Chemical Changes and Structure

You're about to dive into one of the most exciting parts of chemistry! This unit covers everything from reaction rates to atomic structure - basically, how fast things happen and why they happen at all.

Think of it like learning the rules of a game you've been watching your whole life. Once you understand these concepts, you'll start noticing chemistry everywhere - from cooking dinner to understanding why batteries work.

Quick Tip: These topics build on each other, so mastering the basics early will make everything else click into place much easier.

Rate of Reaction Factors

Ever wondered why chips cook faster than whole potatoes? It's all about reaction rates! Four main factors control how fast reactions happen, and once you understand them, you can predict and control chemical reactions.

Temperature is probably the most obvious one. Higher temperatures mean particles move faster, leading to more collisions and quicker reactions. That's why your food lasts ages in the fridge but goes off quickly in a warm room.

Concentration works like a busy corridor - more particles in the same space means more bumping into each other. A 1.0 mol/L acid will react way faster than a 0.1 mol/L acid because there are simply more reactive particles present.

Memory Trick: Think of students moving between classes - more students in a corridor means more collisions!

Particle Size and Catalysts

Here's where things get interesting! Particle size affects reaction rate because smaller particles have a larger surface area. Sugar granules dissolve faster than sugar cubes for exactly this reason - more surface area means more contact with water.

Catalysts are like that friend who stirs up drama but never gets in trouble themselves. They speed up reactions without being used up in the process. A catalyst provides an alternative pathway for the reaction that requires less energy.

The brilliant thing about catalysts is they remain unchanged at the end. Think of a teabag - it helps make your tea but comes out still looking like a teabag!

Real-world Connection: Car catalytic converters use this principle to break down harmful exhaust gases without being consumed themselves.

Following Reactions - Gas Collection

Now you can actually measure how fast reactions happen! There are several clever ways to collect and measure gases produced during reactions, each suited to different types of gases.

Collecting gas over water only works with gases that won't dissolve in water. You simply let the gas push water out of an upturned measuring cylinder - dead simple but effective.

For soluble gases, you'll need a gas syringe instead. This method captures gases that would otherwise dissolve and disappear into the water. You can also measure mass lost by weighing the reaction flask as gases escape - the mass decreases as products leave the system.

Lab Success: Always check if your gas is soluble in water before choosing your collection method - it'll save you from getting rubbish results!

Gas Collection Methods and Rate Calculations

Displacement methods depend on whether your gas is lighter or heavier than air. Upward displacement works for gases less dense than air, while downward displacement suits gases more dense than air.

Calculating reaction rates uses a straightforward formula: Rate = ΔQ/ΔT. The Greek letter delta (Δ) just means "change in", so you're finding change in quantity over change in time.

For example, if Barry collected 30 cm³ of gas in 20 seconds, his rate would be 30/20 = 1.5 cm³ s⁻¹. Simple maths, but it tells you exactly how fast the reaction is happening!

Exam Tip: Always include units in your rate calculations - typically cm³ s⁻¹ for gas collection experiments.

Rate Graphs and Data Analysis

Rate graphs turn your numbers into visual stories about reactions. The steeper the line, the faster the reaction is happening at that moment. Every graph needs proper scales, labels, units, and plotted results - miss any of these and you'll lose marks!

Looking at the example graph, you can see how particle size affects reaction rate. Chalk powder (smaller particles) produces a much steeper initial curve than chalk lumps, proving that surface area really does matter.

The curves eventually flatten out as reactants get used up. This makes perfect sense - fewer reactants available means fewer successful collisions and a slower reaction rate.

Graph Reading: The initial gradient (steepest part) shows the maximum reaction rate when reactant concentrations are highest.

Atomic Theory Basics

Everything around you is made of atoms - tiny building blocks containing three types of particles. Protons (positive charge) and neutrons (no charge) hang out in the central nucleus, while electrons (negative charge) whiz around in energy levels.

The atomic number tells you how many protons an atom has, and this never changes for a particular element. Atoms are naturally neutral because they have equal numbers of positive protons and negative electrons that cancel each other out.

For example, fluorine has an atomic number of 9, so every fluorine atom has exactly 9 protons and 9 electrons. Change the number of protons and you get a completely different element!

Key Insight: The number of protons defines what element you're dealing with - it's like an atom's unique fingerprint.

Electron Arrangement

Energy levels around the nucleus can only hold specific numbers of electrons: first level holds 2, and the next three levels each hold 8. Think of them like parking spaces - once they're full, electrons have to use the next level out.

Drawing electron arrangements becomes easy once you know the rules. Hydrogen (1 electron) goes in the first level, while lithium (3 electrons) fills the first level then puts one in the second level: 2,1.

Elements with the same number of outer electrons have similar chemical properties. That's why hydrogen and lithium both react in similar ways - they each have just one electron in their outer level.

Pattern Spotting: Elements in the same group of the periodic table have the same number of outer electrons and similar chemical behaviour.

Nuclide Notation

Nuclide notation packs loads of information into a compact format. The big number (mass number) tells you protons plus neutrons, while the small number (atomic number) shows just the protons.

For sodium ²³₁₁Na, you've got 11 protons, 11 electrons, and 12 neutrons (23-11=12). It's like a chemical shorthand that instantly tells you the atom's composition.

Once you know the pattern, you can work out any element's composition. Magnesium ²⁴₁₂Mg has 12 protons, 12 electrons, and 12 neutrons. The maths stays the same every time!

Quick Method: Mass number minus atomic number always gives you the number of neutrons.

Ion Formation

Ions are just atoms that have gained or lost electrons, giving them an overall charge. Sodium normally has 11 electrons, but Na⁺ has lost one electron, leaving it with 10 electrons and a positive charge.

Atoms form ions to achieve stable electron arrangements like noble gases. Sodium loses its lone outer electron to get the same arrangement as neon (2,8), while sulfur gains two electrons to match argon's stable 2,8,8 pattern.

It's all about reaching that stable, happy state. Think of it like wanting to be in a complete team rather than being the odd one out - atoms "want" their outer levels to be full.

Memory Hook: Positive ions (like Na⁺) have lost electrons, while negative ions (like S²⁻) have gained electrons.