Writing And Balancing Chemical Equations

Chemical equations are representation of chemical reactions in terms of the symbols and formulae of the elements and compounds involved. In a chemical equation, the reactants are always written on the left hand side while the products are written on the right hand side. For instance, if A and B combines together to give C and D, the equation of the reaction is written as:

       A + B       →        C + D

       Reactants Products

BALANCING CHEMICAL EQUATIONS

All equations must be balanced in order to comply with the law of conservation of matter. Equations are balanced through the use of coefficients in front of the formula and not by changing the subscript numbers within the formulae of the products.

Example 1: Write a balanced equation for the combustion of ammonia gas in air.

Solution:

Step I: Write the reactants and predict the products

              NH_3(g) + O_2(g) → NO_(g) + H₂O_(g)

Step II: The equation is not balanced. Therefore the equation can be balanced by placing the right coefficient in front of each molecule to balance the number of atoms. Thus, the balanced equation is:

              4NH_3(g) + 5O_2(g) → 4NO_(g) + 6H₂O_(g)

Example 2: Write a balanced equation for the combustion of ethane in oxygen.

Solution:

The general formula for the combustion of Alkanes is

             C_xH_y + (x + y/4) O₂ → XCO₂ + y/2 H₂O

The molecular formula for ethane is C₂H₆, so, x=2 and y=6

Substituting x and y into the formula above gives

            C₂H₆ + (2 + 6/4) O₂ →2CO₂ + 6/2 H₂O

            C₂H₆ + 7/2 O₂ →2CO₂ + 3H₂O

The equation is balanced. However, equations are written with whole number coefficients. By multiplying the entire equation by 2, we get

           2C₂H₆ + 7O₂ →4CO₂ + 6H₂O

IMPORTANCE OF CHEMICAL EQUATIONS

1. It gives us information on the product that can be formed from the combination of two or more reactants in a particular reaction.
2. It tells us the physical states of the reactants and products.
3. It indicates the direction of the reaction and whether the reaction is reversible.
4. It tells us the stoichiometry of the reaction (i.e. the relationship between the amount of reactants and products) in terms of mole ratio of the reactants and products involved.

Consider the table below:

Equation	Mole ratio/ Mass ratio
2HCl + CaCO3→ CaCl2 + H2O + CO2	2 mole of HCl and 1 mole of CaCO3produced 1 mole of CaCl2, 1 mole of H2Oand 1 moles of CO2
2HCl + CaCO3 → CaCl2 + H2O + CO2	73g of HCl and 100g of CaCO3produced 111g of CaCl2, 18g of H2Oand 44g of CO2

GENERAL EVALUATION/REVISION

1. Balance the following equations:

(a) KClO_3(s) → KCl_(s) + O_2(g)

(b) ZnCO_3(s) + HCl_(aq) → ZnCl_(aq) + H₂O + CO_2(g)

• What is the volume in dm³ of 8g of oxygen gas at s.t.p?
• State the use of each of the following apparatuses: triangular pipe clay, beehive shelf, bell jar, fume cupboard, dessicator.
• Outline three differences between physical and chemical changes.

READING ASSIGNMENT

New School Chemistry for Senior Secondary Schools by O.Y. Ababio, pg 36-40

WEEKEND ASSIGNMENT

1. A balanced chemical equation obeys which of the laws? A. Law of conservation of matter  B. Law of definite proportion C. Law of multiple proportion D. Boyle’s law
2. The numerical coefficients in a balanced equation give the A. number of mole of reactants and products B. molar mass of the reactants and products C. number of reactants only D. mass ratio of the reactants.
3. A molecule of neon is A. diatomic B. monoatomic C.triatomic D. polyatomic
4. H₂SO₄ + xKOH → K₂SO₄ + yH₂O. The value for x and y in the above equation is A. 1 and 2 B. 2 and 3 C. 2 and 1 D. 4 and2
5. The balanced chemical equation for the reaction between hydrochloric acid and sodium hydroxide isA. NaOH + HCl → NaCl + H₂O       B. NaCl + HCl → NaOH + H₂O

C. NaOH + H₂SO₄ →Na₂SO₄ + H₂O D. H₂SO₄ + KOH → K₂SO₄ + H₂O

THEORY

 1. Balance the following equation: H₂SO₄ + Na₂CO₃ → Na₂SO₄ + H₂O + CO₂

    Ca(OH)₂ + CO₂ → CaCO₃ + H₂O 2. State two information provided by the equation of a chemical reaction.