Netherlands VWO 5 Chemistry
Chapters: 3
1. Chemical processes (part 2)
Conservation laws and cycles · Reaction kinetics · Chemical equilibrium · Energy calculations · Quality of energy · Activation energy in experiments
- The Law of Conservation of Mass – In a chemical reaction, mass is not made and not destroyed. The total mass of the substances before the reaction equals the total mass after it. This is true because atoms are only rearranged: the same atoms, in the same numbers, are there before and after. If a gas leaves or joins from the air, the mass on a balance seems to change, but when we count every substance, the total stays the same. A related rule, the law of definite proportions, says a pure compound always has its elements in the same mass ratio (water is always 1 g hydrogen to 8 g oxygen).
- Rate of a Chemical Reaction – The rate of a reaction tells how fast a reactant is used up or a product is made, per unit time. Rate = −Δ[R]/Δt = +Δ[P]/Δt (unit mol L⁻¹ s⁻¹). The rate law, rate = k[A]^x[B]^y, is found by experiment; x + y is the order. Molecularity is the number of particles that collide in one elementary step.
- Chemical Equilibrium: Kc, Kp, Q and Gibbs Energy – In a closed container a reversible reaction goes both ways. After some time the forward and backward rates become equal, so amounts stop changing, but the reaction does not stop. This is dynamic equilibrium. At equilibrium the ratio of products to reactants (each raised to its coefficient) is a fixed number, the equilibrium constant K. Kc uses concentrations, Kp uses partial pressures, and Kp = Kc(RT)^Δn. Pure solids and liquids are left out of K. The reaction quotient Q tells the direction: Q < K goes forward, Q > K goes backward, Q = K is equilibrium. K and Gibbs energy are linked: ΔG = ΔG° + RT ln Q and ΔG° = −RT ln K.
- Enthalpy, Calorimetry and Hess's Law – Enthalpy H = U + pV. At constant pressure the heat taken in or given out equals ΔH; at constant volume it equals ΔU. We measure ΔU in a sealed bomb calorimeter and ΔH in an open cup calorimeter, using q = C ΔT or q = m c ΔT. For gases, ΔH = ΔU + Δn_g RT. The reaction enthalpy ΔrH is negative for exothermic and positive for endothermic reactions, and ΔrH° = ΣΔfH°(products) − ΣΔfH°(reactants). Hess's law: the total enthalpy change is the same whether a reaction happens in one step or many, so thermochemical equations can be added like algebra.
- Temperature, Activation Energy, Catalysts and Collision Theory – Reactant molecules must climb an energy hill, the activation energy Ea, to become products. The Arrhenius equation k = A e^(−Ea/RT) shows that a small rise in temperature lets many more molecules cross, so k rises fast. A catalyst gives a lower hill without changing ΔH. Collision theory: molecules react only when they collide with enough energy and the right orientation.
2. Synthesis and modelling
Chemical synthesis · Molecular modelling
- Organic Synthesis – Organic synthesis means making a target molecule from simpler starting materials through a planned series of reactions. Each step changes one functional group and needs the right reagent and conditions (temperature, catalyst, solvent, reflux or distillation). Families of compounds are linked (alkane ↔ alkene → haloalkane → alcohol → aldehyde/ketone → carboxylic acid → ester; haloalkane → amine or nitrile), so we can move between them like stations on a map. Chemists plan backwards from the target (retrosynthesis), prefer short routes with high yield and atom economy, and then carry out the reaction, separate, purify and check the product.
- Molecular Modelling: Seeing Molecules in 3D – A molecular model is a small stand-in for a molecule that you can see and turn around. Ball-and-stick models show bonds and angles, space-filling models show size, and computer models can also show how a reaction happens step by step. From the model you can predict the shape of a molecule and follow a reaction mechanism.
3. Innovation and chemical research
Chemical research · Selectivity and specificity · Sustainability · New materials · Research and design task
- How Structure Decides the Properties of Substances – The way particles are held together decides how a substance behaves. Ionic and giant covalent substances have strong bonds all through, so they melt very high. Small molecules have weak forces between molecules, so they melt and boil low. Metals have free electrons, so they conduct.
- Structure and Properties of Materials – What a material does (its property) depends on what is inside it (its structure). Metals conduct because they have free electrons. Solid salt does not conduct because its ions are locked, but melted or dissolved salt does because the ions can move. Sugar dissolves but stays neutral, so it does not conduct. Long plastic chains are strong; a chain with weak links can be cut by bacteria, so it is biodegradable. Chains with OH groups attract water, so they swell into a gel. To predict a property, ask: what are the particles, do they carry charge, can they move, and how strongly do they attract each other or water?
- Enzymes – Enzymes are proteins that act as biological catalysts: they speed up reactions in cells without being used up. Each enzyme has an active site that fits only one kind of substrate (specificity). Enzymes work by lowering the activation energy. They work best at an optimum temperature and pH; too much heat or the wrong pH changes the active site's shape and denatures them.
- Sustainability – Sustainability means meeting our needs today without taking away the ability of future people to meet theirs. It rests on three pillars — environment, society and economy. A resource is used sustainably when we use it no faster than it renews. Today humanity uses about 1.7 Earths' worth of resources a year, and this use is very unequal. Stewardship, careful planning and spatial tools like GIS help us move towards sustainability.
- Materials Innovation: From a Lab Idea to a New Product – Chemical innovation turns an idea into a product in stages: idea, lab test, pilot, factory, market. New materials (alloys, bio-based plastics, smart materials) and new food processes are designed from structure and tested for performance, cost and safety. Scale-up from grams to tonnes and the market decide which ideas succeed.
- The Scientific Method – The scientific method is the careful way scientists find out how the world works. Observe something, ask a testable question, make a hypothesis (a clear, testable guess), test it with a fair experiment (change one variable, measure one, keep the rest the same), repeat and record data, analyse it, draw a conclusion and share it so others can check. Results that fail the test are useful too: they send you back to a new hypothesis.