Based on the definition of a chemical reaction, think about any chemical reaction and;

  1. Write that chemical reaction in words.
  2. Translate the above chemical reaction into symbols.
  3. Which principles do you follow in writing a chemical reaction so that any Chemist who sees it can carry out the same experiment?
  4. After writing a balanced chemical equation for the reaction, how do you interpret it?

Solution:

  1. A general statement for a chemical reaction is: A substance (reactant) A reacts with a substance (reactant) B to produce (to form) products C and D.

For example: Hydrogen gas reacts with oxygen gas to produce (to form) water vapor.

  1. This chemical reaction is represented by a chemical equation:
    2H2(g) + O2(g) → 2H2O(g)

A chemical equation is the expression of a chemical reaction using chemical formulas and symbols. It consists into two parts separated by an arrow, →: (i) before the arrow, we have the chemical formulas of the reactant(s), separated by a "+" symbol; (ii) after the arrow we have the chemical formulas of product(s), also separated by "+" if more than one products are formed, and (iii) the direction of the arrow shows the direction of the reaction: reactants → products

3.   In writing a chemical equation, you must be guided by the following rules:

  • Ensure that the chemical symbols or formulas of the reactants and products are correctly written.
  • Ensure that the “Law of Mass Conservation”, also called “Lavoisier’s law” in honor of the French chemist Antoine Lavoisier who discovered it in 1789, is respected.

This Law states that “during a chemical reaction the mass is neither created nor destroyed”. In other words, the mass of the reactants at the beginning of the reaction will be equal to the mass of the products at the end of the reaction.

This results from the fact that during a chemical reaction, as you can see, atoms do not change, only their combinations change. Ensure that the chemical equation is balanced, i.e. the number of each atom on each side of the chemical equation must be equal. If the number of an atom is not equal on both sides, factors of multiplication on both sides are introduced so that the number of the atoms on both sides is equal; this is in the fulfillment of the Law of mass conservation.

Examples:

  • H2(g) + O2(g) → H2O(g) : chemical symbols well written, but equation not balanced, here O atoms are not equal on both sides of the equation
  • H2(g) + O2(g) → 2H2O(g) : introduction of a multiplication factor 2 in front of H2O; although this makes the number of O on both sides of the equation equal, the number of H is no more equal on both sides!
  • 2H2(g) + O2(g) → 2H2O(g): introduction of a multiplication factor 2 in front of H2; the number of all atoms are equal on both sides, 4H atoms and 2O atoms respectively; this gives the balanced equation of the chemical reaction.

A balanced equation of a chemical reaction is also called a “Stoichiometric equation”.

N.B:

  1. By convention, it is recommended to use the smallest coefficient possible in balancing chemical equations.
  2. It is recommended to avoid, where possible, fractional coefficients.
  3. The coefficient 1 is not written.
  • In some reactions, the conditions of temperature(T or ∆) and pressure(P) can be indicated on the arrow: A + B → T, P → C + D
    When a catalyst is used, this is also indicated above the arrow: → Catalyst →
  • It is recommended to indicate the states of the reactants and products; this is done by putting between brackets after the chemical formula of the reactants and products the letters or symbols: solid (s), liquid (l), aqueous solution (aq), gaseous (g), precipitate (↓) or (s), gas formed (↑).

As you can see, a chemical equation is written by showing all the conditions required so that any chemist all over the world can read it and do it.

  1. Interpretation of a chemical equation:
    CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

Many forms of interpretation are possible:

  • 1 molecule of CH4 + 2 molecules of O2 → 1 molecule of CO2 + 2 molecules of H2O
  • 1 mole of CH4 + 2 moles of O2 → 1 mole of CO2 + 2 moles of H2O; this is the most used.
  • 16 g of CH4 + 2 x 32 g of O2 → 44 g of CO2 + 2 x 18 g of H2O

Notice that the mass conservation principle must be fulfilled:
80g of reactants = 80g of products