UPCAT Chemistry — Chemical Equilibrium & REDOXCheat Sheet
Chemical Equilibrium & REDOX cheat sheet for UPCAT aspirants. If you could only take one sheet of paper into your review session, this is what it would look like. University of the Philippines's most-tested concepts, all in one place.
Exam context
On the UPCAT 2026, the Chemistry subtest carries a "Core" weight in University of the Philippines's pattern. Chemical Equilibrium & REDOX lands at position 7th out of 7 in the standard review order. Target score is UPG ≤ 2.2 typical, and roughly 20 items come from Chemistry on a typical UPCAT paper.
Chemical Equilibrium & REDOX - Cheat sheet
Your last-minute revision companion for Chemical Equilibrium and REDOX reactions - all key formulas, reactions, and concepts for UPCAT success
Sections
Formulas
Formula
K_eq = [products]^coefficients / [reactants]^coefficients
Meaning
K_eq = equilibrium constant, [ ] = molar concentrations, coefficients from balanced equation
Watch Out
Only use concentrations of aqueous and gaseous species, NOT solids or liquids
When To Use
To calculate equilibrium constant or predict equilibrium position
Formula
Q = [products]^coefficients / [reactants]^coefficients
Meaning
Q = reaction quotient using actual concentrations at any time
Watch Out
Q < K means forward reaction, Q > K means reverse reaction
When To Use
To predict which direction reaction will proceed (Q vs K comparison)
Section Title
Chemical Equilibrium Fundamentals
Important Facts
- Equilibrium can only exist in closed systems
- All reactants and products must be present at equilibrium
- Equilibrium can be reached from either direction
- Catalysts speed up both forward and reverse reactions equally
- Temperature change is the only factor that changes K value
Key Definitions
Term
Chemical Equilibrium
Example
N₂ + 3H₂ ⇌ 2NH₃ at constant temperature
Definition
State where forward and reverse reaction rates are equal and concentrations remain constant
Term
Dynamic Equilibrium
Example
Ice melting and freezing simultaneously at 0°C
Definition
Reactions continue occurring but no net change in concentrations
Term
Le Chatelier's Principle
Example
Adding reactant shifts equilibrium toward products
Definition
System shifts to counteract any imposed stress or change
Diagrams To Know
- Concentration vs time graph showing equilibrium establishment
- Energy profile diagram for reversible reactions
Reactions Or Equations
Note
General form of reversible reaction at equilibrium
Equation
aA + bB ⇌ cC + dD
Conditions
Closed system, constant temperature
Formulas
Formula
Electrons lost = Electrons gained
Meaning
Conservation of electrons in balanced REDOX equations
Watch Out
Must balance both atoms AND electrons - check both
When To Use
When balancing REDOX reactions by any method
Section Title
REDOX Fundamentals
Important Facts
- OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
- Oxidizing agent gets reduced, reducing agent gets oxidized
- REDOX involves net transfer of electrons between species
- All combustion reactions are REDOX reactions
- Metabolism and cellular respiration involve REDOX processes
Key Definitions
Term
Oxidation
Example
Zn → Zn²⁺ + 2e⁻ (Zn oxidized from 0 to +2)
Definition
Loss of electrons, increase in oxidation number
Term
Reduction
Example
Cu²⁺ + 2e⁻ → Cu (Cu reduced from +2 to 0)
Definition
Gain of electrons, decrease in oxidation number
Term
Oxidizing Agent
Example
O₂ in combustion reactions
Definition
Species that causes oxidation by accepting electrons (gets reduced itself)
Term
Reducing Agent
Example
H₂ in many reactions
Definition
Species that causes reduction by donating electrons (gets oxidized itself)
Diagrams To Know
- Electron transfer diagram showing oxidation and reduction
- Galvanic cell setup showing electron flow
Reactions Or Equations
Note
Zn is reducing agent, Cu²⁺ is oxidizing agent
Equation
Zn + Cu²⁺ → Zn²⁺ + Cu
Conditions
Aqueous solution
Note
H₂ oxidized (0 to +1), O₂ reduced (0 to -2)
Equation
2H₂ + O₂ → 2H₂O
Conditions
Combustion
Common Values
Value
-2
Symbol
O
Quantity
Oxygen oxidation number
Value
+1
Symbol
H
Quantity
Hydrogen oxidation number
Value
-1
Symbol
F
Quantity
Fluorine oxidation number
Section Title
Oxidation Number Rules
Important Facts
- Free elements have oxidation number = 0 (H₂, O₂, Na)
- Group 1 metals = +1, Group 2 metals = +2, Al = +3
- Oxygen = -2 (except in peroxides where O = -1)
- Hydrogen = +1 (except in metal hydrides where H = -1)
- Fluorine = -1 in all compounds
- Sum of oxidation numbers = 0 in neutral compounds
- Sum of oxidation numbers = charge of ion in polyatomic ions
Diagrams To Know
- Periodic table showing common oxidation states
- Step-by-step oxidation number assignment process
Reactions Or Equations
Note
Sum check: +1 + 5 + 3(-2) = 0 ✓
Equation
KNO₃: K(+1) + N(+5) + 3O(-2) = 0
Conditions
Neutral compound
Note
Sum check: +6 + 4(-2) = -2 ✓
Equation
SO₄²⁻: S(+6) + 4O(-2) = -2
Conditions
Polyatomic ion
Section Title
Balancing REDOX Equations
Important Facts
- Both methods must give same final balanced equation
- Always check: atoms balanced AND charges balanced
- In acidic solution: add H⁺ and H₂O to balance H and O
- In basic solution: add OH⁻ and H₂O to balance H and O
- Electrons cancel out in final balanced equation
Key Definitions
Term
Oxidation Number Method
Example
Al + H₂SO₄ → Al₂(SO₄)₃ + H₂
Definition
Balance by equalizing electrons lost and gained using oxidation number changes
Term
Half-Reaction Method
Example
ClO₃⁻ + I⁻ → I₂ + Cl⁻
Definition
Balance by separating into oxidation and reduction half-reactions
Diagrams To Know
- Step-by-step oxidation number method flowchart
- Half-reaction method procedure diagram
Reactions Or Equations
Note
Al loses 6e⁻ total, 6H⁺ gain 6e⁻ total
Equation
2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
Conditions
Balanced using oxidation number method
Note
Cl gains 6e⁻, 6I⁻ lose 6e⁻ total
Equation
ClO₃⁻ + 6H⁺ + 6I⁻ → Cl⁻ + 3H₂O + 3I₂
Conditions
Acidic solution, balanced using half-reaction method
Formulas
Formula
ΔU = q + w
Meaning
ΔU = change in internal energy, q = heat, w = work
Watch Out
Sign conventions: +q = heat absorbed, +w = work done ON system
When To Use
First law of thermodynamics for energy conservation
Formula
KE = ½mv² = (3/2)RT
Meaning
KE = kinetic energy, m = mass, v = velocity, R = gas constant, T = temperature
Watch Out
Temperature must be in Kelvin for RT equation
When To Use
Relating molecular motion to temperature
Section Title
Thermochemistry Basics
Important Facts
- Energy cannot be created or destroyed (First Law)
- Heat flows from high to low temperature spontaneously
- Work involves moving against opposing force
- Internal energy is sum of all kinetic and potential energies
Key Definitions
Term
Thermochemistry
Example
Heat released in combustion reactions
Definition
Study of energy changes in chemical and physical processes
Term
System
Example
Reactants and products in a chemical reaction
Definition
Part of universe being studied
Term
Surroundings
Example
The calorimeter and room in a heat measurement
Definition
Everything else outside the system
Diagrams To Know
- System vs surroundings diagram
- Energy flow diagram for exothermic/endothermic reactions
Must Remember
- OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons
- Oxidation number rules: Free elements = 0, O = -2, H = +1, Group 1 = +1, Group 2 = +2
- Sum of oxidation numbers = 0 (neutral) or ion charge (polyatomic ions)
- In REDOX: electrons lost = electrons gained (conservation)
- Le Chatelier's Principle: system shifts to oppose applied stress
- Equilibrium: forward rate = reverse rate, concentrations constant
- K > 1 favors products, K < 1 favors reactants
- Only temperature changes the equilibrium constant K
- Catalysts speed up both forward and reverse reactions equally
- Oxidizing agent gets reduced, reducing agent gets oxidized
Last Minute Tips
- For oxidation numbers: Start with known values (O=-2, H=+1) and work backwards using sum rules
- In REDOX balancing: Always check BOTH atom balance AND charge balance at the end
- Le Chatelier problems: Identify the stress first, then predict which way shifts to relieve it
- For equilibrium expressions: Only include [aqueous] and (gas), never [solid] or [liquid]
- Common mistake: Temperature is the ONLY thing that changes K value - pressure/concentration don't change K
Comparison Tables
Rows
Values
- Loss of electrons
- Increases
- Becomes more positive
- OIL (Oxidation Is Loss)
- Zn → Zn²⁺ + 2e⁻
Property
Oxidation
Values
- Gain of electrons
- Decreases
- Becomes more negative
- RIG (Reduction Is Gain)
- Cu²⁺ + 2e⁻ → Cu
Property
Reduction
Columns
- Process
- Electron Change
- Oxidation Number
- Memory Aid
- Example
Table Title
Oxidation vs Reduction
Rows
Values
- Causes oxidation
- Gets reduced itself
- O₂, Cl₂, KMnO₄
Property
Oxidizing Agent
Values
- Causes reduction
- Gets oxidized itself
- H₂, Zn, Na
Property
Reducing Agent
Columns
- Agent Type
- What It Does
- What Happens to It
- Common Examples
Table Title
Oxidizing Agent vs Reducing Agent
Rows
Values
- Shift toward products
- N₂ + 3H₂ ⇌ 2NH₃, add N₂ → more NH₃
Property
Add reactant
Values
- Shift toward reactants
- Add NH₃ → more N₂ + H₂
Property
Add product
Values
- Shift toward endothermic side
- If forward is exothermic, shift left
Property
Increase temperature
Values
- Shift toward fewer gas molecules
- 4 mol gas → 2 mol gas, shift right
Property
Increase pressure
Columns
- Stress Applied
- System Response
- Example
Table Title
Equilibrium Shifts (Le Chatelier's Principle)
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