Unit Four: Chemical Reactions and Stoichiometry

Lesson 2: Balancing Chemical Reactions

• Comptencies
• Starter Activities
• Main Activites

Competencies:

• Balance chemical equations using inspection method
• Balance chemical equations using  the Least Common Multiple Method (LCM)

Materials

Charts for formulas of molecules, picture of bicycle, tricycle and automobile

1. Monocycles

 Teacher’s Note

Writing and balancing chemical equations should begin after students managed to write the correct formula of a compound and understand the qualitative and quantitative meanings of symbols, formulas and equations.

In balancing chemical equations, students do not easily understand why they should not change subscripts as far as they are able to make the number of atoms on both sides equal. This lesson should be given as the second lesson in sub-topic 4.3 Chemical equation after students studied writing chemical equation.

Starter activity (5 minutes)

The lesson can be started by revising writing chemical equation and its qualitative and quantitative meanings. Students may individually be asked the steps of writing an equation. Let them write the word equation of a given reaction and change it to skeletal equation.

For example reactions between: hydrogen and oxygen, sulphur and chlorine, sodium hydroxide and hydrochloric acid, calcium carbonate and sulphuric acid.
Let them tell the qualitative and quantitative meanings of their equations. Many students should take part in this revision.

When describing the qualitative and quantitative meanings students may misuse the terms molecules for non-molecular compounds like NaOH and CaCO3. Help them to use ‘formula unit’ instead of molecule. But they can use mole in both cases.

Evaluation

What are the qualitative and quantitative meanings of the chemical equation?

NaHCO3 + CH3COOH → CH3COONa  + CO2  + H2O

Answer

1. Qualitatively it means, sodium bicarbonate reacts/combines with acetic acid to produce/yield/give sodium acetate, carbon dioxide and water.
2. Quantitatively
   1. One formula unit of sodium bicarbonate  reacts with one molecule of acetic acid to produce one formula unit of sodium acetate, one molecule of carbon dioxide and one molecule of water or
   2. One mole of sodium acetate reacts with one mole of acetic acid to produce one mole of sodium acetate, one mole of carbon dioxide and one mole of water. or
   3. 84g NaHCO3 reacts with 60g CH3COOH to produce 82g CH3COONa, 44g  CO2 and 18g H2O

Main activity (27 minutes)

The analogy of assembling vehicle bodies
Procedure

1. Make  students into groups based on your class size
2. Tell them the following story.

   A factory manufactures vehicle bodies (monocycle, bicycle, tricycle and automobile) and buys tyres from another manufacturer to make a complete vehicle. The tyre manufacturer sells the tyres only in pairs.
   The vehicles producer wants to make sample vehicles with minimum number.

3. Ask the students to discuss in their groups to find out the minimum number of each vehicle the manufacture assembles and the number of each type of tyre, he buys so that no tyre is left unassembled.
4. Let them support their explanation by drawings
5. Let them match/associate the assembly of each of the vehicles to one of the following reactions
   1. Mg  + Cl2  → MgCl2
   2. Li + F2  → LiF
   3. C + H2  →  CH4
   4. Al + Br2→ AlBr3

Evaluation

1. To what parts of the vehicle are the Mg, Li, C and Al do you relate? How about Cl2, F2, H2 and Br2?
2. Can the vehicle be functional if any of the body or the tyre is missing?
3. If you want to make 5 tricycles how many tyres do you buy?
4. If you have 10 tyres how many cars can you assemble?

Concluding activity
Give them more advanced reactions to balance.

1. Sodium reacts with chlorine gas to produce sodium chloride
2. Lead (II) nitrate reacts with potassium iodide to produce lead iodide and potassium nitrate.
3. Potassium chlorate decomposes to form potassium chloride and oxygen
4. Propane burns to produce carbon dioxide and water
5. Sodium reacts with calcium chloride to produce sodium chloride and calcium
6. Hydrogen reacts with oxygen to produce water
7. Carbonic acid decomposes to form carbon dioxide and water
8. Phosphoric acid reacts with calcium hydroxide to produce calcium phosphate and water.

Let them explain why the formulas cannot be changed during balancing. Let them associate balancing equation with the laws of chemical reaction.