Comprehensive Edexcel iGCSE Chemistry Notes

Properties of Ionic and Simple Molecular Compounds

Ionic compounds are like nature's building blocks with some pretty predictable behaviour patterns. They have high melting and boiling points because those electrostatic forces holding opposite charges together are seriously strong - you need loads of energy to break them apart.

You'll recognise ionic compounds by their crystalline appearance (even if they look like powder, they're still crystals under a microscope). They're also surprisingly brittle - give them a small knock and like charges end up next to each other, causing the whole structure to split apart through repulsion.

Most ionic compounds dissolve in water because water molecules are polar and can form strong enough attractions with ions to break the lattice. However, they won't dissolve in organic solvents since these are non-polar. Here's something vital for your exams: ionic solids don't conduct electricity because the ions are stuck in fixed positions, but as liquids or in solution, they conduct brilliantly as the ions can move freely.

Key Exam Tip: Remember it's the ions that carry charge in ionic compounds, not electrons!

Simple molecular structures are much weaker affairs. They tend to be gases or liquids at room temperature with low melting and boiling points because you're only breaking weak intermolecular forces, not the strong covalent bonds within molecules. They don't conduct electricity and generally don't dissolve in water (except for a few exceptions like ethanol and ammonia that actually react with water).

Giant Covalent and Metallic Structures

Diamond is the ultimate example of a giant covalent structure - it's basically one massive molecule! Every carbon atom forms four strong covalent bonds, creating an incredibly hard structure with very high melting and boiling points. Diamond doesn't conduct electricity because all electrons are tightly held in covalent bonds with no freedom to move around.

Graphite is diamond's completely different cousin. It's soft because the layers can slide over each other easily (the attractions between layers are weak). However, it still has high melting and boiling points due to strong covalent bonds within layers. Unlike diamond, graphite conducts electricity because each carbon only bonds to three others, leaving delocalised electrons free to move throughout each layer.

C60 Fullerene looks like a football and behaves more like a simple molecular compound. It has lower melting and boiling points because there are only weak intermolecular forces between the C60 molecules. It doesn't conduct electricity because whilst there are delocalised electrons, they can only move within individual C60 molecules, not between them.

Memory Trick: Think "layers slide" for graphite's softness and "spare electron per carbon" for its conductivity!

Metallically bonded structures are brilliant at conducting electricity thanks to their sea of delocalised electrons. They're also malleable because when you apply force, layers of positive ions can slide over each other without breaking the overall bonding structure.

Electrolysis and Group Reactions

Electrolysis in aqueous solutions follows predictable patterns that'll save you marks in exams. At the cathode, if the metal is more reactive than hydrogen, you'll get hydrogen gas. If it's less reactive (like copper), you'll get the metal itself. At the anode, you usually get oxygen from common compounds, unless there's something more interesting like chlorine present.

Group 1 metals get more exciting as you go down the periodic table. Lithium just fizzes with water, sodium melts into a ball and floats, whilst potassium ignites with a purple flame. Rubidium and caesium are properly violent! They all form soluble metal hydroxides and show characteristic flame colours when burned in air.

Group 7 halogens work in reverse - they get less reactive going down. They all react with hydrogen to form acidic, poisonous hydrogen halides that dissolve in water to make acids. You can prove their reactivity order through displacement reactions - more reactive halogens will kick out less reactive ones from their compounds.

Exam Gold: Remember the colour changes in halogen displacement - bromine is orange, iodine is brown!

The iron and oxygen experiment is a classic way to find the percentage of oxygen in air. As iron rusts, it uses up oxygen, and you can measure the volume change to calculate that air is roughly 21% oxygen.

Chemical Reactions and Industrial Processes

Particle collision theory explains why reactions happen - particles must collide with enough energy (the activation energy) to react successfully. As reactants get used up, there are fewer particles to collide, so the reaction rate naturally decreases over time.

Fractional distillation is like nature's sorting system for crude oil. The heated mixture enters a tower that's hot at the bottom and cool at the top. Smaller molecules with low boiling points condense near the top, whilst larger molecules with high boiling points condense lower down.

Cracking breaks down larger, less useful hydrocarbons into smaller, more valuable ones. Heat the alkanes until they become gases, then pass them over a silica or alumina catalyst at 600-700°C. It's like molecular recycling!

Industrial Insight: Cracking helps oil companies make more petrol and diesel from heavy crude oil fractions.

Alkane reactions are pretty straightforward. Complete combustion with plenty of oxygen gives you carbon dioxide and water. Incomplete combustion (not enough oxygen) produces dangerous carbon monoxide. In UV light, alkanes undergo substitution reactions with halogens, swapping hydrogen atoms for halogen atoms.

Don't forget those solubility rules - they're exam lifesavers! All sodium, potassium, and ammonium compounds are soluble, as are all nitrates. Most chlorides and sulfates are soluble too, with a few key exceptions like silver chloride and barium sulfate.

Alkenes and Ethanol Chemistry

Alkenes are much more reactive than alkanes because of their double bond. Like alkanes, they undergo combustion reactions, but they also do addition reactions that alkanes can't manage. The bromine test is your go-to for identifying alkenes - they turn orange bromine water colourless as bromine adds across the double bond.

Ethanol production happens in two completely different ways, and you need to know both! The industrial method involves hydration of ethene - react ethene with steam using phosphoric acid as a catalyst at 300°C and high pressure (60-70 atmospheres). It's fast and efficient for large-scale production.

Fermentation is the traditional biological route. Yeast enzymes convert sugars into ethanol at around 30°C, but crucially this must happen in the absence of air to prevent aerobic respiration from occurring instead. This method is slower but uses renewable resources and requires less energy input.

Sustainability Note: Fermentation uses renewable plant materials, whilst hydration relies on crude oil - both have their place in modern industry.

The beauty of understanding these reaction mechanisms is that they help you predict what will happen when you mix different chemicals together. Whether it's addition, substitution, or combustion, each reaction type follows predictable patterns that make chemistry much more logical once you grasp the fundamentals.