Ontario Grade 12 Chemistry Study Guide (SCH4U): Organic, Electrochemistry, and Equilibrium

Ontario Grade 12 Chemistry (SCH4U) is the university-preparation chemistry course that serves as the foundation for university-level chemistry, chemical engineering, biochemistry, medicine, and pharmacy programs. The course is distinctive in its breadth — it covers organic chemistry, thermochemistry, kinetics, equilibrium, and electrochemistry across five units — and its depth in each, particularly equilibrium and organic chemistry.

The students who achieve high marks understand that SCH4U rewards precise, systematic problem-solving and clear conceptual understanding over memorisation. Many students attempt to memorise their way through the organic chemistry unit — this approach breaks down on unfamiliar compound names and reactions. Building genuine understanding of why organic reactions occur pays larger dividends.

Organic chemistry: naming, reactions, and isomers

IUPAC nomenclature hierarchy: When naming compounds with multiple functional groups, IUPAC specifies a priority order. The highest priority group is the principal characteristic group (determines the name suffix). Priority order (highest to lowest): carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine. Lower priority groups are named as substituents or prefixes.

Drawing and naming steps:

1. Identify the principal characteristic group → determine suffix
2. Find the longest carbon chain containing the principal group → parent chain name
3. Number the chain so the principal group has the lowest possible locant
4. Name and number all substituents (alphabetically, with multiplying prefixes if needed)
5. For cyclic compounds: prefix cyclo-; for benzene ring: phenyl substituent or benzene as parent

Organic reaction types — reagents and products:

• Addition (alkenes + HX, H₂, Br₂, H₂O): adds across the double bond; Markovnikov's rule (H to the more-substituted C) for HX addition
• Substitution (haloalkanes + OH⁻ → alcohol; or arenes): replaces a substituent
• Elimination (alcohol + conc. H₂SO₄, heat → alkene + H₂O): removes atoms to form double bond
• Esterification (carboxylic acid + alcohol + H⁺ catalyst → ester + water): condensation reaction
• Hydrolysis (ester + H₂O → carboxylic acid + alcohol; saponification with NaOH → carboxylate salt)
• Combustion: complete (CO₂ + H₂O) vs incomplete (CO or C)

Isomers: Structural isomers have the same molecular formula but different structural formulas. Geometric (cis-trans) isomers require a double bond or ring and different groups on each doubly bonded carbon. Optical (chiral) isomers require a chiral carbon (four different substituents) — the two non-superimposable mirror images are enantiomers.

Thermochemistry and kinetics

Hess's Law: The enthalpy change of a reaction is independent of the path taken. You can add (or subtract) individual reaction enthalpy changes to find the enthalpy change of a target reaction. Strategy: arrange known reactions so their sum equals the target reaction. Reverse a reaction to change the sign of ΔH; multiply a reaction to multiply ΔH.

Standard enthalpies of formation (ΔH°f): ΔH°rxn = Σ[ΔH°f(products)] − Σ[ΔH°f(reactants)]. The standard enthalpy of formation of an element in its standard state is zero by definition.

Collision theory and activation energy: Reactions occur when molecules collide with sufficient energy (≥ activation energy Ea) and correct orientation. Arrhenius equation: k = Ae^(−Ea/RT). Temperature increase raises the fraction of molecules with sufficient energy. Catalysts lower Ea (provide an alternative reaction pathway) without being consumed.

Use the Cornell Notes Tool for thermochemistry: enthalpy diagrams in the main column, the Hess's Law calculation in the cue column, and the physical interpretation in the summary.

Chemical equilibrium

Le Chatelier's Principle and Kc:

A system at equilibrium responds to stress by shifting to partially relieve it. The equilibrium constant Kc is fixed at a given temperature — it does not change when concentrations change (the position of equilibrium shifts, but Kc stays the same). Kc only changes when temperature changes.

Ksp and solubility: The solubility product expression for a sparingly soluble salt. AgCl ⇌ Ag⁺ + Cl⁻ → Ksp = [Ag⁺][Cl⁻]. From molar solubility s: if AgCl has molar solubility s, then [Ag⁺] = [Cl⁻] = s and Ksp = s². Common ion effect: adding a common ion shifts equilibrium to reduce solubility (Le Chatelier).

pH calculations:

• Strong acid: [H⁺] = concentration of acid directly (complete dissociation). pH = −log[H⁺].
• Weak acid: set up Ka = [H⁺][A⁻]/[HA], use ICE table, solve for [H⁺], calculate pH.
• Buffer (Henderson-Hasselbalch): pH = pKa + log([A⁻]/[HA]).

Electrochemistry

Galvanic cells: Spontaneous redox reaction produces electrical energy. Identify the anode (oxidation) and cathode (reduction) from the standard reduction potential table — the half-reaction with higher reduction potential undergoes reduction. Cell potential: E°cell = E°cathode − E°anode.

Electrolytic cells: Applied voltage drives a non-spontaneous redox reaction. The amount of substance deposited or dissolved is calculated using Faraday's laws: n = Q/F = It/F moles of electrons; moles of substance = n/number of electrons per ion.

The Spaced Repetition Flashcard Tool is highly effective for functional group identification and organic reaction types. Build one card per reaction type: front = starting material + reagent, back = product + reaction type name. See the Ontario Grade 12 Physics study guide for the parallel strategies on the other major Grade 12 science course.