Energy Changes in Chemical Reactions

Ever wondered why hand warmers heat up or why injury packs get cold? It's all about energy transfer during chemical reactions. When reactions happen, energy either gets released or absorbed.

Exothermic reactions give out heat energy to their surroundings, causing temperatures to rise. You'll see this in combustion (like burning fuel), neutralisation reactions, hand warmers, and self-heating cans. The key thing to remember: exothermic means energy exits the system.

Endothermic reactions take in heat energy from their surroundings, making temperatures drop. Think thermal decomposition, sports injury packs, and cooling systems. Here's where bond energies become crucial - breaking bonds always requires energy (endothermic), whilst forming bonds releases energy (exothermic).

Remember: Bond breaking = endothermic (+energy needed), Bond forming = exothermic (-energy released)

Acids, Bases and Chemical Changes

The pH scale might look complicated, but it's actually dead simple. Lower pH means more acidic, higher pH means more alkaline, and pH 7 is neutral - like pure water.

Strong acids completely break apart (ionise) in water, releasing loads of H+ ions. Weak acids only partially ionise, creating a reversible reaction. When acids meet bases, you get neutralisation: acid + base → salt + water.

Redox reactions involve the transfer of electrons. Oxidation means gaining oxygen, whilst reduction means losing it. The reactivity series ranks metals from most reactive (sodium) to least reactive (gold). In electrolysis, reduction happens at the negative electrode (cathode) where metals form, and oxidation occurs at the positive electrode (anode) where non-metals form.

Key Pattern: Acid + metal oxide/carbonate/metal = salt + water (+ carbon dioxide for carbonates, + hydrogen for metals)

Quantitative Chemistry and Calculations

Chemistry isn't just about reactions - it's about precise measurements and calculations that help predict what happens when chemicals react.

Relative formula mass (Mr) is simply adding up all the atomic masses in a compound's formula. Percentage mass tells you what fraction of a compound is made up of a particular element - use the formula: (Ar × number of atoms / Mr of compound) × 100.

A mole represents 6.02 × 10²³ particles (Avogadro's number) - it's chemistry's way of counting atoms and molecules. Concentration measures how much substance is dissolved in a given volume: concentration = mass ÷ volume.

Conservation of mass means atoms can't disappear - the total mass before a reaction equals the total mass after. Limiting reactants determine when reactions stop - once one chemical runs out, the reaction can't continue, leaving the other chemical in excess.

Pro Tip: In exam calculations, always show your working and include units - even if your final answer is wrong, you can still pick up marks!

Chemical Bonding and Structure

Understanding how atoms stick together explains why materials behave so differently. There are three main types of chemical bonding, each creating substances with unique properties.

Ionic bonding happens between metals and non-metals. Metals lose electrons to become positive ions, whilst non-metals gain electrons to become negative ions. These oppositely charged ions attract each other, forming giant ionic lattices with high melting points that don't conduct electricity when solid.

Covalent bonding occurs between non-metals that share electron pairs to get full outer shells. Simple covalent compounds exist as separate molecules, but giant covalent structures like diamond and graphite have atoms bonded throughout the entire structure with strong covalent bonds.

Metallic bonding involves delocalised electrons that move freely between metal atoms, explaining why metals conduct electricity and can form alloys.

State Changes: Solids have fixed positions (just vibrate), liquids move freely, and gases move randomly at high speed.

Atomic Structure and the Periodic Table

Atoms contain three tiny particles: protons (+1 charge), neutrons (no charge), and electrons (-1 charge). This simple structure explains how the entire periodic table works.

Metals are typically strong, conduct electricity, have high melting points, and get more reactive as you go down Group 1. Non-metals are often dull, don't conduct electricity, have lower melting points, and in Group 7 become less reactive going down.

The periodic table has evolved significantly. Early versions arranged elements by atomic weight, but Mendeleev cleverly left gaps for undiscovered elements. Today's table arranges elements by increasing atomic number (number of protons).

Separation techniques help us study pure substances: chromatography separates mixtures, evaporation removes solvents, crystallisation forms pure crystals, and distillation separates liquids. Remember that compounds have elements chemically bonded, whilst mixtures just have elements mixed together physically.

Historical Timeline: Dalton's sphere → Thomson's plum pudding → Rutherford's nuclear model → Bohr's electron shells → Modern quantum model