Gas Phase Reaction Calculator

Calculator

Enter reaction and conditions

Use coefficients and moles for: aA + bB ⇌ cC + dD

All calculations assume ideal-gas behavior.

Species A label

Example: H2, CO, NH3

Species B label

Product C label

Product D label

Coefficients a, b

a b

Use 0 if a species is absent.

Coefficients c, d

c d

Initial moles n₀(A)

Initial moles n₀(B)

Initial moles n₀(C)

Initial moles n₀(D)

Temperature (K)

Condition mode

Volume mode recalculates pressure using nRT/V.

Total pressure (used in pressure mode)

Volume (L, used in volume mode)

Constant type

Constant value

Use a value at the entered temperature.

Results appear above this form after submit.

Example

Example data table

Sample input/output snapshot for: H₂ + I₂ ⇌ 2HI at 700 K, fixed pressure.

Field	Example value
Coefficients (a,b,c,d)	1, 1, 2, 0
Initial moles (A,B,C,D)	1.00, 1.00, 0.00, 0.00
Temperature	700 K
Total pressure	1.00 bar
Kp	~50 (illustrative)
Key outputs	ξ, equilibrium moles, mole fractions, partial pressures

Note: Example constant is illustrative; use a reliable source for real systems.

Formulas

Formulas used

aA + bB ⇌ cC + dD, extent ξ: nA = nA0 − aξ, nB = nB0 − bξ, nC = nC0 + cξ, nD = nD0 + dξ.
Total moles: nT = nA + nB + nC + nD.
Ideal-gas total pressure (volume mode): P = nT R T / V, with R = 0.08314472 L·bar/(mol·K).
Mole fraction: yᵢ = nᵢ / nT, partial pressure: Pᵢ = yᵢ P.
Reaction quotient (pressure form): Qp = (P_C^c P_D^d)/(P_A^a P_B^b).
Equilibrium condition: Qp = Kp.
Convert constants: Kp = Kc (RT)^{Δn}, where Δn = (c+d) − (a+b).

The calculator solves for ξ numerically (bisection with feasibility bounds) to satisfy the equilibrium condition.

Guide

How to use this calculator

Enter coefficients for A, B, C, and D. Use 0 if absent.
Enter initial moles for each species.
Set temperature and choose pressure or volume mode.
Enter Kp or Kc at that temperature.
Press Calculate to view equilibrium moles and pressures.
Use download buttons to export CSV or PDF reports.

FAQs

Frequently asked questions

1) What does this calculator compute?

It estimates equilibrium composition for a gas reaction using Kp or Kc, then reports extent, mole fractions, partial pressures, and total pressure.

2) Can I use reactions with missing species?

Yes. Set the missing species coefficient to 0 and keep its initial moles at 0. The remaining terms still form a valid equilibrium expression.

3) What is the difference between pressure mode and volume mode?

Pressure mode holds total pressure constant. Volume mode holds container volume constant and updates total pressure with nRT/V as the reaction shifts.

4) How does it convert Kc and Kp?

It applies Kp = Kc(RT)^{Δn}. Δn is products minus reactants stoichiometric sums, using the same coefficients you enter.

5) Why do I get “no feasible extent” errors?

The entered coefficients and initial moles may force negative moles for every possible extent. Recheck signs, coefficients, and starting amounts for each species.

6) Is non-ideal behavior supported?

This tool assumes ideal gases. For high pressures or strong interactions, use fugacities or activity coefficients from an equation of state.

7) Can I trust the result without a real K value?

You need a reliable Kp or Kc at your temperature. The solver is mathematical; accuracy depends on constant quality and whether ideal-gas assumptions apply.