anions:

• Halide: Cl-, Br -, and I

  except Ag+, Hg2+, and Pb2+ compounds

• Nitrate 1NO3 -2, perchlorate 1ClO4 -2, acetate 1CH3CO2 -2, and sulfate 1SO4 2-2

  • except Ba2+, Hg2+, and Pb2+ sulfates.

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

When acids and bases are mixed in the correct proportion, both acidic and basic properties disappear because of a neutralization reaction.

• The most common kind of neutralization reaction occurs between an acid (generalized as HA) and a metal hydroxide (generalized as MOH) to yield water and a salt.

• The H + ion from the acid combines with the OH- ion from the base to give neutral H2O, whereas the anion from the acid 1A-2 combines with the cation from the base 1M+2 to give the salt.

An oxidation is defined as the loss of one or more electrons by an atom, and a reduction is the gain of one or more electrons. Thus, an oxidation-reduction reaction, or redox reaction, is one in which electrons are transferred from one atom to another.

Oxidation and reduction always occur together. Whenever one substance loses an electron (is oxidized), another substance must gain that electron (be reduced).

• . The substance that gives up an electron and causes the reduction is called a reducing agent.

• The substance that gains an electron and causes the oxidation is called an oxidation agent.

Reducing agent :

• Loses one or more electrons

• Causes reduction

• Undergoes oxidation

• Becomes more positive (less negative) (May gain oxygen atoms)

Oxidizing agent :

• Gains one or more electrons

• Causes oxidation

• Undergoes reduction

• Becomes more negative (less positive) (May lose oxygen atoms).

The alkali metals and alkaline earth metals are the most powerful reducing agents (electron donors).

• This is due in part to the fact that alkali metals and alkaline earth metals have low ionization energies.

The reactive non-metals at the top right of the periodic table have the highest ionization energies and are extremely weak reducing agents but powerful oxidizing agents (electron acceptors).

In reactions involving metals and non-metals, metals tend to lose electrons while non-metals tend to gain electrons.

In reactions involving non-metals, the “more metallic” element (farther down and/or to the left in the periodic table) tends to lose electrons, and the “less metallic” element (up and/or to the right) tends to gain electrons

Corrosion is the deterioration of a metal by oxidation, such as the rusting of iron in moist air. The economic consequences of rusting are enormous: it has been estimated that up to one-fourth of the iron produced in the United States is used to replace bridges, buildings, and other structures that have been destroyed by corrosion.

Combustion is the burning of a fuel by rapid oxidation with oxygen in air.

Respiration is the process of breathing and using oxygen for the many biological redox reactions that provide the energy required by living organisms.

Bleaching makes use of redox reactions to decolorize or lighten coloured material.

Metallurgy, the science of extracting and purifying metals from their ores, makes use of numerous redox processes.

• Worldwide, approximately 800 million tons of iron are produced each year by reduction of the mineral hematite, Fe2O3, with carbon monoxide.

Oxidation number (or oxidation state), which indicates whether the atom is neutral, electron-rich, or electron-poor.

Oxidation numbers do not necessarily imply ionic charges.

• An atom in its elemental state has an oxidation number of 0.

• A monatomic ion has an oxidation number equal to its charge.

• In a molecular compound, an atom usually has the same oxidation number it would have if it were a monatomic ion.

• The sum of the oxidation numbers in a neutral compound is 0.