Chemical reactions: conventions and symbols

The reaction of hydrogen gas and oxygen gas produces water. This reaction requires an input of energy and can be written as:

• a word equation (shown below in blue) or
• a chemical equation (shown below in black using chemical formulas).

This is an exothermic reaction because energy is released. Contrast to the above exothermic reaction is the reverse reaction, the decomposition reaction of water.

The decomposition reaction of water, as given below, is an endothermic reaction. To decompose water into its elements, energy is required. A process to decompose water is by electrolysis.

The following table summarizes the symbols and terms used in chemical equations:

Reactants and products

Reactants are what is written on the left-hand-side of the arrow.a
products are what is written on the right-hand-side of the arrow.

Arrow

An arrow separates the reactants from the products.

• indicates that the reaction proceeds from left to right. It is a one way reaction.

• indicates that the reaction can proceed in both directions. When the products are formed, they recombine and form the reactants. In time, when the rate of the forward reaction equals the rate of the reverse reaction, a reaction equilibrium is reached.

Physical states

The physical state of each species in the chemical reaction are written with a bracket.

• (s) a solid substance,
• (l) a liquid substance,
• (g) a gaseous substance,
• (aq) an aqueous solution (substance dissolved in water)

Coefficients

Coefficients are numbers written in front of each substance in a balanced chemical reaction. If nothing is written in front, the default value is 1.

eg. the 2 written in front of H₂ (g) is a coefficient. the 2 written in front of H₂O (l) is a coefficient.

In the decomposition reaction of water, it means that:

• there are two molecules H₂ (g) and 1 molecule of O₂ (g) formed from the decomposition of two molecules of H₂O (l). OR, scaled up,
• there are two moles H₂ (g) and 1 mole of O₂ (g) formed from the decomposition of two moles of H₂O (l).

Let's count the number of each type of atoms on both sides of the decomposition reaction of water:

• The number of hydrogen atoms on the left-hand-side of the chemical equation:
  • Each water molecule has 2 hydrogen atoms.The coefficient of '2' tells us that there are two water molecules at the start of the reaction.

            Therefore, there are 2 H atoms x 2 = 4 hydrogen atoms on the left-hand-side

• The number of oxygen atoms on the left-hand-side of the chemical equation:
  • each water molecule has 1 oxygen atom. The coefficient of '2' tells us that there are two water molecules at the start of the reaction.

           Therefore, there are 1 O atom x 2 = 2 oxygen atoms on the left-hand-side

• The number of hydrogen atoms on the right-hand-side of the chemical equation:
  • two H2 (g) molecules are formed. Each H₂ (g) molecules has 2 hydrogen atoms. The coefficient of '2' tells us that there are two H₂ (g) produced.

           Therefore, there are 2 H atoms x 2 =  4 hydrogen atoms on the right-hand-side

• The number of oxygen atoms on the right-hand-side of the chemical equation:
  • one O₂ (g) molecules is formed. Each O₂ (g) molecules has 2 oxygen atoms. There is no coefficient written in from of the O₂ (g). By default, the coefficient is '1'.

           Therefore, there are 2 O atoms x 1 = 2 oxygen atoms on the right-hand-side

Note: The number of each type of atoms on both sides of the chemical equation is the SAME.

No of atoms:	LHS	RHS
Hydrogen atoms	4	4
Oxygen atoms	2	2

This is not a coincident. The above reaction is called a 'balanced' chemical reaction. All chemical reactions must be 'balanced' because they must obey the Law of Conservation of Mass.

Content suitability

BCIT courses: CHEM 0011