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Home›Chemistry

Chemical reactions

Burning gas, rusting iron and baking soda fizzing in vinegar are all atoms trading partners. Balance equations with atom-count bars, weigh a reaction in a closed and an open container, and watch particles collide faster as the temperature rises.

Progress

About this course

Melt an ice cube and you still have water. Dissolve sugar in tea and the sugar molecules are still sugar, only spread out. These are physical changes. A chemical change always produces matter that differs from what was there before: rust is a different substance from the iron, oxygen and water it came from. The course is about what stays the same through such a change, which is every atom.

The six units are what a chemical reaction is, balancing equations, conservation of mass, types of reaction, energy in reactions, and reaction rates. Write hydrogen burning as H₂ + O₂ → H₂O and count: two oxygen atoms on the left and only one on the right, so one has gone missing, which cannot happen. Conservation of mass gets a worked example in moles. CH₄ + 2O₂ → CO₂ + 2H₂O has 80.039 g of reactants, and the products add to 80.039 g as well. A log weighs a few kilograms and the ash far less, while a nail grows heavier as it rusts, because the scale was not weighing the gases. Types of reaction include single replacement, as when a copper wire in silver nitrate solution grows silver crystals while the liquid turns blue. The atom-count bars in the lab show each reaction keeping every atom it started with.

Energy and rate close the course. If the products have less enthalpy than the reactants, ΔH is negative and the reaction is exothermic. Collision theory explains rates with three ideas: particles must meet, meet the right way round, and hit hard enough to reach the activation energy. In the particle model the lab uses a lowered energy barrier so reactions appear within seconds.

Before you start

Equations here are written with formulas such as CH₄ and Fe₂O₃, so you should know what atoms and molecules are. Atoms and Chemical bonds cover that ground. The conservation of mass unit also uses molar masses in g/mol, which the bonds course introduces.

When you finish

You will be able to balance equations such as Fe + O₂ → Fe₂O₃, show that mass is conserved when methane burns, and sort reactions into synthesis, decomposition, single and double replacement, combustion and redox. You can also work out the heat from burning two moles of methane, and explain why sawdust burns faster than a log and why food keeps longer in a fridge.

  1. 1What is a chemical reaction?One methane molecule and two oxygen molecules become one carbon dioxide and two water molecules, and not a single atom is lost on the way.
  2. 2Balancing chemical equationsHydrogen and oxygen make water only in the ratio 2 : 1 : 2, because one O₂ molecule carries enough oxygen for two water molecules.
  3. 3Conservation of massBurn 16.04 g of methane in 64.00 g of oxygen and you get 44.01 g of carbon dioxide and 36.03 g of water: 80.04 g before and after.
  4. 4Types of chemical reactionsDrop a copper wire into silver nitrate solution and silver crystals grow on it while the liquid turns blue: copper has swapped places with silver.
  5. 5Energy in chemical reactionsBurning one mole of methane, 16 g, releases 890.8 kJ of heat, enough to heat about 2.7 litres of water from 20 °C to 100 °C.
  6. 6Reaction ratesFor many reactions, raising the temperature by 10 °C roughly doubles the rate, which is why food keeps for days in a fridge but spoils on a warm counter.

Related courses

  • States of matter
    Why does ice float, why does water boil at 90 °C high in the mountains, and why does a syringe get harder to push? Move the temperature and watch particles lock into a crystal, slide as a liquid and fly apart as a gas, then follow a real heating curve and Boyle's law with numbers from OpenStax and NIST.
  • Atoms
    Everything you can touch is made of atoms. The three particle types used to describe atoms are protons, neutrons and electrons; ordinary hydrogen has no neutron. Build atoms from hydrogen to calcium in 3D and see how the count of each particle decides what the atom is.
  • The periodic table
    All 118 known elements fit on one chart of 18 columns and 7 rows. Raise the tiles in 3D by mass, size or electronegativity and the patterns that chemists use every day show up as hills and valleys.
  • Chemical bonds
    Why does salt form cubes, why is water bent, and why can copper be drawn into wire? Turn 14 molecules in 3D, switch between ball-and-stick and space-filling views, and measure the bond angles that decide how each substance behaves.
Sources for this course (14)
  • OpenStax Chemistry 2e (openstax.org/books/chemistry-2e): 1.2 law of conservation of matter, 1.3 chemical change (rust, combustion, cooking and rotting)
  • 4.1 balancing by inspection (CH4 + 2O2, 2H2O to 2H2 + O2, 2N2 + 5O2 to 2N2O5)
  • 4.2 precipitation (PbI2 from KI and Pb(NO3)2, AgCl insoluble)
  • redox (Na + Cl2, copper wire in silver nitrate)
  • combustion, 5.1 specific heat of water 4.184 J/g °C, 5.3 enthalpy (ΔH < 0 exothermic; ΔHc CH4 −890.8, H2 −285.8, ethanol −1366.8, methanol −726.1 kJ/mol; 1.00 L isooctane 33,100 kJ; ozone formation endothermic)
  • 12.2 factors affecting rates (sodium and iron in air, calcium and sodium in water, sawdust, rate roughly doubles per 10 °C, phosphorus in pure oxygen, air about 20% oxygen, H2O2 decomposition)
  • 12.5 collision theory and Arrhenius equation (R = 8.314 J/mol/K, catalytic converters)
  • 12.7 catalysis and enzymes
  • NIST Chemistry WebBook methane ΔcH°gas −890.7 ± 0.4 kJ/mol (webbook.nist.gov)
  • NIST CODATA Avogadro constant 6.022 140 76 × 10²³ mol⁻¹ exact (physics.nist.gov)
  • IUPAC CIAAW abridged standard atomic weights (H 1.0080, C 12.011, N 14.007, O 15.999, Na 22.990, Ca 40.078, Fe 55.845, Sn 118.71; ciaaw.org)
  • ACS International Historic Chemical Landmark, The Chemical Revolution of Antoine-Laurent Lavoisier (acs.org, Traité élémentaire de chimie 1789, quotation on conservation of mass)
  • Lavoisier, memoir on the calcination of tin in closed vessels, Mémoires de l'Académie Royale des Sciences for 1774, published 1777 (via web.lemoyne.edu/giunta/classicalcs/lavoisier.html)
  • PubChem sodium bicarbonate and acetic acid pages (pubchem.ncbi.nlm.nih.gov).

Checked: 2026-10-17