Chemical Reactions

Chemical Reactions

How atoms rearrange into new substances, and how to write it down

A candle burns, a nail rusts, bread browns in the toaster. In each case the atoms you started with end up arranged into new substances. That is a chemical reaction, and a chemical equation is the shorthand chemists use to describe it.[1]

On this page we'll learn to spot a reaction, write and balance its equation, sort reactions into five common types, and use moles to work out how much of each substance takes part. Try the equation balancer and the mole calculator, then finish with the quiz. You'll need what you learned in Chemical Bonding: reactions are bonds breaking and new bonds forming.

Signs of a Chemical Change

In a physical change, like melting ice, the substance stays the same. In a chemical change, new substances form.[1]

Clues to look for

  • Gas bubbles forming when nothing is being boiled
  • A color change, like a shiny nail turning rust-brown
  • A solid (a precipitate) appearing when two clear solutions mix
  • The mixture getting hotter or colder by itself
  • Light given off, as in a flame
  • A new smell

Clues, not proof

Each clue can fool you. Boiling water makes bubbles, but it's still water. Dissolving some salts makes a solution cold, which is how instant cold packs work, yet no new substance forms.[8] The real test is whether a new substance with new properties has formed.

Reading a Chemical Equation

A chemical equation lists the starting substances, the reactants, on the left and the new substances, the products, on the right, joined by an arrow that means “react to form.”[2] Here is hydrogen burning in oxygen to make water:

2H2(g) + O2(g) → 2H2O(l)

Conservation of Mass and Balancing

In his 1789 textbook Traité élémentaire de chimie, the French chemist Antoine-Laurent Lavoisier wrote, in Robert Kerr's 1790 English translation, that “in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment.”[3] We now call this the law of conservation of mass. Atoms are not created or destroyed in a reaction, only rearranged, so a correct equation must have the same number of each kind of atom on both sides.[2]

+ → 2H₂ + O₂4 H, 2 O atoms2H₂O4 H, 2 O atoms

Balancing step by step

  1. Write the correct formulas for every reactant and product. Never change them after this.
  2. Count the atoms of each element on each side.
  3. Change coefficients only, one element at a time. Elements that appear in just one substance per side are easiest to start with; leave lone elements such as O2 until last.
  4. Recount everything after every change.
  5. Finish with the lowest whole-number coefficients. If every coefficient can be divided by the same number, divide.

Worked example: methane burning

CH4 + O2 → CO2 + H2O

Carbon: 1 on each side, fine. Hydrogen: 4 on the left, 2 on the right, so put a 2 in front of H2O. Oxygen: now 2 + 2 = 4 on the right, so put a 2 in front of O2.

CH4 + 2O2 → CO2 + 2H2O

Check: 1 C, 4 H and 4 O on each side.

Equation Balancer

Pick an equation, then use the + and − buttons to change the coefficients. The counters show the atoms on each side.

Five Common Reaction Types

Many high school courses sort reactions into these five patterns. They don't cover every reaction, but they help you predict products.

Synthesis

Two or more substances combine into one. A + B → AB

2Na + Cl2 → 2NaCl

Decomposition

One substance breaks into simpler ones. AB → A + B

CaCO3(s) →heat CaO(s) + CO2(g)

Heating limestone makes lime and carbon dioxide.

Single replacement

An element takes the place of another in a compound. A + BC → AC + B

Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

Double replacement

Two compounds swap partners. AB + CD → AD + CB

AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

The white solid AgCl is a precipitate.

Combustion

A fuel reacts with oxygen, usually releasing heat and light. Burning a hydrocarbon completely gives CO2 and H2O.

CH4 + 2O2 → CO2 + 2H2O

A note on redox

In many reactions electrons move from one atom to another. Losing electrons is oxidation and gaining them is reduction, and the two always happen together. In 2Na + Cl2 → 2NaCl, sodium is oxidized and chlorine is reduced.[4] The synthesis, single replacement and combustion examples above are all redox reactions; the double replacement one is not. College textbooks often sort reactions as precipitation, acid–base and redox instead.[4]

Moles and Simple Stoichiometry

Atoms are far too small to count one by one, so chemists count them in batches called moles. One mole contains exactly 6.022 140 76 × 1023 particles. This is the Avogadro number, fixed exactly when the SI was revised in 2018.[5][6][14]

The molar mass of a substance is the mass of one mole, in grams per mole (g/mol). Add up the atomic masses in the formula:[7][9]

SubstanceWorkingMolar mass
H22 × 1.0082.016 g/mol
O22 × 15.99931.998 g/mol
H2O2 × 1.008 + 15.99918.015 g/mol
CO212.011 + 2 × 15.99944.009 g/mol

The coefficients in a balanced equation give the mole ratio. In 2H2 + O2 → 2H2O, 2 mol of H2 react with 1 mol of O2 to make 2 mol of H2O.[10]

Gram to gram: how much water from 4.0 g of hydrogen?

  1. Grams to moles: 4.0 g ÷ 2.016 g/mol = 1.98 mol H2
  2. Mole ratio: 1.98 mol H2 × (2 mol H2O ÷ 2 mol H2) = 1.98 mol H2O
  3. Moles to grams: 1.98 mol × 18.015 g/mol ≈ 36 g H2O

The 1.98 mol of H2 also needs 0.99 mol (about 32 g) of O2. And 4.0 g + 32 g ≈ 36 g: mass is conserved.

Limiting reactants. Real mixtures rarely have the exact ratio. If you mix 4 mol of H2 with 1 mol of O2, the oxygen runs out first after reacting with only 2 mol of H2. Oxygen is the limiting reactant: it decides that 2 mol of water form, and 2 mol of H2 are left over.[11] The Stoichiometry unit goes much further.

Mole Calculator

Pick a substance and type a mass. The calculator shows the moles and the number of particles.

18.015 g of water is 1.000 mol, about 6.022 × 10²³ molecules.

Atomic masses: H 1.008, C 12.011, N 14.007, O 15.999, Na 22.990, Cl 35.45.[9] For NaCl the “particles” are formula units, since salt has no separate molecules.

Reaction Energy

Breaking bonds takes energy and forming bonds releases it. A reaction that releases heat to its surroundings is exothermic, like the burning acetylene in a welding torch. A process that absorbs heat is endothermic, like the cold pack mentioned earlier.[8] The heat change at constant pressure is called the enthalpy change, ΔH: negative for exothermic, positive for endothermic.[12]

Even exothermic reactions usually need a push to get started. A match has to be struck; a candle needs a flame. That minimum energy colliding particles need to react is the activation energy, Ea.[13]

Exothermic: energy released EnergyReaction progress ReactantsProducts Ea ΔH < 0 Endothermic: energy absorbed EnergyReaction progress ReactantsProducts Ea ΔH > 0

The hump is the activation energy. The gap between reactants and products is ΔH. These are sketches, not to scale.

How Do We Know?

Equations look neat on paper. Here is some of the evidence that they describe what atoms really do.

Weighing everything

In the 1770s, burning was explained by “phlogiston,” a substance thought to escape from things as they burned. Yet metals heated in air got heavier, not lighter. Lavoisier showed that phosphorus and sulfur gain weight when they burn because they combine with part of the air, the part he named oxygen. In 1783 he made water from oxygen and hydrogen and then split water back into them, showing water is a compound.[15] Careful weighing, before and after, is what turned chemistry into a measuring science.

Counting atoms in a silicon sphere

How can anyone know how many particles are in a mole? Scientists made nearly perfect spheres of highly purified silicon, measured their mass and volume, used X-rays to measure the spacing between atoms in the crystal, and used mass spectrometers to measure the mix of silicon isotopes. Combining these let them, in effect, count the atoms.[6] That work helped fix the Avogadro number exactly.[5]

Measuring heat

Whether a reaction is exothermic or endothermic is measured with a thermometer. In a calorimeter, the heat released or absorbed changes the temperature of a known mass of water, and since it takes 4.184 J to warm 1 g of water by 1 °C, the temperature change tells you the energy.[8]

Test Yourself

Ten questions. Click an answer to check it and read why. The balancer and calculator above are allowed!

The Reaction Challenge

Think About It

Questions to puzzle over in class or at home.

  1. A log burns down to a small pile of ash. Does that break the law of conservation of mass? Where did the mass go, and how could you prove it?
  2. Why are you allowed to change coefficients but never subscripts when you balance an equation?
  3. Gasoline and oxygen can sit together in a car's tank without reacting. Use the idea of activation energy to explain why.
  4. Why do you think scientists decided to fix the Avogadro number at an exact value instead of measuring it again and again?

Next unit: The Mole and Stoichiometry →

Back to all chemistry lessons

Sources

  1. OpenStax, Chemistry 2e, “1.3 Physical and Chemical Properties” (physical and chemical change). https://openstax.org/books/chemistry-2e/pages/1-3-physical-and-chemical-properties
  2. OpenStax, Chemistry 2e, “4.1 Writing and Balancing Chemical Equations” (reactants, products, coefficients, state symbols, balancing). https://openstax.org/books/chemistry-2e/pages/4-1-writing-and-balancing-chemical-equations
  3. Antoine-Laurent Lavoisier, Traité élémentaire de chimie (Paris, 1789); English translation by Robert Kerr, Elements of Chemistry (Edinburgh, 1790), Project Gutenberg eBook #30775. The quotation is from the chapter on vinous fermentation. https://www.gutenberg.org/ebooks/30775
  4. OpenStax, Chemistry 2e, “4.2 Classifying Chemical Reactions” (precipitation, acid–base and oxidation–reduction reactions). https://openstax.org/books/chemistry-2e/pages/4-2-classifying-chemical-reactions
  5. BIPM, “SI base unit: mole” (one mole contains exactly 6.022 140 76 × 1023 elementary entities). https://www.bipm.org/en/si-base-units/mole
  6. NIST, “Redefining the Mole” (silicon spheres, X-ray and mass spectrometry measurements, 2018 SI vote). https://www.nist.gov/si-redefinition/mole
  7. OpenStax, Chemistry 2e, “3.1 Formula Mass and the Mole Concept.” https://openstax.org/books/chemistry-2e/pages/3-1-formula-mass-and-the-mole-concept
  8. OpenStax, Chemistry 2e, “5.1 Energy Basics” (exothermic and endothermic processes, cold packs, specific heat of water 4.184 J/g °C). https://openstax.org/books/chemistry-2e/pages/5-1-energy-basics
  9. PubChem, “Periodic Table of Elements” (atomic masses). https://pubchem.ncbi.nlm.nih.gov/periodic-table/
  10. OpenStax, Chemistry 2e, “4.3 Reaction Stoichiometry” (mole ratios, gram-to-gram calculations). https://openstax.org/books/chemistry-2e/pages/4-3-reaction-stoichiometry
  11. OpenStax, Chemistry 2e, “4.4 Reaction Yields” (limiting reactant). https://openstax.org/books/chemistry-2e/pages/4-4-reaction-yields
  12. OpenStax, Chemistry 2e, “5.3 Enthalpy.” https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy
  13. OpenStax, Chemistry 2e, “12.5 Collision Theory” (activation energy). https://openstax.org/books/chemistry-2e/pages/12-5-collision-theory
  14. NIST, “CODATA Value: Avogadro constant.” https://physics.nist.gov/cgi-bin/cuu/Value?na
  15. American Chemical Society, “The Chemical Revolution of Antoine-Laurent Lavoisier,” International Historic Chemical Landmark (phosphorus and sulfur, oxygen, water). https://www.acs.org/education/whatischemistry/landmarks/lavoisier.html

Atomic masses are from the PubChem periodic table, the same data used on our periodic table page.