What is a chemical reaction? How substances change

A chemical reaction is a process in which one or more substances, the reactants, turn into different substances, the products, because their atoms rearrange. Bonds between atoms break and new bonds form, but the atoms themselves are not created, destroyed or changed into other elements. That is what separates a chemical reaction from melting ice, which changes a substance's form, and from a nuclear reaction, which changes the atoms' nuclei.

In short

• In a reaction, atoms swap partners. Every atom that goes in comes out, so the total mass doesn't change.
• A chemical equation is a count of atoms, and balancing it means making the count the same on both sides.
• Whether a reaction gives off heat or soaks it up depends on whether the bonds it forms are stronger or weaker than the bonds it breaks.

How can you tell a chemical reaction has happened?

The sure test is that a new substance exists afterward, with different properties from what you started with. Iron and oxygen become rust, a reddish powder that is neither metal nor gas. In practice chemists look for clues, and each can be misleading on its own:

Clue	Usually a reaction when…	Can mislead when…
Gas bubbles	Baking soda fizzes in vinegar, making carbon dioxide	Water boils: the bubbles are still water, a physical change
New color	A cut apple browns as its compounds react with oxygen	Food coloring spreads in water, which is just mixing
Heat or light	Wood burns, a glow stick glows	A light bulb heats up from electric current, with no new substance
A solid appears in a liquid	Two clear solutions mix and a cloudy solid, a precipitate, forms	Salt crystals appear as salty water evaporates
Hard to reverse	A cooked egg can't be uncooked	Some reactions do run backward easily, such as charging a battery

The question to ask is always whether the substance itself has changed, not whether something looks different. Ice melting, sugar dissolving and water boiling are physical changes: the molecules are the same before and after.

What is happening to the atoms?

Reactions happen when particles collide hard enough and in the right orientation for bonds to break and new ones to form. The bonds involved are the subject of the chemical bond note; a reaction is those bonds being traded. In every case the work is done by the atoms' outer electrons, which are shared, handed over or rearranged.

Chemists sort reactions into a few broad families. In combustion, a fuel combines with oxygen, releasing heat and light. In redox (reduction–oxidation) reactions, electrons move from one substance to another; rusting, batteries and the way cells burn food are all redox. In acid–base reactions, a hydrogen ion passes from an acid to a base, which is what pH measures. In precipitation reactions, dissolved ions combine into a solid that falls out of solution. Many reactions belong to more than one family: burning is a redox reaction too.

How do you balance a chemical equation? A worked example

Methane, CH₄, is the main ingredient of natural gas. When it burns in oxygen, O₂, it makes carbon dioxide, CO₂, and water, H₂O. Written straight out:

CH₄ + O₂ → CO₂ + H₂O

That is unbalanced: it has 4 hydrogen atoms on the left and only 2 on the right. Balance it one element at a time, changing only the numbers in front of each formula, never the formulas themselves:

1. Carbon: 1 on the left, 1 on the right. Already balanced.
2. Hydrogen: 4 on the left, 2 on the right. Put a 2 in front of the water: CH₄ + O₂ → CO₂ + 2H₂O. Now 4 and 4.
3. Oxygen: the right side now has 2 in CO₂ plus 2 in the two waters, 4 in all. The left has 2. Put a 2 in front of the oxygen: CH₄ + 2O₂ → CO₂ + 2H₂O.
4. Check: left, 1 C, 4 H, 4 O. Right, 1 C, 4 H, 4 O.

The balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O, and it is a recipe in molecules: one methane molecule needs two oxygen molecules.

Now check the mass, using molar masses (the mass of one mole, 6.02 × 10²³ molecules): methane 16.04 g, oxygen 32.00 g, carbon dioxide 44.01 g, water 18.02 g.

• Left: 16.04 + 2 × 32.00 = 80.04 grams.
• Right: 44.01 + 2 × 18.02 = 80.05 grams, the same to within rounding.

So 16 grams of methane burned in 64 grams of oxygen gives 44 grams of carbon dioxide and 36 grams of water. Nothing appears and nothing vanishes. Antoine Lavoisier established this law, the conservation of mass, in the late 1700s by weighing reactions carefully in sealed vessels. When wood burns and leaves a light pile of ash, the missing mass has left as carbon dioxide and water vapor; burn it in a sealed jar and the jar weighs the same after.

Why do some reactions release heat and others absorb it?

Breaking bonds always takes energy, and forming bonds always releases it. A reaction's overall energy change is the difference. Burning methane is exothermic: the bonds in carbon dioxide and water are stronger than those in methane and oxygen, so energy comes out. Measured carefully, burning one mole of methane, 16 grams, releases about 890 kilojoules, according to the NIST Chemistry WebBook. That is about 56 kJ per gram, which is why natural gas heats so many homes.

Endothermic reactions run the other way: their products hold more energy than their reactants, so they need a supply of it. Photosynthesis is the great example, using light to turn carbon dioxide and water into sugar and oxygen. Respiration, the way living cells release energy from food, runs nearly the same reaction in reverse, and the sum of such reactions in a body is its metabolism.

Here is where the simple picture of conserved mass bends slightly. By E = mc², the 890 kJ that leaves burning methane carries away a little mass: 890,000 J divided by c², about 9 × 10¹⁶, is roughly 1 × 10⁻¹¹ kg, or 10 nanograms out of 80 grams. That is about one part in ten billion, far too small for any balance to detect, which is why chemists treat mass as exactly conserved.

What controls how fast a reaction goes?

Most reactions need a push to get started, even when they release energy overall. A gas stove doesn't light itself: methane and oxygen can sit mixed until a spark supplies the activation energy, the energy needed to start breaking the first bonds. After that, the heat released keeps the flame going.

How fast a reaction runs depends on how often particles collide with enough energy:

• Temperature: hotter particles move faster and hit harder. A common rule of thumb is that many reactions roughly double in speed for every 10 °C rise, which is why food keeps longer in the fridge.
• Concentration: more particles in the same space means more collisions.
• Surface area: a pile of flour burns slowly, but flour dust in the air can explode, because every grain is exposed to oxygen at once.
• Catalysts: a catalyst speeds up a reaction by offering an easier route, with a lower activation energy, and comes out unchanged at the end.

Catalysts are everywhere. A car's catalytic converter uses metals such as platinum and palladium to turn poisonous carbon monoxide into carbon dioxide. Most nitrogen fertilizer depends on an iron catalyst that combines nitrogen from the air with hydrogen to make ammonia. And every living cell runs on catalysts made of protein, called enzymes, which can make a reaction millions of times faster or more.

Do reactions always run to completion?

Not always. Many reactions are reversible: as products build up, they start reacting back into the reactants, until the forward and backward reactions run at the same rate. At that point, called equilibrium, the amounts stop changing even though both reactions continue. Conditions can shift the balance one way or the other, which is how chemical engineers coax more product out of a reaction and how a rechargeable battery is charged and discharged.

Check yourself

• Without looking back, explain why boiling water is not a chemical reaction even though it makes bubbles.
• Balance the equation for hydrogen burning in oxygen to make water, and check that the atoms match.
• Why does a gas stove need a spark if burning methane releases energy?

Sources

• Writing and balancing chemical equations — OpenStax, Chemistry 2e
• Catalysis — OpenStax, Chemistry 2e
• Methane: gas phase thermochemistry data — NIST Chemistry WebBook
• Conservation of mass — Wikipedia
• Chemical reaction — Wikipedia