ChemistryDifficulty 1-4

Chemical Reactions

A chemical reaction rearranges atoms: bonds in the starting substances (reactants) break and new bonds form, making new substances (products) with different properties. No atoms are created or destroyed, so mass is conserved and a chemical equation must balance. For a reaction to happen at all, particles must collide in the right orientation with at least the activation energy, which is why temperature changes reaction rates.

Stoichiometry and mass balance

Because coefficients are mole ratios and atoms are conserved, mass balance holds for any closed system: if 16.04 g of CH₄ reacts completely with 64.00 g of O₂, the products total 80.04 g. Open systems only appear to violate this when gases enter or leave unmeasured — the point of Lavoisier's careful weighing.

Kinetics in brief

Collision theory identifies two requirements — orientation and energy above Eₐ. Temperature acts mainly on the second, which is why rates are so sensitive to temperature and why refrigeration slows spoilage reactions.

Info: Industrial chemistry depends on controlling these factors: the Haber synthesis of ammonia from nitrogen and hydrogen runs at controlled temperature and pressure with a catalyst.

Common misconception: A balanced equation says nothing about how fast a reaction goes or by what steps; it is a statement of conservation, not of mechanism.

Full explanation — the complete reference version every reading depth is based on

What happens in a reaction

In a chemical reaction, the atoms of the reactants are rearranged into products — new substances with new properties. Burning methane in oxygen produces carbon dioxide and water. Melting ice, by contrast, is not a reaction: the water molecules stay H₂O, so it is a physical change.

\mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O}

Combustion of methane: 1 C, 4 H and 4 O atoms on each side.

Balancing equations

Because atoms are neither created nor destroyed, every element must appear the same number of times on both sides. You balance an equation by changing the coefficients in front of formulas, never the small subscripts — changing H₂O to H₂O₂ would describe a different substance.

Worked example

Start with aluminium and chlorine forming aluminium chloride: Al + Cl₂ → AlCl₃.
Chlorine: 2 on the left, 3 on the right. The lowest common multiple is 6, so write 3Cl₂ and 2AlCl₃.
Aluminium is now 1 on the left and 2 on the right, so write 2Al.
Check: 2Al + 3Cl₂ → 2AlCl₃ has 2 Al and 6 Cl on each side.

Coefficients are ratios: in methane combustion 1 methane reacts with 2 oxygen to give 1 carbon dioxide and 2 water, whether you count molecules, dozens or moles.

Common misconception: When wood burns or a candle shrinks, it can look as though matter is destroyed. It is not: the atoms leave as gases such as carbon dioxide and water vapour. Lavoisier showed by weighing everything, including the gases, that mass is conserved.

Why reactions happen at different speeds

Particles must collide with the correct orientation and with at least a minimum energy — the activation energy. At higher temperatures more collisions have enough energy, so reactions generally go faster; this is why milk sours more quickly at room temperature than in a refrigerator.

Why it matters

Reactions make the materials of modern life. Fritz Haber's synthesis of ammonia from nitrogen and hydrogen, run with a catalyst at controlled temperature and pressure and recognised by the 1918 Nobel Prize in Chemistry, gave a way to make ammonia for artificial fertiliser. Living cells run reactions too: aerobic respiration is summarised by the balanced equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (6 C, 12 H and 18 O atoms on each side), and photosynthesis by its reverse. A battery, meanwhile, uses a chemical reaction to push electrons round a circuit.

Warning: Real reactions can release heat, gases or corrosive products. Never mix household products to 'see what happens'; only carry out experiments from a trusted, safety-checked source with a responsible adult.

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Photosynthesis — Related to
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Sources and methodology

In a chemical reaction, atoms are rearranged to form new substances with new properties. (awaiting scientific review)
- 1.5: Chemical Reactions and Equations (Chemistry: Physical Principles), CC BY 4.0 — Textbook
A balanced chemical equation reflects the law of conservation of mass: atoms are neither created nor destroyed in a chemical reaction, only rearranged, so each element has the same number of atoms on both sides. (awaiting scientific review)
- 1.5: Chemical Reactions and Equations (Chemistry: Physical Principles), CC BY 4.0 — Textbook
Chemical equations are balanced by changing the coefficients in front of formulas, never the subscripts, because changing a subscript would change the identity of the substance. (awaiting scientific review)
- 1.5: Chemical Reactions and Equations (Chemistry: Physical Principles), CC BY 4.0 — Textbook
In the combustion of methane, CH₄ + 2O₂ → CO₂ + 2H₂O, methane, oxygen, carbon dioxide and water react and form in the ratio 1:2:1:2, whether counted in molecules or in moles. (awaiting scientific review)
- 1.5: Chemical Reactions and Equations (Chemistry: Physical Principles), CC BY 4.0 — Textbook
Antoine-Laurent Lavoisier popularised the principle of conservation of mass, demonstrating by carefully weighing all the components of a reaction, including gases, that matter is neither created nor destroyed, only transformed. (awaiting scientific review)
- Antoine-Laurent Lavoisier (Scientific Biographies) — Science media
For a reaction to occur, reactant particles must collide with the correct orientation and with at least a minimum energy, called the activation energy. (awaiting scientific review)
- 6.6: Collision Theory, Activation Energy, and the Arrhenius Equation (Chemistry: Physical Principles), CC BY 4.0 — Textbook
Changes of state such as melting and boiling are physical changes, not chemical changes: water vapour has the same chemical composition, H₂O, as liquid water or ice. (awaiting scientific review)
- Phases of Matter (Beginner's Guide to Aeronautics) — Government or standards body
Fritz Haber received the 1918 Nobel Prize in Chemistry for synthesising ammonia from nitrogen and hydrogen, a reaction run in the presence of a catalyst whose product is used to make fertiliser. (awaiting scientific review)
- The Nobel Prize in Chemistry 1918: Fritz Haber — Facts — Other (unclassified)
Chemical reactions generally proceed faster at higher temperatures; milk, for example, turns sour much more quickly at room temperature than in a refrigerator. (awaiting scientific review)
- 6.2.3.1: Arrhenius Equation (Stephen Lower), CC BY — Textbook

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

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