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Equilibrium Constant Calculator

Computes the concentration equilibrium constant Kc of a reaction aA + bB ⇌ cC + dD from equilibrium concentrations raised to their stoichiometric coefficients, converts it to Kp with Kp = Kc·(RT)^Δn for gas-phase reactions, and reports log K, the standard Gibbs energy ΔG° = −RT·ln K and which side the equilibrium favours.

When to use

You have equilibrium concentrations of the species of a balanced reaction with up to two reactants and two products and need Kc, Kp, ΔG° or a statement of whether products or reactants are favoured.

Do not use when: You need to predict equilibrium concentrations from a known K and initial amounts (an ICE-table problem), the pH of a weak acid (use weak-acid-ph), or K from a cell potential (use nernst-equation).

Formula

Kc = concentration_c^c × concentration_d^d / (concentration_a^a × concentration_b^b); Δn = c + d − a − b; Kp = Kc × (R × T)^Δn with R = 0.082057 L·atm/(mol·K); ΔG° = −R × T × ln K with R = 8.314 J/(mol·K); log_kc = log10 Kc

Concentrations are used in place of activities, so K is dimensionless relative to the 1 mol/L (Kc) or 1 atm (Kp) standard state; species with coefficient 0 are omitted, as pure solids, liquids and the solvent should be. K is temperature-dependent: the entered temperature only converts Kc to Kp and to ΔG°, it does not extrapolate K to another temperature (that needs ΔH° via the van 't Hoff equation).

Inputs

ParameterTypeUnitRequiredDescription
coefficient_aintegerdefault 1Stoichiometric coefficient of reactant A. Range: ≥ 1, ≤ 20
concentration_anumbermol/LyesEquilibrium concentration of reactant A (omit pure solids, liquids and the solvent from the expression by not entering them). Range: > 0, ≤ 1000000
coefficient_bintegerdefault 0Stoichiometric coefficient of reactant B; 0 when there is no second reactant. Range: ≥ 0, ≤ 20
concentration_bnumbermol/Ldefault 0Equilibrium concentration of reactant B (required when coefficient_b > 0). Range: ≥ 0, ≤ 1000000
coefficient_cintegerdefault 1Stoichiometric coefficient of product C. Range: ≥ 1, ≤ 20
concentration_cnumbermol/LyesEquilibrium concentration of product C. Range: > 0, ≤ 1000000
coefficient_dintegerdefault 0Stoichiometric coefficient of product D; 0 when there is no second product. Range: ≥ 0, ≤ 20
concentration_dnumbermol/Ldefault 0Equilibrium concentration of product D (required when coefficient_d > 0). Range: ≥ 0, ≤ 1000000
temperature_knumberKdefault 298.15Absolute temperature used for Kp = Kc(RT)^Δn and ΔG° = −RT ln K (298.15 K = 25 °C). Range: > 0, ≤ 100000

Outputs

OutputTypeUnitDescription
kcnumber[C]^c [D]^d / ([A]^a [B]^b) with concentrations in mol/L (dimensionless by convention).
kc_scientificstringKc in scientific notation.
kpnumberKc × (R·T)^Δn with R = 0.082057 L·atm/(mol·K): the pressure-based constant with partial pressures in atm (equal to Kc when Δn = 0).
kp_scientificstringKp in scientific notation.
delta_n_gasintegerc + d − a − b: change in moles of gas per reaction as written.
rt_l_atm_per_molnumberL·atm/mol0.082057 × temperature_k, the factor raised to Δn.
log_kcnumberBase-10 logarithm of Kc.
delta_g_kj_molnumberkJ/mol−R·T·ln Kc with R = 8.314 J/(mol·K), standard state 1 mol/L; negative when products are favoured.
delta_g_kp_kj_molnumberkJ/mol−R·T·ln Kp (standard state 1 atm), the value to use for gas-phase reactions; identical to delta_g_kj_mol when Δn = 0.
reaction_directionstringWhether products or reactants are favoured (K > 1, K < 1 or K ≈ 1).
reaction_textstringThe reaction as interpreted, e.g. 1 A + 3 B ⇌ 2 C.

Example

H2 + I2 ⇌ 2 HI with [H2] 0.1, [I2] 0.1, [HI] 0.7 mol/L at 298.15 K: {"coefficient_a":1,"concentration_a":0.1,"coefficient_b":1,"concentration_b":0.1,"coefficient_c":2,"concentration_c":0.7} → {"kc":49,"kp":49,"delta_n_gas":0,"log_kc":1.6902,"delta_g_kj_mol":-9.648,"reaction_direction":"K > 1: products favoured at equilibrium","reaction_text":"1 A + 1 B ⇌ 2 C"}

N2 + 3 H2 ⇌ 2 NH3 at 500 K with Kc = 0.5 ([N2] 1, [H2] 1, [NH3] 0.70711): {"coefficient_a":1,"concentration_a":1,"coefficient_b":3,"concentration_b":1,"coefficient_c":2,"concentration_c":0.70711,"temperature_k":500} → {"kc":0.5,"delta_n_gas":-2,"kp":0.00029703,"kp_scientific":"2.970e-4","rt_l_atm_per_mol":41.0287,"delta_g_kj_mol":2.881,"delta_g_kp_kj_mol":33.764,"reaction_direction":"K < 1: reactants favoured at equilibrium","reaction_text":"1 A + 3 B ⇌ 2 C"}

GET https://tttkmbb.com/api/v1/calculate/equilibrium-constant?coefficient_a=1&concentration_a=0.1&coefficient_b=1&concentration_b=0.1&coefficient_c=2&concentration_c=0.7

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Sources

FAQ

How do I enter a reaction with a solid or the solvent?

Leave it out: pure solids, pure liquids and water as solvent have activity 1 and do not appear in K. Set the unused coefficient to 0 (B or D) or renumber the species so A and C are the ones that appear.

Kp in atm or bar?

Kp here uses partial pressures in atm (R = 0.082057 L·atm/(mol·K)). For the IUPAC 1 bar standard state multiply by 1.01325^Δn, or equivalently use R = 0.083145 L·bar/(mol·K).

How does K change with temperature?

This calculator does not extrapolate K. Use the van 't Hoff equation ln(K2/K1) = −(ΔH°/R)(1/T2 − 1/T1): K rises with temperature for endothermic reactions and falls for exothermic ones.

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