Chemical Reactions and Stoichiometry

Grade 9 · Science · Study Guide

Learn how atoms rearrange during chemical reactions and how to use balanced equations to predict quantities of reactants and products.

A chemical reaction is a process in which substances (reactants) are transformed into new substances (products) through the breaking and forming of chemical bonds. Evidence that a chemical reaction has occurred includes color change, gas production, precipitate formation, temperature change, or light emission. Crucially, in a chemical reaction, atoms are neither created nor destroyed — they are rearranged. This is the Law of Conservation of Mass: the total mass of reactants always equals the total mass of products.

Chemical reactions are represented by balanced chemical equations. A balanced equation has equal numbers of each type of atom on both sides. Coefficients (numbers in front of formulas) are adjusted to balance equations — the subscripts within a formula are never changed. For example: 2H₂ + O₂ → 2H₂O shows that 2 molecules of hydrogen gas react with 1 molecule of oxygen to produce 2 molecules of water. Common reaction types include synthesis (A + B → AB), decomposition (AB → A + B), single replacement (A + BC → AC + B), and double replacement (AB + CD → AD + CB). Combustion reactions burn a fuel in oxygen, producing carbon dioxide and water.

Stoichiometry uses the mole concept and molar ratios from balanced equations to calculate the amounts of reactants or products. The mole is a counting unit: 1 mole = 6.022 × 10²³ particles (Avogadro's number). The molar mass of an element (g/mol) is its atomic mass from the periodic table.

Types of Chemical Reactions
Type	General Form	Example
Synthesis	A + B → AB	2Na + Cl₂ → 2NaCl
Decomposition	AB → A + B	2H₂O → 2H₂ + O₂
Single Replacement	A + BC → AC + B	Zn + 2HCl → ZnCl₂ + H₂
Double Replacement	AB + CD → AD + CB	AgNO₃ + NaCl → AgCl + NaNO₃
Combustion	Fuel + O₂ → CO₂ + H₂O	CH₄ + 2O₂ → CO₂ + 2H₂O

Key terms

Reactant: A starting substance in a chemical reaction; written on the left side of the equation.
Product: A substance formed in a chemical reaction; written on the right side of the equation.
Law of Conservation of Mass: Mass is neither created nor destroyed in a chemical reaction; total mass of reactants equals total mass of products.
Balanced Equation: A chemical equation where the number of each type of atom is equal on both sides.
Coefficient: Number placed in front of a chemical formula in an equation to balance it; indicates the relative number of moles.
Mole: SI unit for amount of substance; 1 mole = 6.022 × 10²³ particles (Avogadro's number).
Stoichiometry: The calculation of quantities (moles, grams, liters) in chemical reactions using balanced equations and molar ratios.

Flashcards

Reactant: Starting material on the left side of a chemical equation.
Product: Substance formed; on the right side of a chemical equation.
Law of Conservation of Mass: Atoms are rearranged but never created or destroyed in a reaction — total mass stays constant.
Synthesis Reaction: A + B → AB; two or more substances combine to form one product.
Decomposition Reaction: AB → A + B; one compound breaks down into two or more simpler substances.
Mole: 6.022 × 10²³ particles; SI unit for measuring amount of chemical substance.

Practice questions

In the equation 2H₂ + O₂ → 2H₂O, what are the reactants?
1. H₂O only
2. H₂ and O₂
3. H₂O and O₂
4. H₂ only
Show answer
Answer: H₂ and O₂
Reactants are the starting materials, written to the LEFT of the arrow. Products are written to the right. In 2H₂ + O₂ → 2H₂O, hydrogen and oxygen are reactants; water is the product.
In a balanced chemical equation, you can change the subscripts in a formula to make both sides equal.
Show answer
Answer: False
To balance an equation you change COEFFICIENTS (numbers in front of formulas), never subscripts. Changing subscripts would create a different compound entirely (e.g., H₂O vs H₂O₂).
The law that states mass is neither created nor destroyed in a chemical reaction is the Law of Conservation of ___.
Show answer
Answer: Mass
The Law of Conservation of Mass (Lavoisier, 1789) states that matter cannot be created or destroyed in a chemical reaction — only rearranged. This is why equations must be balanced.
What type of reaction is: AB → A + B?
1. Synthesis
2. Combustion
3. Decomposition
4. Single replacement
Show answer
Answer: Decomposition
Decomposition breaks one compound (AB) into simpler components (A + B). Water splitting by electrolysis (2H₂O → 2H₂ + O₂) is a classic example.
How many atoms of hydrogen are on each side of the balanced equation: 2H₂ + O₂ → 2H₂O?
1. 2
2. 4
3. 6
4. 8
Show answer
Answer: 4
Left side: 2H₂ = 2 × 2 = 4 hydrogen atoms. Right side: 2H₂O = 2 × 2 = 4 hydrogen atoms. The equation is balanced — equal atoms on each side.

Fun facts

Avogadro's number (6.022 × 10²³) is so enormous that a mole of popcorn kernels would cover the entire United States 9 miles deep.
The rusting of iron is a chemical reaction (4Fe + 3O₂ → 2Fe₂O₃) that releases heat so slowly you can't feel it — but it is still an exothermic reaction.
Baking soda (NaHCO₃) and vinegar (acetic acid) undergo a double-replacement reaction that releases CO₂ gas — the familiar fizzing in a science fair volcano.

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