Learning path: Chemical Reactions

Your path

1. MoleculesDirect prerequisite

   A molecule is a group of atoms held together by covalent bonds that acts as a single particle — such as water, H₂O, or carbon dioxide, CO₂. A molecular formula says which atoms a molecule contains and how many; its three-dimensional shape, which VSEPR theory predicts from repelling electron groups, strongly affects how it behaves. Chemists count molecules in moles: exactly 6.022 140 76 × 10²³ of them.

   Take the Molecules quiz

2. Chemical ReactionsYour goal

   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.

   Take the Chemical Reactions quiz

Where to go next

Concepts that build directly on Chemical Reactions:

• Acids and Bases: Acids and bases are two complementary families of substances. In the Brønsted–Lowry picture (1923), an acid donates a proton (H⁺) and a base accepts one, so every acid–base reaction is a proton transfer. Strong acids such as hydrochloric acid ionise essentially completely in water, while weak acids such as acetic acid ionise only slightly. When an acid and a base react in the right proportions they neutralise each other, typically forming a salt and — for hydroxide bases — water.
• Batteries: A battery stores energy chemically and releases it as electricity: in each cell, a chemical reaction pushes electrons out of the negative electrode (anode), through the circuit, and into the positive electrode (cathode). Single-use (primary) cells cannot be recharged; rechargeable (secondary) cells use reversible reactions. Lithium-ion cells, honoured by the 2019 Nobel Prize in Chemistry, power phones and laptops; because their lithium ions move in and out of the electrodes without consuming them, they can be recharged hundreds of times.
• Reaction Energy: Every chemical reaction involves an energy change, because breaking bonds absorbs energy and forming bonds releases it. If more energy is released than absorbed, the reaction is exothermic (ΔH < 0) and warms its surroundings; if the reverse, it is endothermic (ΔH > 0). Separately, almost every reaction must first climb an energy barrier, the activation energy, which sets how fast it goes — and which a catalyst lowers without being used up.