Equilibrium Constants: Kc and Kp

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Core Idea

The equilibrium constant Kc is expressed in terms of molar concentrations; Kp is expressed in terms of partial pressures (for gaseous equilibria). The two are related by Kp = Kc(RT)^Δn, where Δn is the change in moles of gas. Both are temperature-dependent and dimensionless (or have derived units depending on stoichiometry).

Common Misconceptions

Thinking Kc and Kp are the same or that they have the same numerical value. Forgetting that only gases contribute to Kp, and only solutes contribute to Kc (water is typically omitted).

Explainer

When you studied chemical equilibrium, you learned that reversible reactions reach a state where the forward and reverse rates are equal — and that the ratio of product concentrations to reactant concentrations at equilibrium is constant at a given temperature. Kc and Kp are both ways of expressing that ratio; the difference is only in what units you use to measure "how much."

Kc uses molar concentrations (mol/L). For a general reaction aA + bB ⇌ cC + dD, the expression is Kc = [C]^c[D]^d / ([A]^a[B]^b). The brackets denote equilibrium concentrations. Notice that every concentration is raised to its stoichiometric coefficient — that exponent comes directly from the rate-law derivation underpinning equilibrium. A large Kc means products are heavily favored at equilibrium; a small Kc means reactants dominate.

Kp applies specifically to gaseous equilibria and replaces concentrations with partial pressures. Since the ideal gas law tells you P = nRT/V = (n/V)RT = [gas]·RT, you can convert between the two: Kp = Kc(RT)^Δn, where Δn is the change in moles of gas (products minus reactants). If Δn = 0 (same number of gas moles on each side), Kp equals Kc. If more moles of gas are produced, Kp > Kc; if fewer, Kp < Kc. Tracking the sign of Δn is one of the most error-prone steps — count carefully.

A critical rule often forgotten: pure solids and pure liquids do not appear in equilibrium expressions. The reason is that their "concentration" is essentially fixed (determined by their density, which barely changes), so it is folded into the constant K itself. Only gaseous species enter Kp, and only dissolved species enter Kc. In heterogeneous equilibria — reactions mixing phases — this distinction is essential to writing a correct expression.

Both constants are temperature-dependent but pressure-independent. Adding or removing reactants shifts the position of equilibrium (the reaction quotient Q changes) but does not change K itself. This is the foundation for Le Chatelier's principle, which you will explore next: K stays fixed while Q adjusts until it again equals K.

Practice Questions 3 questions

Prerequisite Chain

Counting to 10 → Counting to 20 → Understanding Zero → The Number Zero → Counting to Five → One-to-One Correspondence → Combining Small Groups Within 5 → Addition Within 10 → Addition Within 20 → Two-Digit Addition Without Regrouping → Two-Digit Addition with Regrouping → Addition Within 100 → Repeated Addition as Multiplication → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Two-Digit by One-Digit Division → Division with Remainders → Remainders and Quotients in Division → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Reading and Writing Decimals → Comparing and Ordering Decimals → Adding and Subtracting Decimals → Multiplying Decimals → Dividing Decimals → Dividing Fractions → Mixed Number Arithmetic → Order of Operations → Integer Order of Operations → Variable Expressions → Combining Like Terms → One-Step Equations → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Iterated Integrals and Fubini's Theorem → Double Integrals in Cartesian Coordinates → Double Integrals over Rectangular Regions → Double Integrals in Polar Coordinates → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Green's Theorem → Surface Integrals and Flux of Vector Fields → Surface Integrals and Flux of Vector Fields → Divergence Theorem: Flux and Outflow → Divergence Theorem → Electric Flux → Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → The Electromagnetic Spectrum → Blackbody Radiation and Planck's Law → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → Heisenberg Uncertainty Principle → Wavefunction and the Born Rule → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Scattering Theory → Introduction to Scattering Theory → Partial Wave Analysis in Scattering → Spin Angular Momentum → Electron Spin and Intrinsic Magnetic Moment → Stern-Gerlach Experiment: Spin Quantization and Measurement → Electron Diffraction and Matter Wave Properties → Davisson-Germer Experiment: Crystal Diffraction of Electrons → Electron Diffraction and Matter Wave Interference → Wavefunctions and Probability Density Interpretation → Quantum Superposition and Linear Combinations of States → Quantum Operators and Observables → Canonical Commutation Relations and Uncertainty → Heisenberg Uncertainty Principle and Measurement Limits → Time-Independent Schrödinger Equation and Eigenvalues → Hydrogen Atom in Quantum Mechanics → Spectral Lines and Energy Transitions → Selection Rules for Atomic Transitions → LS and jj Coupling Schemes in Multi-Electron Atoms → Pauli Exclusion Principle and Antisymmetric Wavefunctions → Electron Configuration and the Aufbau Principle → The Periodic Table and Atomic Electronic Structure → The Periodic Table → Electron Configuration → Periodic Trends → Ionization Energy → Ionic Bonding → Lewis Structures → Resonance Structures and Delocalized Electrons → Resonance and Formal Charge → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Equilibrium Constants: Kc and Kp

Longest path: 166 steps · 740 total prerequisite topics

Prerequisites (4)

Chemical Equilibriumhard Gas Lawssoft Solving Multi-Step Equationssoft Exponential Functions and Graphssoft

Leads To (18)

ATP Hydrolysis and Cellular Free Energysoft Action Potential Phasessoft Allosteric Enzyme Regulationsoft Citric Acid Cycle: Mechanism and Stoichiometrysoft Fermentation Pathways and Metabolic End-Productssoft Fractional Crystallization and Magmatic Differentiationsoft Glycolysis: Mechanism and Regulationsoft Hemoglobin Cooperativity and the Oxygen-Hemoglobin Dissociation Curvesoft Metabolic Integration: Coordinating Pathwayssoft Metamorphic Equilibrium and Phase Diagramssoft Michaelis-Menten Enzyme Kineticssoft Mutation-Selection Balancesoft Ocean Carbonate System and Buffering Capacityhard Oxidative Phosphorylation and Chemiosmotic Couplingsoft Oxygen Transport and Hemoglobin Dynamicssoft Reaction Quotient (Q) and Equilibrium Directionhard Resting Membrane Potentialsoft The Iron-Carbon Phase Diagram and Steel Microstructuressoft