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LearnEdexcel4CH12017 specificationPaper 2C · Paper 2 (all content)Principles of chemistry

Paper 2C · Module

Principles of chemistry

14 subtopics≈ 500 min in total

Start the module: Separation and purification Open graphing calculator

Separation and purification
Choose apparatus for precision: a burette or pipette for accurate volumes, a gas syringe for gas volumes, a balance for mass. Choose a separation method from the properties of the mixture: filtration removes an insoluble solid from a liquid; crystallisation gets a soluble solid from its solution; simple distillation gets a solvent from a solution; fractional distillation separates miscible liquids with different boiling points; chromatography separates dissolved substances such as dyes.
≈ 35 min
Solids, liquids and gases
Everything is made of tiny particles that are always moving. In a solid they vibrate in fixed positions; in a liquid they are close together but slide past one another; in a gas they are far apart and move quickly in all directions. Heating gives particles more energy; at melting and boiling points that energy is used to overcome the forces between particles, so the temperature stays constant.
≈ 35 min
Formulae and equations
A chemical formula shows which elements a substance contains and in what ratio. A balanced equation has the same number of each type of atom on both sides, because atoms are rearranged, never created or destroyed. State symbols (s), (l), (g), (aq) show the physical state. Ionic equations show only the ions that actually react.
≈ 35 min
Arrangement and periodic trends
Elements are arranged in order of proton number. Vertical columns are groups: elements in a group have the same number of outer electrons and similar reactions. Horizontal rows are periods: elements in a period have the same number of electron shells. Metals are on the left, non-metals on the right; across a period, elements change from metallic to non-metallic.
≈ 30 min
Elements, compounds and mixtures
An element contains only one type of atom. A compound contains two or more elements chemically bonded in a fixed ratio, with properties different from its elements. A mixture contains two or more substances not chemically joined; they can be separated by physical methods. A pure substance melts and boils at a sharp, fixed temperature; impurities make it melt over a range and at a lower temperature.
≈ 25 min
Electrolysis
Electrolysis uses electricity to break down an ionic compound that is molten or dissolved (the electrolyte). Positive ions (cations) move to the negative cathode and gain electrons (reduction). Negative ions (anions) move to the positive anode and lose electrons (oxidation). In aqueous solutions, water's $\mathrm{H^+}$ and $\mathrm{OH^-}$ compete: hydrogen forms at the cathode unless the metal is less reactive than hydrogen; oxygen forms at the anode unless a halide is concentrated.
≈ 45 min
Relative masses and the mole
Relative formula mass $M_r$ is the sum of the relative atomic masses in a formula. A mole is the amount of substance containing $6.02\times10^{23}$ particles (the Avogadro constant); one mole has a mass in grams equal to $M_r$ . So $n=\frac{m}{M_r}$ . The empirical formula is the simplest whole-number ratio of atoms, found by dividing each mass by $A_r$ and simplifying.
≈ 40 min
Chromatography
Paper chromatography separates soluble substances. Spots are placed on a pencil baseline above the solvent level; the solvent moves up the paper, carrying each substance a different distance depending on its solubility and attraction to the paper. A pure substance gives one spot. Substances are identified by comparing with known samples, or by the $R_f$ value $=\frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$ . Colourless substances need a locating agent.
≈ 30 min
Atomic structure and isotopes
An atom has a tiny nucleus of protons (charge $+1$ , mass $1$ ) and neutrons (charge $0$ , mass $1$ ), surrounded by electrons (charge $-1$ , mass about $\frac1{1840}$ ) in shells. The proton number $Z$ identifies the element; the mass number $A$ = protons + neutrons. Electrons fill shells 2, 8, 8, … . Isotopes are atoms of the same element with different numbers of neutrons; relative atomic mass is the abundance-weighted mean of their masses.
≈ 40 min
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. Gases diffuse faster than liquids. Lighter particles (lower relative molecular mass) move faster at the same temperature, so they diffuse more quickly; higher temperature also speeds up diffusion.
≈ 25 min
Ions and ionic bonding
Metal atoms lose outer electrons to form positive ions; non-metal atoms gain electrons to form negative ions, each reaching a full outer shell. An ionic bond is the strong electrostatic attraction between oppositely charged ions. Ionic compounds form giant lattices, so they have high melting points, and they conduct electricity only when molten or dissolved, because then the ions can move.
≈ 35 min
Mole calculations
Every mole calculation follows the same path: convert what you know into moles, use the mole ratio from the balanced equation, then convert back into what you want. Gases: $n=\frac{V}{24}$ ( $\mathrm{dm^3}$ at room temperature and pressure). Solutions: $n=c\times V$ ( $V$ in $\mathrm{dm^3}$ ). Percentage yield $=\frac{\text{actual}}{\text{theoretical}}\times100$ .
≈ 50 min
Simple molecules and covalent bonds
When non-metal atoms bond, they share pairs of electrons to complete their outer shells. A shared pair is a covalent bond: strong. Simple molecules (like $\mathrm{H_2O}$ , $\mathrm{CO_2}$ , $\mathrm{CH_4}$ ) are held to each other only by weak intermolecular forces, so they have low melting and boiling points, and they don't conduct electricity because they have no charged particles free to move.
≈ 35 min
Giant structures and metallic bonding
In giant covalent structures every atom is covalently bonded into one huge network. Diamond (each C bonded to 4 others) is extremely hard with a very high melting point. Graphite has layers of C bonded to 3 others, with delocalised electrons, so it conducts and its layers slide. Silicon(IV) oxide is like diamond. In metals, positive ions sit in a sea of delocalised electrons: they conduct, are malleable and have high melting points.
≈ 40 min