Edexcel IGCSE Chemistry: Principles of Chemistry — Revision Notes

Condensed for the final weeks. For the full explanation, use the Principles of Chemistry study guide.

States of matter

State	Arrangement	Movement
Solid	Regular, close-packed	Vibrate about fixed positions
Liquid	Close, irregular	Slide past one another
Gas	Far apart, random	Move rapidly in all directions

Diffusion — particles moving from high to low concentration. It is faster in gases than liquids, faster at higher temperature (more kinetic energy), and faster for lighter particles. The ammonia–hydrogen chloride tube experiment demonstrates the last point: the white ring forms nearer the HCl end because NH₃ has the lower relative molecular mass and diffuses faster.

Atomic structure

Particle	Relative mass	Charge
Proton	1	+1
Neutron	1	0
Electron	1/1836	−1

Atomic number = protons. Mass number = protons + neutrons.

Isotopes — same number of protons, different numbers of neutrons. They have identical chemical properties because chemical behaviour depends on electron arrangement, which is unchanged; only physical properties such as density differ.

Electron shells fill 2, 8, 8. Group number = outer electrons; period number = number of shells.

Bonding

Bond	Between	Held by
Ionic	Metal + non-metal	Electrostatic attraction between oppositely charged ions
Covalent	Non-metal + non-metal	Shared pair attracted to both nuclei
Metallic	Metal atoms	Positive ions and delocalised electrons

Every definition must include electrostatic attraction — “atoms share electrons” describes the arrangement, not the force.

Properties explained:

• Ionic compounds have high melting points (strong forces throughout a giant lattice) and conduct only when molten or dissolved, because only then are the ions free to move.
• Simple molecular substances have low melting points because only weak intermolecular forces break — not the covalent bonds. Saying “the covalent bonds break” is the most heavily penalised error in the topic.
• Giant covalent: diamond is hard with four bonds per carbon and no free electrons, so it does not conduct. Graphite conducts because each carbon forms only three bonds, leaving one delocalised electron per atom free to move; its layers are held by weak forces so they slide, making it a lubricant.
• Metals conduct in both solid and molten states because the delocalised electrons are always mobile. The same delocalised-electron structure also explains why metals are malleable: the layers of positive ions can slide over one another without breaking any specific bond, since the delocalised electrons hold the whole structure together regardless of how the ions are arranged.

Calculations

moles = mass / Mr
moles = concentration x volume(dm3)
moles of gas = volume(dm3) / 24        at room temperature and pressure

percentage yield  = (actual / theoretical) x 100

Empirical formula method: divide each mass or percentage by the relative atomic mass, then divide all results by the smallest, then scale to whole numbers.

Worked example. A compound contains 40.0 g of calcium, 12.0 g of carbon and 48.0 g of oxygen (Ar: Ca = 40, C = 12, O = 16). Moles: Ca = 40.0 ÷ 40 = 1.0; C = 12.0 ÷ 12 = 1.0; O = 48.0 ÷ 16 = 3.0. Dividing through by the smallest value (1.0) gives the ratio Ca : C : O = 1 : 1 : 3, so the empirical formula is CaCO₃.

Percentage yield is never 100% because of incomplete reactions (particularly reversible reactions that do not go to completion), some product being lost during separation and purification, and some reactants reacting in unexpected side reactions.

Electrolysis

• Cathode is negative, attracting cations, which are reduced (gain electrons).
• Anode is positive, attracting anions, which are oxidised (lose electrons).

OIL RIG — Oxidation Is Loss, Reduction Is Gain of electrons. PANIC — Positive Anode, Negative Is Cathode.

Selective discharge in aqueous solution:

• At the cathode, the less reactive cation is discharged. Metals below hydrogen in the reactivity series are deposited; metals above it leave hydrogen evolved instead.
• At the anode, halides are discharged in preference to OH⁻; otherwise oxygen from OH⁻. Concentration matters: concentrated NaCl gives chlorine, very dilute gives oxygen.

Electrolysis of molten compounds gives the metal and the non-metal directly, with no water to complicate the discharge.

Exam traps

• Saying covalent bonds break when a simple molecular substance melts.
• Saying ionic solids conduct electricity.
• Omitting “electrostatic attraction” from a bonding definition.
• Forgetting that a reversible reaction not going to completion is one of the standard reasons the calculated amount of product is not obtained.
• Forgetting that diffusion rate depends on relative molecular mass.
• Confusing the polarity of the electrodes — in electrolysis the cathode is negative.
• Saying isotopes have different chemical properties.

Self-test

1. Why does the white ring form nearer the HCl end in the diffusion experiment?
2. Why do isotopes have identical chemical properties?
3. Why does graphite conduct but diamond not?
4. State three reasons why the percentage yield of a reaction is less than 100%.
5. In electrolysis, which electrode is negative and what happens there?

Answers: 1. Ammonia has a lower relative molecular mass than hydrogen chloride, so it diffuses faster and travels further before they meet. 2. Chemical properties depend on the arrangement of electrons, which is the same for all isotopes of an element; only the number of neutrons differs. 3. Each carbon in graphite forms only three covalent bonds, leaving one delocalised electron per atom free to move along the layers; in diamond all four outer electrons are used in bonding. 4. The reaction may not go to completion because it is reversible; some product may be lost during separation and purification; some reactants may react in unexpected side reactions. 5. The cathode is negative; cations are attracted to it and are reduced by gaining electrons.