Chemical Reactions Overview

Study Notes

A chemical reaction is a process where atoms are rearranged to form new substances.
Reactants are the starting substances, and products are the resulting substances.
Reactions can be represented by word equations (e.g., copper + oxygen → copper oxide) or balanced symbol equations (e.g., 2Cu + O₂ → 2CuO).
A balanced symbol equation shows the formula of each substance, how atoms rearrange, and the relative number of atoms in each substance.

All chemical reactions involve the transfer of energy to or from the surroundings.
Energy Transfer: Reactions can either absorb energy (endothermic) or release energy (exothermic).
- Endothermic reactions absorb energy from their surroundings, causing a decrease in temperature.
- Exothermic reactions release energy to their surroundings, causing an increase in temperature.
Conservation of Mass: In a chemical reaction, atoms are rearranged, but not created or destroyed. The total mass of reactants equals the total mass of products.
Physical Changes: Changes of state (e.g., water to ice to steam) are not chemical reactions but are physical changes—they are reversible.
Signs that a chemical reaction is happening: Seeing flames or sparks, noticing a smell, hearing fizzing, or feeling temperature changes.

Thermal Decomposition: A decomposition reaction where a substance breaks down into simpler substances when heat is applied.
Combustion: A type of oxidation reaction where a fuel reacts with oxygen to release energy in the form of light and heat. Fossil fuels (petrol, diesel, coal) are non-renewable sources of energy, producing carbon dioxide when burned, which is harmful. Hydrogen can also be combusted, releasing only water as a byproduct.
Oxidation: A reaction where a substance reacts with oxygen.

Some reactions are fast, others slow. Catalysts can speed up reactions without being used up in the reaction. Different reactions require different catalysts.

Balanced symbol equation: Shows the chemical formulas and the number of atoms of each element involved in a reaction.
Catalyst: A substance that speeds up a reaction without being consumed.
Chemical reaction: A process where atoms rearrange to form new substances.
Combustion: A reaction that releases energy, usually in the form of heat and light.
Conservation of mass: The total mass of reactants is always equal to the total mass of products.
Decomposition: A reaction where a substance breaks down into simpler substances.
Endothermic: A reaction that absorbs energy from its surroundings.
Exothermic: A reaction that releases energy to its surroundings.
Fossil fuel: A fuel formed from the remains of ancient plants and animals.
Fuel: A substance that reacts to release energy when burned.
Non-renewable: Resources that cannot be replenished over a short time, such as fossil fuels.
Oxidation: A reaction where a substance combines with oxygen.
Physical change: A change of state, where the substance remains the same.
Product: The new substance(s) formed during a chemical reaction.
Reactant: A substance that undergoes changes during a reaction.
Reversible: A change that can be returned to its original state.
Thermal decomposition: A decomposition reaction that requires an input of heat.
Word equation: A description of a chemical reaction using words.