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⚛️ Chemistry · Chemical Reactions

Memory tricks for VSEPR, polarity & intermolecular forces

From the five reaction types to balancing equations to Le Chatelier's principle — these memory tricks lock in the rules for predicting how, and how fast, a chemical reaction actually proceeds.

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Memory Tricks

Proven mnemonics — fast to learn, hard to forget.

⚛️ Chemical Reactions — 9 Memory Tricks  ·  Click any card to expand · Save favorites · Switch to Flashcard or Quiz mode below
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Redox
OIL RIG
Oxidation Is Loss · Reduction Is Gain (of electrons)
OIL RIG is the most tested mnemonic in all of chemistry. Oxidation Is Loss of electrons — the species being oxidized loses electrons and its oxidation number increases. Reduction Is Gain of electrons — the species being reduced gains electrons and its oxidation number decreases. They always happen together.
📖 Full Lesson → Difficulty: Intermediate
Oxidizing agent
The species that causes oxidation by accepting electrons — it gets REDUCED itself. "OA gets reduced."
Reducing agent
The species that causes reduction by donating electrons — it gets OXIDIZED itself. "RA gets oxidized."
LEO says GER
Alternative: Lose Electrons = Oxidized · Gain Electrons = Reduced. Both mnemonics work — pick your favorite.
Oxidation numbers
Increase = oxidized. Decrease = reduced. Free elements = 0. O usually = -2. H usually = +1. Monatomic ion = its charge.
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Alex breaks down OIL RIG, oxidation numbers, and the oxidizing/reducing agent trap — 3:04.
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🃏 Redox
OIL RIG
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🃏 Answer
Oxidizing agent — The species that causes oxidation by accepting electrons — it gets REDUCED itself. "OA gets reduced."
Reducing agent — The species that causes reduction by donating electrons — it gets OXIDIZED itself. "RA gets oxidized."
LEO says GER — Alternative: Lose Electrons = Oxidized · Gain Electrons = Reduced. Both mnemonics work — pick your favorite.
Oxidation numbers — Increase = oxidized. Decrease = reduced. Free elements = 0. O usually = -2. H usually = +1. Monatomic ion = its charge.
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Reaction Types
SDSDC (S=Synthesis, D=Decomposition, S=Single replacement, D=Double replacement, C=Combustion)
Synthesis · Decomposition · Single replacement · Double replacement · Combustion
Every reaction in general chemistry fits one of five categories. SDSDC: Synthesis (A+B→AB) · Decomposition (AB→A+B) · Single Replacement (A+BC→AC+B) · Double Replacement (AB+CD→AD+CB) · Combustion (fuel+O₂→CO₂+H₂O). Identify the type first — then predict the products.
📖 Full Lesson → Difficulty: Beginner
Synthesis
Two or more substances combine to form one product. 2H₂ + O₂ → 2H₂O. Always A + B → AB pattern.
Decomposition
One compound breaks into two or more simpler substances. 2H₂O → 2H₂ + O₂. Opposite of synthesis.
Single replacement
One element displaces another from a compound. Zn + CuSO₄ → ZnSO₄ + Cu. Use activity series to predict if it occurs.
Double replacement
Ions of two compounds exchange partners. Often produces a precipitate, gas, or water. AgNO₃ + NaCl → AgCl↓ + NaNO₃.
Combustion
Hydrocarbon + O₂ → CO₂ + H₂O (complete) or CO + H₂O (incomplete). Always exothermic — releases energy.
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🃏 Reaction Types
SDSDC
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🃏 Answer
SDSDC (S=Synthesis, D=Decomposition, S=Single replacement, D=Double replacement, C=Combustion)
Synthesis — Two or more substances combine to form one product. 2H₂ + O₂ → 2H₂O. Always A + B → AB pattern.
Decomposition — One compound breaks into two or more simpler substances. 2H₂O → 2H₂ + O₂. Opposite of synthesis.
Single replacement — One element displaces another from a compound. Zn + CuSO₄ → ZnSO₄ + Cu. Use activity series to predict if it occurs.
Double replacement — Ions of two compounds exchange partners. Often produces a precipitate, gas, or water. AgNO₃ + NaCl → AgCl↓ + NaNO₃.
Combustion — Hydrocarbon + O₂ → CO₂ + H₂O (complete) or CO + H₂O (incomplete). Always exothermic — releases energy.
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Balancing
COACH (C=Count atoms, O=Only change Coefficients, A=Atoms conserved, C=Check both sides, H=Hydrogen and oxygen last)
Count atoms · Only change Coefficients · Atoms conserved · Check both sides · Hydrogen and oxygen last
COACH gives you the systematic approach to balancing any equation. Count atoms on each side · Only change Coefficients (never subscripts) · Atoms are Conserved — matter can't be created or destroyed · Check both sides when done · Balance Hydrogen and Oxygen last (they appear in the most compounds).
📖 Full Lesson → Difficulty: Beginner
Never change subscripts
Changing H₂O to H₃O changes the compound entirely. Only the big numbers (coefficients) in front of formulas can change.
Start with metals
Balance elements that appear in only one reactant and one product first. Leave H and O for last — they appear everywhere.
Use fractions if needed
It's fine to use ½ or ¾ as intermediate coefficients — multiply everything by the denominator at the end to clear fractions.
Polyatomic ions
If a polyatomic ion appears unchanged on both sides, balance it as a unit — don't split it into atoms.
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🃏 Balancing
COACH
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🃏 Answer
COACH (C=Count atoms, O=Only change Coefficients, A=Atoms conserved, C=Check both sides, H=Hydrogen and oxygen last)
Never change subscripts — Changing H₂O to H₃O changes the compound entirely. Only the big numbers (coefficients) in front of formulas can change.
Start with metals — Balance elements that appear in only one reactant and one product first. Leave H and O for last — they appear everywhere.
Use fractions if needed — It's fine to use ½ or ¾ as intermediate coefficients — multiply everything by the denominator at the end to clear fractions.
Polyatomic ions — If a polyatomic ion appears unchanged on both sides, balance it as a unit — don't split it into atoms.
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Equilibrium
LE CHAT
Le Chatelier's principle — stress the system, it shifts to relieve the stress
LE CHAT: if a system at equilibrium is stressed, it shifts to counteract the stress. Add reactant → shifts right. Remove product → shifts right. Increase pressure → shifts toward fewer moles of gas. Increase temperature → shifts in the endothermic direction. Used to maximize yield in industrial reactions (Haber process).
📖 Full Lesson → Difficulty: Intermediate
Concentration
Add reactant or remove product → shifts right (forward). Remove reactant or add product → shifts left (reverse).
Pressure
Increase pressure (decrease volume) → shifts toward side with fewer moles of gas. Equal moles of gas = no shift.
Temperature
Treat heat as a reactant (endothermic) or product (exothermic). Increase temp → shifts away from heat. Decrease → shifts toward heat.
Catalyst
Speeds up both forward and reverse reactions equally — reaches equilibrium faster but does NOT shift the equilibrium position.
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🃏 Equilibrium
LE CHAT
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🃏 Answer
Concentration — Add reactant or remove product → shifts right (forward). Remove reactant or add product → shifts left (reverse).
Pressure — Increase pressure (decrease volume) → shifts toward side with fewer moles of gas. Equal moles of gas = no shift.
Temperature — Treat heat as a reactant (endothermic) or product (exothermic). Increase temp → shifts away from heat. Decrease → shifts toward heat.
Catalyst — Speeds up both forward and reverse reactions equally — reaches equilibrium faster but does NOT shift the equilibrium position.
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Reaction Rates
CATS (C=Concentration, A=Area/surface area, T=Temperature, S=Surface area catalyst/catalyst)
Concentration · Area (surface) · Temperature · Surface area catalyst
Reaction rate depends on collision frequency and energy. CATS: Concentration (more particles = more collisions) · Area/surface area (powders react faster than chunks) · Temperature (faster molecules = more energetic collisions) · catalyst (lowers activation energy). All four increase the rate of successful collisions.
📖 Full Lesson → Difficulty: Intermediate
Collision theory
Reactions occur when particles collide with sufficient energy (≥ activation energy) and proper orientation. Rate = collision frequency × fraction with enough energy.
Temperature effect
Rule of thumb: every 10°C rise roughly doubles reaction rate. Higher temperature → more molecules exceed activation energy threshold.
Catalyst mechanism
Provides an alternative reaction pathway with lower activation energy. Is regenerated — not consumed. Enzymes are biological catalysts.
Activation energy
Minimum energy needed for a reaction to occur. Shown on energy diagrams as the "hill" between reactants and products. Exothermic: products lower than reactants.
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🃏 Reaction Rates
CATS
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🃏 Answer
CATS (C=Concentration, A=Area/surface area, T=Temperature, S=Surface area catalyst/catalyst)
Collision theory — Reactions occur when particles collide with sufficient energy (≥ activation energy) and proper orientation. Rate = collision frequency × fraction with enough energy.
Temperature effect — Rule of thumb: every 10°C rise roughly doubles reaction rate. Higher temperature → more molecules exceed activation energy threshold.
Catalyst mechanism — Provides an alternative reaction pathway with lower activation energy. Is regenerated — not consumed. Enzymes are biological catalysts.
Activation energy — Minimum energy needed for a reaction to occur. Shown on energy diagrams as the "hill" between reactants and products. Exothermic: products lower than reactants.
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Activity Series
Please Stop Calling Me A Zebra — I Like Copper's Silver Nature
K · Na · Ca · Mg · Al · Zn · Fe · Ni · Sn · Pb · H · Cu · Ag · Au · Pt
The activity series ranks metals by reactivity. Metals higher on the list displace metals lower on it from ionic solutions. Please Stop Calling Me A Zebra — I Like Copper's Silver Nature gives you K→Na→Ca→Mg→Al→Zn→Fe→Ni→Sn→Pb→H→Cu→Ag→Au→Pt from most to least reactive.
📖 Full Lesson → Difficulty: Intermediate
Rule
Metal A displaces Metal B from solution only if A is above B on the activity series. Zn displaces Cu; Cu cannot displace Zn.
Hydrogen's position
Metals above H react with acids to produce H₂ gas. Metals below H (Cu, Ag, Au, Pt) do NOT react with dilute acids.
Most reactive
K, Na, Ca react violently with water producing H₂ gas and metal hydroxide. Never store in water or open air.
Least reactive
Au, Pt are noble metals — resistant to corrosion and most chemical reactions. That's why gold jewelry lasts forever.
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🃏 Activity Series
Please Stop Calling Me A Zebra — I Like Copper's Silver Nature
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🃏 Answer
Rule — Metal A displaces Metal B from solution only if A is above B on the activity series. Zn displaces Cu; Cu cannot displace Zn.
Hydrogen's position — Metals above H react with acids to produce H₂ gas. Metals below H (Cu, Ag, Au, Pt) do NOT react with dilute acids.
Most reactive — K, Na, Ca react violently with water producing H₂ gas and metal hydroxide. Never store in water or open air.
Least reactive — Au, Pt are noble metals — resistant to corrosion and most chemical reactions. That's why gold jewelry lasts forever.
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Precipitation
SNAP (N=Nitrates always soluble, A=Alkali metals always soluble, P=Perchlorates always soluble — the "S" frames the mnemonic)
Solubility rules: Nitrates Always soluble · Alkali metals Always soluble · Perchlorates Always soluble
SNAP covers the always-soluble categories. Beyond SNAP: most chlorides are soluble EXCEPT AgCl, PbCl₂, Hg₂Cl₂ · most sulfates soluble EXCEPT BaSO₄, PbSO₄, CaSO₄ · most carbonates, phosphates, and hydroxides are INSOLUBLE except with alkali metals or NH₄⁺. Insoluble = precipitate (↓).
📖 Full Lesson → Difficulty: Intermediate
Always soluble
All nitrates (NO₃⁻) · All alkali metal salts (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) · All ammonium salts (NH₄⁺) · All perchlorates.
Usually soluble
Chlorides (except Ag⁺, Pb²⁺, Hg₂²⁺) · Sulfates (except Ba²⁺, Pb²⁺, Ca²⁺, Hg₂²⁺) · Acetates.
Usually insoluble
Carbonates · Phosphates · Hydroxides · Sulfides — all insoluble UNLESS combined with alkali metals or NH₄⁺.
Net ionic equation
Remove spectator ions (those that don't participate). The net ionic equation shows only the ions that actually react to form the precipitate.
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🃏 Precipitation
Solubility rules — SNAP
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🃏 Answer
SNAP (N=Nitrates always soluble, A=Alkali metals always soluble, P=Perchlorates always soluble — the "S" frames the mnemonic)
Always soluble — All nitrates (NO₃⁻) · All alkali metal salts (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) · All ammonium salts (NH₄⁺) · All perchlorates.
Usually soluble — Chlorides (except Ag⁺, Pb²⁺, Hg₂²⁺) · Sulfates (except Ba²⁺, Pb²⁺, Ca²⁺, Hg₂²⁺) · Acetates.
Usually insoluble — Carbonates · Phosphates · Hydroxides · Sulfides — all insoluble UNLESS combined with alkali metals or NH₄⁺.
Net ionic equation — Remove spectator ions (those that don't participate). The net ionic equation shows only the ions that actually react to form the precipitate.
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Exo vs Endo
EXO exits · ENDO enters
Exothermic = heat exits system (ΔH negative) · Endothermic = heat enters system (ΔH positive)
EXO exits — heat leaves the system, surroundings get warm, ΔH is negative. ENDO enters — heat flows into the system from surroundings, surroundings get cold, ΔH is positive. Combustion is always exothermic. Photosynthesis is endothermic. Hand warmers = exo. Ice packs = endo.
📖 Full Lesson → Difficulty: Beginner
ΔH negative
Exothermic — products have LESS energy than reactants. Energy released. Feels hot. Combustion, neutralization, respiration.
ΔH positive
Endothermic — products have MORE energy than reactants. Energy absorbed. Feels cold. Photosynthesis, melting ice, cooking.
Energy diagram
Exothermic: products lower than reactants on the diagram. Endothermic: products higher. Activation energy is the hill in both cases.
Bond energy connection
Breaking bonds requires energy (endothermic). Forming bonds releases energy (exothermic). ΔH = energy to break bonds − energy released forming bonds.
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🃏 Exo vs Endo
Exothermic vs endothermic — which way does heat flow?
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🃏 Answer
EXO exits · ENDO enters
ΔH negative — Exothermic — products have LESS energy than reactants. Energy released. Feels hot. Combustion, neutralization, respiration.
ΔH positive — Endothermic — products have MORE energy than reactants. Energy absorbed. Feels cold. Photosynthesis, melting ice, cooking.
Energy diagram — Exothermic: products lower than reactants on the diagram. Endothermic: products higher. Activation energy is the hill in both cases.
Bond energy connection — Breaking bonds requires energy (endothermic). Forming bonds releases energy (exothermic). ΔH = energy to break bonds − energy released forming bonds.
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Equilibrium Constant
K = Products over Reactants (raised to their coefficients)
Keq = [products]^coefficients ÷ [reactants]^coefficients
K expression: products in the numerator, reactants in the denominator, each raised to the power of its coefficient. K > 1 → products favored (equilibrium lies to the right). K < 1 → reactants favored (lies left). K = 1 → roughly equal. Pure solids and liquids are NOT included in K expressions.
📖 Full Lesson → Difficulty: Advanced
Writing Keq
For aA + bB ⇌ cC + dD: Keq = [C]^c[D]^d / [A]^a[B]^b. Use molar concentrations for Kc or partial pressures for Kp.
Excluded species
Pure solids (s) and pure liquids (l) have constant concentrations — they're excluded from K expressions. Only aqueous (aq) and gas (g) species are included.
Reaction quotient Q
Q uses same expression as K but with non-equilibrium concentrations. Q < K → reaction proceeds right. Q > K → reaction proceeds left. Q = K → at equilibrium.
Kp vs Kc
Kp uses partial pressures; Kc uses molar concentrations. Related by: Kp = Kc(RT)^Δn where Δn = moles gas products − moles gas reactants.
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🃏 Equilibrium Constant
The equilibrium constant K — how is it written?
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🃏 Answer
K = Products over Reactants (raised to their coefficients)
Writing Keq — For aA + bB ⇌ cC + dD: Keq = [C]^c[D]^d / [A]^a[B]^b. Use molar concentrations for Kc or partial pressures for Kp.
Excluded species — Pure solids (s) and pure liquids (l) have constant concentrations — they're excluded from K expressions. Only aqueous (aq) and gas (g) species are included.
Reaction quotient Q — Q uses same expression as K but with non-equilibrium concentrations. Q < K → reaction proceeds right. Q > K → reaction proceeds left. Q = K → at equilibrium.
Kp vs Kc — Kp uses partial pressures; Kc uses molar concentrations. Related by: Kp = Kc(RT)^Δn where Δn = moles gas products − moles gas reactants.
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