Quick Notes

Quick Notes

Chemistry Chapter 3: Mole Concept, Chemical Formulae and Equations
3.4 Chemical Equation
3.4 Chemical Equation
CHEMISTRY • Form 4 • Chapter 3: The Mole Concept, Chemical Formula and Equation

3.4Chemical Equation

Chemical equations provide a concise and precise method for chemists to represent chemical reactions. Learn to write, balance and interpret them, then use mole ratios to solve stoichiometric problems.

Learning Objectives

  • Write balanced chemical equations with correct formulae and physical states.
  • Interpret chemical equations qualitatively and quantitatively.
  • Solve stoichiometry numerical problems involving mass, number of particles, and gas volumes at STP or room conditions.

Flowchart of 5 Steps to Write a Balanced Chemical Equation

A clear visual roadmap from a word description to a fully balanced stoichiometric equation with physical states.

1
Word equation

Hydrogen + Oxygen → Water

2
Chemical formulae

H2 + O2 → H2O

3
Atom count inspection

Left: 2 H, 2 O  •  Right: 2 H, 1 O — oxygen is unbalanced.

4
Balancing coefficients

2H2 + O2 → 2H2O

5
Physical state insertion

2H2(g) + O2(g) → 2H2O(l)

Brief Explanation

Understand the structure, balancing rules and interpretation of chemical equations.

A chemical equation uses chemical symbols and formulae to describe a chemical reaction accurately. Reactants are positioned on the left and products on the right of an arrow indicating "produces".

Structure of a Chemical Equation

Reactants are written on the left, products on the right, separated by an arrow (→) meaning "produces". Reaction conditions such as heating may be shown around the arrow; the Greek letter delta (Δ) indicates that heating is required.

Physical State Symbols

State symbols are usually indicated in parentheses after each chemical formula: (s) solid, (l) liquid, (g) gas, and (aq) aqueous solution.

State symbol Meaning
(s)Solid
(l)Liquid
(g)Gas
(aq)Aqueous solution — substance dissolved in water
The law of conservation of mass states that matter can neither be created nor destroyed. Therefore, a chemical equation must be balanced by adjusting integer coefficients placed directly in front of formulae so that the count of each type of atom matches across both sides. Never alter chemical subscripts.

Qualitative Interpretation

For 2Na(s) + Cl2(g) → 2NaCl(s), the qualitative aspect identifies the reactants — solid sodium metal and chlorine gas — and the product, solid sodium chloride, together with their physical states.

Quantitative Interpretation

The coefficients represent both individual entity counts and relative quantities in moles. For the same reaction: 2 atoms of Na react with 1 molecule of Cl2 to produce 2 formula units of NaCl; equivalently, 2 mol of Na react with 1 mol of Cl2 to produce 2 mol of NaCl.

Stoichiometric conversion tools:
  • Number of moles = mass (g) ÷ molar mass (g mol−1)
  • Number of particles = number of moles × Avogadro constant (NA = 6.02 × 1023 mol−1)
  • Volume of gas = number of moles × molar volume
  • Molar volume at STP = 22.4 dm3 mol−1
  • Molar volume at room conditions = 24 dm3 mol−1

Try to Answer First

Answer mentally, then check the explanation.

1

What does the symbol (aq) placed after a chemical formula represent?

Answer: It represents an aqueous solution, meaning that the substance is dissolved in water.
2

Balance the following skeleton equation: Fe2O3(s) + H2(g) → Fe(s) + H2O(l)

Answer: Fe2O3(s) + 3H2(g) → 2Fe(s) + 3H2O(l)
3

Decomposition of calcium carbonate: CaCO3(s) → CaO(s) + CO2(g). Calculate the mass of CaCO3 needed to produce 1.2 dm3 of CO2 gas at room conditions. [Relative atomic mass: C = 12, O = 16, Ca = 40; Molar volume at room conditions = 24 dm3 mol−1]

Answer: Moles of CO2 = 1.2 dm3 ÷ 24 dm3 mol−1 = 0.05 mol. From the equation, 1 mol of CaCO3 produces 1 mol of CO2, so 0.05 mol of CaCO3 is required. Molar mass of CaCO3 = 40 + 12 + 3(16) = 100 g mol−1. Mass of CaCO3 = 0.05 mol × 100 g mol−1 = 5.0 g.

Common Mistakes

Avoid these errors when writing and using chemical equations.

!
Changing subscripts instead of coefficients. Students frequently alter formula subscripts to balance equations, for example writing H2O2 instead of 2H2O to balance oxygen, which changes the chemical identity of the substance.
!
Confusing gas molar volumes at STP and room conditions: using 24 dm3 mol−1 when STP (22.4 dm3 mol−1) is specified, or vice versa.
!
Omitting or misinterpreting physical state symbols: forgetting state symbols, confusing (l) with (aq), or omitting the Greek letter Δ when heat is required.
!
Ignoring stoichiometric mole ratios during conversions: directly multiplying or equating the calculated moles of a reactant to a product without applying the coefficient ratio from the balanced equation.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

Coefficients indicate the mass ratio of reacting substances.

Correct Fact

Coefficients indicate the mole ratio or entity ratio, not mass ratios. Reactants do not combine in simple mass ratios of their coefficients because different substances have different molar masses.

Misconception

The total number of molecules on the reactant side must equal the total number of molecules on the product side.

Correct Fact

Only the total number of atoms of each element is conserved. The total number of molecules can change during a reaction; for example, in N2 + 3H2 → 2NH3, 4 reactant molecules form 2 product molecules.

Misconception

The arrow symbol (→) denotes an equal sign (=).

Correct Fact

The arrow denotes a chemical transformation meaning "produces" or "yields", showing the direction of the reaction from reactants to products.

Summary

Key facts to remember.

  • A chemical equation represents a reaction using reactants on the left, products on the right, and an arrow (→) meaning "produces".
  • State symbols indicate physical forms: (s) solid, (l) liquid, (g) gas, and (aq) aqueous solution. A delta (Δ) indicates heating.
  • The law of conservation of mass requires chemical equations to be balanced by placing coefficients in front of formulae so atom totals match.
  • Stoichiometry uses coefficients as mole ratios to convert between masses, gas volumes at STP or room conditions, and numbers of particles.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

What does the Greek letter delta (Δ) written below or above the reaction arrow in a chemical equation signify?

2

In the balanced equation 2H2O2(l) → 2H2O(l) + O2(g), what volume of oxygen gas at STP is produced from the complete decomposition of 30.6 g of hydrogen peroxide, H2O2? [Relative atomic mass: H = 1, O = 16; Molar volume at STP = 22.4 dm3 mol−1]

B. Fill in the Blanks

3

Chemical equations must be balanced to satisfy the law of conservation of ________, which states that matter can neither be created nor destroyed.

4

The coefficient placed in front of AgNO3(aq) to balance the equation Zn(s) + AgNO3(aq) → Zn(NO3)2(aq) + 2Ag(s) is ________.

C. Match the Term or Symbol with Its Description

Drag each term on desktop. On mobile, tap a term and then tap the matching box.

Answer Choices

(aq) Stoichiometry Delta (Δ) Coefficient

Description Matching

1Denotes a substance dissolved in water to form an aqueous solution
2The quantitative study of the composition and ratios of substances involved in a chemical reaction
3Symbol written with the arrow to show that heating is required for the reaction
4Number placed in front of a chemical formula to show the mole ratio and balance atom counts
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Chemical equation Reactant Product Coefficient Law of conservation of mass Physical state Delta (Δ) Stoichiometry Mole ratio