Symbols, Formula And Equations II SS1 Chemistry Lesson Note

LEARNING OBJECTIVES

By the end of this lesson, students should be able to:

1. Balance chemical equations using appropriate methods
2. State the law of conservation of mass
3. Write balanced chemical equations from word equations
4. Identify different types of chemical reactions
5. Apply the mole concept to chemical equations
6. Solve stoichiometric problems involving chemical equations

LESSON CONTENT

A chemical equation represents a chemical reaction using symbols and formulae.

Parts of a Chemical Equation:

• Reactants → substances that react (left side)
• Products → substances formed (right side)
• Arrow (→) → means “yields” or “produces”

Example:

2H₂ + O₂ → 2H₂O
(Reactants → Products)

2. LAW OF CONSERVATION OF MASS

Statement: Matter cannot be created or destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.

Application: In a balanced equation, the number of atoms of each element must be the same on both sides.

Example:

• Unbalanced: H₂ + O₂ → H₂O
  1. Left: 2H, 2O
  2. Right: 2H, 1O (NOT BALANCED)
• Balanced: 2H₂ + O₂ → 2H₂O
  1. Left: 4H, 2O
  2. Right: 4H, 2O (BALANCED ✓)

3. BALANCING CHEMICAL EQUATIONS

METHOD 1: INSPECTION METHOD (Trial and Error)

Steps:

1. Write the unbalanced equation
2. Count atoms of each element on both sides
3. Add coefficients to balance each element
4. Check that all elements are balanced
5. Ensure coefficients are in the smallest whole number ratio

Example 1: Balance the equation for the formation of water

Step 1: H₂ + O₂ → H₂O (unbalanced)

Step 2: Count atoms

• Left side: H = 2, O = 2
• Right side: H = 2, O = 1

Step 3: Balance oxygen by putting 2 before H₂O
H₂ + O₂ → 2H₂O

Step 4: Recount

• Left side: H = 2, O = 2
• Right side: H = 4, O = 2

Step 5: Balance hydrogen by putting 2 before H₂
2H₂ + O₂ → 2H₂O (BALANCED ✓)

Final check:

• Left: 4H, 2O
• Right: 4H, 2O ✓

Example 2: Balance: Fe + O₂ → Fe₂O₃

Step 1: Count atoms

• Left: Fe = 1, O = 2
• Right: Fe = 2, O = 3

Step 2: Balance Fe first – put 2 before Fe
2Fe + O₂ → Fe₂O₃

Step 3: Now count oxygen

• Left: O = 2
• Right: O = 3

Step 4: Find common multiple of 2 and 3 = 6
Put 3 before O₂ and 2 before Fe₂O₃
2Fe + 3O₂ → 2Fe₂O₃

Step 5: Recount Fe

• Left: Fe = 2
• Right: Fe = 4

Step 6: Put 4 before Fe
4Fe + 3O₂ → 2Fe₂O₃ (BALANCED ✓)

Final check:

• Left: 4Fe, 6O
• Right: 4Fe, 6O ✓

METHOD 2: ALGEBRAIC METHOD

For more complex equations, use algebraic coefficients.

Example: Balance C₃H₈ + O₂ → CO₂ + H₂O

Step 1: Assign coefficients as variables

aC₃H₈ + bO₂ → cCO₂ + dH₂O

Step 2: Balance each element

C: 3a = c

H: 8a = 2d

O: 2b = 2c + d

Step 3: Let a = 1 (simplest starting point)

C: 3(1) = c → c = 3

H: 8(1) = 2d → d = 4

O: 2b = 2(3) + 4 → 2b = 10 → b = 5

Step 4: Balanced equation

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O ✓

4. TYPES OF CHEMICAL REACTIONS

A. SYNTHESIS (COMBINATION) REACTION

Two or more substances combine to form one product.

General form: A + B → AB

Examples:

2Mg + O₂ → 2MgO

N₂ + 3H₂ → 2NH₃

2Na + Cl₂ → 2NaCl

B. DECOMPOSITION REACTION

One substance breaks down into two or more simpler substances.

General form: AB → A + B

Examples:

2H₂O → 2H₂ + O₂ (electrolysis)

2KClO₃ → 2KCl + 3O₂ (heating)

CaCO₃ → CaO + CO₂ (heating)

C. SINGLE DISPLACEMENT (SUBSTITUTION) REACTION

One element replaces another in a compound.

General form: A + BC → AC + B

Examples:

Zn + 2HCl → ZnCl₂ + H₂

Mg + CuSO₄ → MgSO₄ + Cu

Cl₂ + 2KBr → 2KCl + Br₂

D. DOUBLE DISPLACEMENT (DOUBLE DECOMPOSITION) REACTION

Two compounds exchange partners.

General form: AB + CD → AD + CB

Examples:

NaCl + AgNO₃ → NaNO₃ + AgCl

HCl + NaOH → NaCl + H₂O

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

E. COMBUSTION REACTION

A substance reacts rapidly with oxygen, releasing energy.

General form: Fuel + O₂ → CO₂ + H₂O + Energy

Examples:

CH₄ + 2O₂ → CO₂ + 2H₂O

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

5. STATE SYMBOLS IN EQUATIONS

Chemical equations often include state symbols:

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous solution (dissolved in water)

Examples:

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

CaCO₃(s) → CaO(s) + CO₂(g)

2H₂(g) + O₂(g) → 2H₂O(l)

6. INFORMATION FROM BALANCED EQUATIONS

A balanced equation provides:

1. Qualitative information – which substances react and what is formed
2. Quantitative information – the ratio of moles of reactants and products
3. Physical states of reactants and products (if shown)

Example: 2H₂(g) + O₂(g) → 2H₂O(l)

This tells us:

• 2 molecules of hydrogen react with 1 molecule of oxygen
• 2 moles of hydrogen react with 1 mole of oxygen
• The ratio is 2:1:2
• The products and reactants are in specific physical states

7. STOICHIOMETRY AND CHEMICAL EQUATIONS

Stoichiometry is the calculation of quantities in chemical reactions based on balanced equations.

Key Concepts:

• The coefficients represent mole ratios
• Use molar masses to convert between mass and moles
• Use Avogadro’s number for particle calculations

Example Problem:

Question: How many moles of oxygen are needed to react completely with 4 moles of hydrogen?

Equation: 2H₂ + O₂ → 2H₂O

Solution: From the equation, the mole ratio is: 2 moles H₂ : 1 mole O₂

Therefore: 4 moles H₂ × (1 mole O₂ / 2 moles H₂) = 2 moles O₂

Answer: 2 moles of O₂ are needed.

8. WRITING EQUATIONS FROM WORD EQUATIONS

Steps:

1. Identify reactants and products
2. Write correct chemical formulae
3. Balance the equation
4. Add state symbols if required

Example 1: Word equation: Sodium reacts with chlorine to form sodium chloride

Step 1: Reactants: sodium (Na), chlorine (Cl₂) Product: sodium chloride (NaCl)

Step 2: Na + Cl₂ → NaCl (unbalanced)

Step 3: 2Na + Cl₂ → 2NaCl (balanced)

Example 2: Word equation: Calcium carbonate decomposes on heating to form calcium oxide and carbon dioxide

Step 1: Reactant: calcium carbonate (CaCO₃) Products: calcium oxide (CaO), carbon dioxide (CO₂)

Step 2: CaCO₃ → CaO + CO₂

Step 3: Already balanced (1:1:1 ratio)

Step 4: CaCO₃(s) → CaO(s) + CO₂(g)

WORKED EXAMPLES

Example 1: Balancing Equations

Balance: Al + O₂ → Al₂O₃

Solution:

Step 1: Count atoms

Left: Al = 1, O = 2

Right: Al = 2, O = 3

Step 2: Balance Al – put 2 before Al

2Al + O₂ → Al₂O₃

Step 3: Balance O – need 6 oxygen atoms (LCM of 2 and 3)

Put 3 before O₂ and 2 before Al₂O₃

2Al + 3O₂ → 2Al₂O₃

Step 4: Now Al is unbalanced

Left: Al = 2

Right: Al = 4

Step 5: Put 4 before Al

4Al + 3O₂ → 2Al₂O₃ ✓

Check: Left (4Al, 6O) = Right (4Al, 6O)

Example 2: Stoichiometry

Question: Calculate the mass of oxygen required to react completely with 6g of magnesium. (Mg = 24, O = 16)

Equation: 2Mg + O₂ → 2MgO

Solution:

Step 1: Calculate moles of Mg

Moles = mass/molar mass = 6/24 = 0.25 moles

Step 2: Use mole ratio from equation

2 moles Mg : 1 mole O₂

0.25 moles Mg : ? moles O₂

Moles of O₂ = 0.25 × (1/2) = 0.125 moles

Step 3: Calculate mass of O₂

Mass = moles × molar mass

Mass = 0.125 × 32 = 4g

Answer: 4g of oxygen is required.

CLASS ACTIVITIES

Activity 1: Balancing Equations Practice

Balance the following equations:

1. Na + H₂O → NaOH + H₂
2. Fe + HCl → FeCl₂ + H₂
3. N₂ + H₂ → NH₃
4. C₄H₁₀ + O₂ → CO₂ + H₂O
5. Ca + O₂ → CaO

Activity 2: Identify Reaction Types

Classify each reaction:

1. 2KClO₃ → 2KCl + 3O₂
2. Fe + CuSO₄ → FeSO₄ + Cu
3. 2Mg + O₂ → 2MgO
4. AgNO₃ + NaCl → AgCl + NaNO₃
5. CH₄ + 2O₂ → CO₂ + 2H₂O

Activity 3: Word to Chemical Equations

Convert to balanced chemical equations:

1. Hydrogen reacts with nitrogen to form ammonia
2. Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas
3. Methane burns in oxygen to form carbon dioxide and water

EVALUATION QUESTIONS

Section A: Multiple Choice

1. The law of conservation of mass states that:

1. a) Mass increases during a reaction
2. b) Mass decreases during a reaction
3. c) Mass remains constant during a reaction
4. d) Mass can be created

1. In the equation 2H₂ + O₂ → 2H₂O, the coefficient 2 before H₂O represents:

1. a) 2 atoms of water
2. b) 2 molecules of water
3. c) 2 grams of water
4. d) 2 liters of water

1. Which of the following is a decomposition reaction?

1. a) 2Mg + O₂ → 2MgO
2. b) 2H₂O → 2H₂ + O₂
3. c) Zn + CuSO₄ → ZnSO₄ + Cu
4. d) HCl + NaOH → NaCl + H₂O

1. The state symbol (aq) stands for:

1. a) Air quality
2. b) Aqueous solution
3. c) Aquatic
4. d) Adequate quantity

1. In a balanced chemical equation, what must be equal on both sides?

1. a) Number of molecules
2. b) Number of atoms of each element
3. c) Number of compounds
4. d) Total mass only

1. What type of reaction is: A + BC → AC + B?

1. a) Combination
2. b) Decomposition
3. c) Single displacement
4. d) Double displacement

1. The equation N₂ + 3H₂ → 2NH₃ shows that nitrogen and hydrogen react in the mole ratio of:

1. a) 1:1
2. b) 1:3
3. c) 2:3
4. d) 3:2

1. Which equation is balanced?

1. a) H₂ + O₂ → H₂O
2. b) 2H₂ + O₂ → 2H₂O
3. c) H₂ + O₂ → 2H₂O
4. d) 2H₂ + 2O₂ → 2H₂O

1. When balancing equations, we change:

1. a) Subscripts
2. b) Coefficients
3. c) Chemical symbols
4. d) Chemical formulae

1. A combustion reaction always involves:

1. a) Water
2. b) Oxygen
3. c) Hydrogen
4. d) Carbon only

Section B: Theory Questions

1. State the law of conservation of mass.
2. Explain why chemical equations must be balanced.
3. Distinguish between a coefficient and a subscript in a chemical equation.
4. List and define FOUR types of chemical reactions with one example each.
5. What information can be obtained from a balanced chemical equation?

Section C: Practical Problems

1. Balance the following equations:

1. a) Fe + O₂ → Fe₃O₄
2. b) Al + HCl → AlCl₃ + H₂
3. c) C₂H₆ + O₂ → CO₂ + H₂O
4. d) Ca(OH)₂ + H₃PO₄ → Ca₃(PO₄)₂ + H₂O

1. Convert the following word equations to balanced chemical equations:

1. a) Copper reacts with oxygen to form copper(II) oxide
2. b) Silver nitrate solution reacts with sodium chloride solution to form silver chloride precipitate and sodium nitrate solution
3. c) Propane (C₃H₈) burns in oxygen to form carbon dioxide and water

1. Calculate:

1. a) How many moles of oxygen are needed to react with 5 moles of carbon in the reaction: C + O₂ → CO₂?
2. b) If 4.8g of magnesium reacts completely with oxygen to form magnesium oxide, what mass of oxygen is required? (Mg = 24, O = 16) Equation: 2Mg + O₂ → 2MgO