HSC Chemistry flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Core charge

The net positive charge experienced by an outer electron, equal to nuclear charge minus inner-shell electrons. It increases across a period and explains trends in radius and ionisation energy.

First ionisation energy

The energy required to remove one mole of electrons from one mole of gaseous atoms. It increases across a period and decreases down a group as outer electrons are further from the nucleus.

Electron configuration order

Electrons fill orbitals in order of increasing energy: 1s 2s 2p 3s 3p 4s 3d 4p. Chromium and copper are exceptions, achieving half-filled or filled d subshells for extra stability.

Amphoteric oxide

An oxide that reacts with both acids and bases to form salts, such as aluminium oxide or zinc oxide. Amphoteric behaviour appears near the metals and nonmetals boundary of the periodic table.

Polar covalent bond

A covalent bond with an electronegativity difference roughly between 0.4 and 1.7, giving unequal electron sharing and a permanent dipole. Larger differences give predominantly ionic bonding.

VSEPR geometry prediction

Electron domains around a central atom repel to maximise separation. Two domains give linear, three trigonal planar, four tetrahedral, five trigonal bipyramidal and six octahedral arrangements.

Limiting reagent

The reactant that is fully consumed first and therefore determines the maximum amount of product formed. Comparing mole ratios with the balanced equation identifies it.

Mole calculation

Amount in moles equals mass divided by molar mass, or concentration multiplied by volume in litres. These relationships underpin stoichiometric calculations in synthesis and titration.

Dispersion forces

Weak attractions from temporary, fluctuating dipoles in all molecules. They strengthen with more electrons and greater surface contact, raising melting and boiling points.

Dipole-dipole interactions

Attractions between the permanent partial charges of polar molecules. They are stronger than dispersion forces of similar molar mass but weaker than hydrogen bonds.

Hydrogen bonding

A strong intermolecular attraction between hydrogen bonded to nitrogen, oxygen or fluorine and a lone pair on a nearby electronegative atom. It raises boiling points and aids solubility.

Branching effect on boiling point

Increased branching shortens the molecule and reduces surface contact, weakening dispersion forces. Isomers with more branching therefore have lower boiling points than straight-chain isomers.

Like dissolves like

Polar and ionic solutes dissolve readily in polar solvents through ion-dipole and hydrogen-bonding interactions. Nonpolar solutes dissolve in nonpolar solvents through dispersion forces.

Chromatography Rf value

The ratio of the distance travelled by a solute to the distance travelled by the solvent front. It depends on relative affinity for the mobile and stationary phases.

Fractional distillation

Separation of miscible liquids by repeated vaporisation and condensation in a fractionating column. It is effective when component boiling points differ by roughly 25 degrees Celsius or more.

Gas solubility in water

Gas solubility decreases as temperature rises and increases with the gas partial pressure above the liquid, as described by Henry's law.

Enthalpy change convention

Delta H equals the enthalpy of products minus reactants. Negative values mean energy is released to the surroundings (exothermic); positive values mean energy is absorbed (endothermic).

Hess's law

The overall enthalpy change of a reaction is independent of the pathway taken. Enthalpy changes of individual steps can be added to find an unknown change.

Bond enthalpy calculation

Enthalpy change equals the sum of bond energies broken minus the sum of bond energies formed. A negative result indicates an exothermic reaction.

Activation energy

The minimum energy colliding particles need to react. Catalysts provide an alternative pathway with lower activation energy without altering the overall enthalpy change.

Collision theory

Reactions occur when particles collide with sufficient energy and correct orientation. Higher concentration, temperature and surface area increase the frequency or fraction of effective collisions.

Le Chatelier's principle

When a system at equilibrium is disturbed, it shifts to oppose the change. Concentration and pressure changes alter position, but catalysts and inert gases do not shift equilibrium.

Equilibrium constant K

The ratio of product concentrations raised to their coefficients to reactant concentrations raised to theirs, at equilibrium. It depends only on temperature for a given reaction.

Gibbs free energy

Delta G equals delta H minus temperature times delta S. A negative value indicates a spontaneous process, favouring higher temperature when entropy change is positive.

Percentage yield

The actual mass or moles of product obtained divided by the theoretical maximum, multiplied by one hundred. It reflects losses from side reactions and purification steps.

Atom economy

The molar mass of the desired product divided by the total molar mass of all reactants, multiplied by one hundred. It measures how much reactant mass ends up in the target product.

Reflux technique

Heating a reaction mixture with a vertical condenser so vapour condenses and returns to the flask. It prevents loss of volatile reactants during prolonged heating.

Recrystallisation

Purification by dissolving a solid in a hot solvent, cooling to crystallise, then filtering. Impurities remain dissolved or are excluded from the growing crystal lattice.

Oxidation of alcohols

Primary alcohols oxidise to aldehydes under distillation with acidified dichromate, or to carboxylic acids under reflux. Secondary alcohols oxidise to ketones; tertiary alcohols resist oxidation.

Green chemistry principles

Design goals include high atom economy, less hazardous reagents and solvents, renewable feedstocks, energy efficiency and waste minimisation, reducing environmental impact of synthesis.

Retrosynthetic planning

Working backwards from a target molecule to simpler precursors, choosing reactions with high yield, good selectivity and few steps.