Synthesis, Decomposition, Single Replacement, Double Replacement, Combustion. Every reaction you encounter in general chemistry fits one of these five categories.
S
Synthesis — A + B → AB (two things combine)
D
Decomposition — AB → A + B (one thing breaks apart)
S
Single Replacement — A + BC → AC + B
D
Double Replacement — AB + CD → AD + CB
C
Combustion — fuel + O₂ → CO₂ + H₂O
⚗️ Reactions
OIL RIG
Oxidation and Reduction
OIL RIG — never confuse oxidation and reduction again
Oxidation Is Loss (of electrons). Reduction Is Gain (of electrons). The two always happen together — redox reactions. The species that loses electrons is oxidized; the one that gains is reduced.
⚗️ Reactions
LEO says GER
Electron Transfer in Redox
LEO GER — an alternative to OIL RIG
Lose Electrons = Oxidized. Gain Electrons = Reduced. Both mnemonics work — pick the one that sticks. Many instructors prefer OIL RIG; both appear on exams.
⚗️ Reactions
Combustion always makes CO₂ + H₂O
Combustion Reaction Products
Complete combustion always produces the same two products
Any hydrocarbon + sufficient O₂ → CO₂ + H₂O. Always. If oxygen is limited, incomplete combustion occurs and CO (carbon monoxide) forms instead of CO₂.
⚗️ Reactions
Activity Series: top metals replace bottom metals
Activity Series of Metals
More active metals displace less active metals from solution
Metals higher on the activity series can displace metals lower on it from ionic solutions. Zinc will displace copper from CuSO₄. Copper cannot displace zinc. Know the top 10.
Balancing Chemical Equations
Balancing equations: atoms in = atoms out. Never change subscripts, only coefficients.
Balancing Chemical Equations
The golden rule — atoms are conserved in every chemical reaction
Law of Conservation of Mass: matter is neither created nor destroyed. Count atoms on each side. Change coefficients (the big numbers in front) to balance — NEVER change subscripts inside a formula. H₂ + O₂ → H₂O becomes 2H₂ + O₂ → 2H₂O.
Exothermic: products have less energy than reactants → energy released as heat. Combustion, respiration, hand warmers. ΔH is negative. Endothermic: products have MORE energy → energy absorbed. Photosynthesis, ice packs, cooking an egg. ΔH is positive. 'EXOthermic = EXits heat.'
Factors Affecting Reaction Rate
Reaction rate increases with: more concentration, higher temperature, catalyst, more surface area
Factors Affecting Reaction Rate
Four ways to speed up a chemical reaction
Concentration: more particles → more collisions. Temperature: faster-moving particles → more energetic collisions. Catalyst: lowers activation energy without being consumed. Surface area: more exposed particles → more collisions (powders react faster than chunks).
Catalysts
Catalyst lowers activation energy — speeds reaction without being consumed
Catalysts
What a catalyst does — and what it doesn't do
Catalyst provides an alternative reaction pathway with lower activation energy. Reaction goes faster but the catalyst is regenerated at the end — not used up. Doesn't change ΔH (energy difference between reactants and products). Enzymes are biological catalysts.
Le Chatelier's Principle
Le Chatelier's Principle: stress the system → equilibrium shifts to relieve the stress
Le Chatelier's Principle
If you push a reaction at equilibrium, it pushes back
Add reactant → shift right (forward). Remove product → shift right. Increase pressure → shift toward fewer moles of gas. Increase temperature → shift in endothermic direction. Used to maximize product yield in industrial reactions (Haber process).
Precipitation Reactions
Precipitation reaction: two solutions mix → insoluble solid (precipitate) forms
Precipitation Reactions
When mixing two solutions produces a solid
Pb(NO₃)₂ + 2KI → PbI₂↓ + 2KNO₃. The ↓ symbol means precipitate (solid falling out of solution). Use solubility rules to predict: most nitrates are soluble. Most chlorides are soluble EXCEPT AgCl, PbCl₂, Hg₂Cl₂.
Oxidation Number Rules
Oxidation numbers: free element = 0. Monatomic ion = charge. Oxygen usually = -2. Hydrogen usually = +1.
Oxidation Number Rules
Assign oxidation numbers to identify what's being oxidized and reduced
Rules in order of priority: 1) Free element = 0. 2) Monatomic ion = its charge. 3) Fluorine = -1 always. 4) Oxygen = -2 (except peroxides). 5) Hydrogen = +1 (except metal hydrides). 6) Sum of oxidation numbers = overall charge of species.