Chemical Reactions and Equations

Reactants, products, and evidence

A chemical reaction rearranges atoms. Starting substances are reactants. New substances are products. An arrow means “yields.”

Evidence may include gas formation, a new solid, a lasting color change, or heat or light. A phase change alone is not a chemical reaction because no new substance forms.

Read an equation from left to right. In 2H2 + O2 → 2H2O, hydrogen and oxygen are reactants and water is the product. The plus sign separates substances; it does not mean ordinary arithmetic addition.

Balancing without changing substances

Atoms are conserved in ordinary reactions, so each element must have the same atom count on both sides. Balance an equation by changing coefficients, the numbers before formulas.

Never change a subscript to balance. A subscript is part of a substance’s identity. Example: 2H2 + O2 → 2H2O has four H atoms and two O atoms on each side.

Worked example. Balance N2 + H2 → NH3. First place 2 before NH3 to match two N atoms. This gives six H atoms on the right, so place 3 before H2. The balanced equation is N2 + 3H2 → 2NH3.

Count atoms on both sides

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Reaction patterns: supporting vocabulary

Use these names as a compact reference. Prioritize identifying reactants/products, conserving atoms and using coefficient ratios over memorizing a long taxonomy.

In synthesis, smaller substances combine: A + B → AB. In decomposition, one substance splits: AB → A + B. In replacement reactions, atoms or ions trade partners.

Combustion uses oxygen and releases energy. Complete combustion of a hydrocarbon makes carbon dioxide and water.

A single replacement has the pattern A + BC → AC + B. A double replacement has the pattern AB + CD → AD + CB. Pattern names describe how particles rearrange; they do not replace the need to balance.

Coefficients are particle ratios

Coefficients show ratios of particles or moles. In 2H2 + O2 → 2H2O, two units of hydrogen react with one unit of oxygen to make two units of water.

A coefficient multiplies every atom in its formula. Count atoms first, then adjust coefficients until both sides match.

Worked example. In 2CO + O2 → 2CO2, the ratio is 2:1:2. If four CO particles react completely with enough oxygen, they need two O2 particles and can make four CO2 particles.

A mole is an amount containing about 6.022 × 10²³ specified entities. Specify atoms, molecules, or formula units. Molar mass is grams per mole, found by adding the atomic masses in a formula. Given H = 1 g/mol and O = 16 g/mol, water has molar mass 2(1) + 16 = 18 g/mol.

Worked amount-to-reaction example: 36 g H2O ÷ 18 g/mol = 2 mol H2O. In 2H2 + O2 → 2H2O, making 2 mol water needs 1 mol oxygen when enough hydrogen is available. The 2:1:2 coefficients are mole or molecule ratios, not gram ratios. Multi-step limiting-reagent and yield calculations are optional chemistry extension, not required here.

Use a supplied value of Avogadro’s number when converting particles and moles; memorizing its digits is not a study goal.

Your quick summary

• Reactants appear before the arrow; products appear after it.
• Balanced equations conserve each kind of atom.
• Only coefficients may be changed when balancing.
• Synthesis combines; decomposition separates.
• Coefficients show particle ratios.