Y12 WACE ATAR CHEM 2026

CHEM Topic 1 - Reaction rate

The change in concentration, mass, volume or another measurable quantity of a reactant or product per unit time.

CHEM Topic 1 - Average reaction rate formula

Average rate = change in amount or concentration / change in time. Use a positive value for a rate.

CHEM Topic 1 - Instantaneous reaction rate

The rate at one particular moment; it is the gradient of a tangent to an amount/concentration-time graph.

CHEM Topic 1 - System

The chemicals being studied in a reaction or physical change.

CHEM Topic 1 - Surroundings

Everything outside the system that can exchange energy, and sometimes matter, with it.

CHEM Topic 1 - Open system

A system that can exchange both matter and energy with its surroundings.

CHEM Topic 1 - Closed system

A system that can exchange energy but not matter with its surroundings.

CHEM Topic 1 - Exothermic reaction

A reaction that releases heat to the surroundings; products have lower enthalpy than reactants and delta H is negative.

CHEM Topic 1 - Endothermic reaction

A reaction that absorbs heat from the surroundings; products have higher enthalpy than reactants and delta H is positive.

CHEM Topic 1 - Enthalpy

The chemical potential energy stored in a system, usually discussed as an enthalpy change, delta H.

CHEM Topic 1 - Activation energy (Ea)

The minimum energy colliding particles need for a successful reaction.

CHEM Topic 1 - Transition state

A short-lived, high-energy arrangement of atoms at the top of the activation-energy barrier.

CHEM Topic 1 - Collision theory

Reactions occur only when particles collide with sufficient energy (at least Ea) and the correct orientation.

CHEM Topic 1 - Successful collision

A collision with energy equal to or above the activation energy and a suitable orientation.

CHEM Topic 1 - Effect of higher concentration on rate

More particles per unit volume means more frequent collisions, so more successful collisions per second and a faster rate.

CHEM Topic 1 - Effect of greater surface area on rate

More exposed particles are available to collide, increasing collision frequency and reaction rate.

CHEM Topic 1 - Effect of higher temperature on rate

Particles move faster and collide more often; crucially, a greater proportion have energy at least equal to Ea, so rate increases.

CHEM Topic 1 - Effect of catalyst on rate

A catalyst provides an alternative pathway with lower activation energy, so a larger fraction of collisions is successful.

CHEM Topic 1 - Effect of nature of reactants on rate

Reactions with lower activation energies and easier bond rearrangements are generally faster.

CHEM Topic 1 - Why a catalyst is not consumed

It is regenerated by the end of the reaction and is not used up overall.

CHEM Topic 1 - Catalyst on energy profile

It lowers the activation energy for both forward and reverse reactions but does not change delta H.

CHEM Topic 1 - Dynamic equilibrium

In a closed system, forward and reverse reactions continue at equal rates, so macroscopic concentrations and properties remain constant.

CHEM Topic 1 - Conditions for dynamic equilibrium

The reaction must be reversible, occur in a closed system, and have both forward and reverse reactions occurring.

CHEM Topic 1 - What is equal at equilibrium

The forward and reverse reaction rates are equal; concentrations are constant but not necessarily equal.

CHEM Topic 1 - Equilibrium position

The relative amounts/concentrations of reactants and products at equilibrium.

CHEM Topic 1 - Reversible reaction symbol

Use the double arrow, ⇌, to show both forward and reverse reactions can occur.

CHEM Topic 1 - Homogeneous equilibrium

All reactants and products are in the same phase.

CHEM Topic 1 - Heterogeneous equilibrium

Reactants and/or products are in more than one phase.

CHEM Topic 1 - Equilibrium law expression

For aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b at a specified temperature.

CHEM Topic 1 - What is excluded from Kc

Pure solids and pure liquids, because their effective concentrations remain constant.

CHEM Topic 1 - Meaning of large Kc

Equilibrium lies to the right: products predominate relative to reactants.

CHEM Topic 1 - Meaning of small Kc

Equilibrium lies to the left: reactants predominate relative to products.

CHEM Topic 1 - Does Kc change with concentration, pressure, volume or catalyst?

No. At a fixed temperature, Kc is constant; these changes can alter equilibrium position only.

CHEM Topic 1 - Does Kc change with temperature?

Yes. Temperature changes the equilibrium constant because it changes the relative reaction rates/position at equilibrium.

CHEM Topic 1 - Le Chatelier's principle

When a system at equilibrium is disturbed, it shifts in the direction that partly opposes the imposed change.

CHEM Topic 1 - Add a reactant at equilibrium

The forward rate initially increases, the system shifts right, and reactant is consumed while products increase until rates are equal again.

CHEM Topic 1 - Remove a reactant at equilibrium

The reverse rate initially exceeds forward rate, so the system shifts left to replace some reactant.

CHEM Topic 1 - Add a product at equilibrium

The reverse rate initially increases and the system shifts left.

CHEM Topic 1 - Remove a product at equilibrium

The forward rate initially exceeds reverse rate and the system shifts right.

CHEM Topic 1 - Decrease volume of a gaseous equilibrium

Pressure/concentrations rise; equilibrium shifts to the side with fewer moles of gas. No shift if gaseous moles are equal.

CHEM Topic 1 - Increase volume of a gaseous equilibrium

Pressure/concentrations fall; equilibrium shifts to the side with more moles of gas. No shift if gaseous moles are equal.

CHEM Topic 1 - Increase pressure at equilibrium

Equivalent to decreasing volume for a gas system: shifts to fewer gas moles, unless both sides have equal gas moles.

CHEM Topic 1 - Temperature increase at equilibrium

Favors the endothermic direction because heat is effectively added.

CHEM Topic 1 - Temperature decrease at equilibrium

Favors the exothermic direction because heat is effectively removed.

CHEM Topic 1 - Catalyst added to equilibrium

Both forward and reverse rates increase equally; equilibrium position, concentrations and Kc do not change, but equilibrium is reached faster.

CHEM Topic 1 - Inert gas at constant volume

No effect on equilibrium position because reactant/product partial pressures do not change.

CHEM Topic 1 - Inert gas at constant pressure

Volume increases and partial pressures of reacting gases fall; apply the volume rule to predict any shift.

CHEM Topic 1 - Concentration-time graph at equilibrium

Lines become horizontal because concentrations become constant, not because reactions stop.

CHEM Topic 1 - Rates-time graph at equilibrium

Forward rate decreases and reverse rate increases until they meet; thereafter they are equal and constant.

CHEM Topic 1 - Haber process equation

N2(g) + 3H2(g) ⇌ 2NH3(g), delta H = negative (exothermic).

CHEM Topic 1 - Haber process conditions

Iron catalyst, about 450 degrees C and high pressure (about 200 atm); compromise conditions balance rate, yield, cost and safety.

CHEM Topic 1 - Haber temperature compromise

Low temperature gives higher ammonia equilibrium yield, but rate is too slow; 450 degrees C gives an economically useful rate with acceptable yield.

CHEM Topic 1 - Haber pressure compromise

High pressure favors fewer gas moles and ammonia yield, but extremely high pressures are expensive and hazardous.

CHEM Topic 1 - Haber ammonia removal

Ammonia is cooled and condensed/removed, shifting equilibrium right to make more ammonia.

CHEM Topic 1 - Contact process overall equation

2SO2(g) + O2(g) ⇌ 2SO3(g), delta H = negative.

CHEM Topic 1 - Contact process conditions

Vanadium(V) oxide catalyst, about 450 degrees C and low/moderate pressure (about 1-2 atm).

CHEM Topic 1 - Why low pressure in Contact process

Although high pressure favors SO3, the gain is small and the cost of compression is not worthwhile.

CHEM Topic 1 - Mole

The amount of substance containing 6.02 x 10^23 specified particles.

CHEM Topic 1 - Avogadro constant

6.02 x 10^23 mol^-1.

CHEM Topic 1 - Moles from mass

n = m / M, where n is moles, m is mass in g and M is molar mass in g mol^-1.

CHEM Topic 1 - Particles from moles

N = n x NA.

CHEM Topic 1 - Concentration formula

c = n / V, where V is in litres (dm^3).

CHEM Topic 1 - Moles in solution

n = cV, with concentration in mol L^-1 and volume in L.

CHEM Topic 1 - Dilution equation

c1V1 = c2V2, provided the amount of solute remains constant.

CHEM Topic 1 - Molar gas volume at SLC

24.5 L mol^-1 (use only when conditions are SLC).

CHEM Topic 1 - Gas moles at SLC

n = V / 24.5 when V is in L.

CHEM Topic 1 - Limiting reagent

The reactant completely consumed first; it determines the maximum amount of product formed.

CHEM Topic 1 - Excess reagent

A reactant present in more than the stoichiometric amount; some remains after reaction.

CHEM Topic 1 - Theoretical yield

The maximum product amount calculated from the limiting reagent and the balanced equation.

CHEM Topic 1 - Actual yield

The product amount actually obtained experimentally.

CHEM Topic 1 - Percentage yield

Percentage yield = actual yield / theoretical yield x 100%.

CHEM Topic 1 - Why percentage yield is below 100%

Incomplete reaction, competing reactions, product loss during transfer/purification, impure reactants or measurement error.

CHEM Topic 1 - Percentage atom economy

Atom economy = Mr of desired product / total Mr of all products x 100%; higher atom economy means less waste.

CHEM Topic 1 - Gravimetric analysis

A quantitative method where a precipitate is formed, filtered, washed, dried, weighed and used in stoichiometric calculations.

CHEM Topic 2 - Arrhenius acid

A substance that produces H+ (or H3O+) ions in aqueous solution.

CHEM Topic 2 - Arrhenius base

A substance that produces OH- ions in aqueous solution.

CHEM Topic 2 - Bronsted-Lowry acid

A proton donor.

CHEM Topic 2 - Bronsted-Lowry base

A proton acceptor.

CHEM Topic 2 - Conjugate acid-base pair

Two species differing by one proton; acid loses H+ to form its conjugate base, while base gains H+ to form its conjugate acid.

CHEM Topic 2 - Amphiprotic species

A species able to donate or accept a proton, for example HCO3- or H2O.

CHEM Topic 2 - Strong acid

An acid that ionises essentially completely in water.

CHEM Topic 2 - Weak acid

An acid that only partially ionises in water and establishes an equilibrium.

CHEM Topic 2 - Strong base

A base that dissociates completely to produce OH- in water, e.g. soluble metal hydroxides.

CHEM Topic 2 - Weak base

A base that reacts only partially with water to form OH-, e.g. NH3.

CHEM Topic 2 - Strong acid examples

HCl, HNO3 and H2SO4 (first ionisation).

CHEM Topic 2 - Common weak acid examples

CH3COOH, H2CO3 and H3PO4.

CHEM Topic 2 - Acid ionisation equation

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq).

CHEM Topic 2 - Acid ionisation constant

Ka = [H3O+][A-] / [HA]; a larger Ka means a stronger acid.

CHEM Topic 2 - Monoprotic acid

An acid with one ionisable proton per molecule, e.g. HCl or CH3COOH.

CHEM Topic 2 - Polyprotic acid

An acid with more than one ionisable proton; ionisation occurs stepwise and usually Ka1 > Ka2 > Ka3.

CHEM Topic 2 - Neutralisation net ionic equation

H+(aq) + OH-(aq) -> H2O(l).

CHEM Topic 2 - Acid plus metal carbonate

Acid + carbonate -> salt + water + carbon dioxide; bubbling of CO2 occurs.

CHEM Topic 2 - Acid plus reactive metal

Acid + metal -> salt + hydrogen gas; bubbling occurs and H2 gives a squeaky pop with a lit splint.

CHEM Topic 2 - Acid plus metal oxide/hydroxide

Acid + base -> salt + water.

CHEM Topic 2 - Ionisation of water

2H2O(l) ⇌ H3O+(aq) + OH-(aq).

CHEM Topic 2 - Ionic product of water

Kw = [H3O+][OH-] = 1.0 x 10^-14 at 25 degrees C.

CHEM Topic 2 - Neutral pH at 25 degrees C

pH 7; [H3O+] = [OH-] = 1.0 x 10^-7 mol L^-1.

CHEM Topic 2 - pH formula

pH = -log10[H3O+] (or [H+]).

CHEM Topic 2 - pOH formula

pOH = -log10[OH-].

CHEM Topic 2 - pH and pOH relationship

pH + pOH = 14 at 25 degrees C.