Reaction Selectivity Calculator
Solve S = Fdesired ÷ Fundesired from product flows, or predict selectivity from rate laws and Arrhenius parameters — with yield & conversion, conversion tables, and live charts.
Product Distribution
Enter values and calculate to see the product split.
Advanced Engineering Results optional
Compares the calculated selectivity with a target you define. Acceptable selectivity depends on your economics, separation cost, and safety constraints — there is no universal pass mark.
Works backwards from your last calculation. Flows: maximum undesired flow / minimum desired flow. Rate laws: the CA that gives the target. Arrhenius: the temperature that gives the target, if one exists in the model.
For A → D and A → U: conversion X = (FA0 − FA)/FA0, selectivity on reactant consumed S(D,A) = a·Fd/(FA0 − FA), and yield Y = a·Fd/FA0 = X·S(D,A). All four amounts must use the same basis (all flows or all amounts). a = mol A consumed per mol D formed.
Compare two operating conditions (for example, before and after a catalyst or temperature change) using their measured product flows.
Conversion Tables
Enter selectivity in any one form and see every equivalent. ΔΔG‡ = RT·ln S is the difference in activation free energy that would produce that rate ratio for competing pathways at temperature T (positive favours the desired product).
| Quantity | Value |
|---|---|
| Enter a value above. | |
Reference: common selectivity levels (ΔΔG‡ at 298.15 K)
| S (D : U) | φ (%) | Undesired (%) | log₁₀ S | ΔΔG‡ (kJ/mol) | ΔΔG‡ (kcal/mol) |
|---|
S is dimensionless, so Fdesired and Fundesired can use any unit as long as both use the same one. Use this converter to bring data from different sources onto a common basis.
| Unit | Equivalent value |
|---|---|
| Enter a value above. | |
Reference factors
| Unit | Type | Multiply by | To get |
|---|
| Energy unit | Value |
|---|---|
| Enter a value. | |
| Temperature scale | Value |
|---|---|
| Enter a value. | |
Reference Tables
| Quantity | Expression | Where it applies |
|---|---|---|
| Overall selectivity | S = Fd / Fu | Exit flows or batch amounts of D and U |
| Instantaneous selectivity | S = rD / rU | Local rates at a given composition and T |
| Power-law parallel | S = (kD/kU)·CA^(αD−αU) | A → D, A → U, rate = k·CA^α |
| Temperature dependence | S = (AD/AU)·exp[−(ED−EU)/RT]·CA^(αD−αU) | Arrhenius rate constants |
| Fractional selectivity | φ = Fd/(Fd+Fu) = S/(1+S) | D and U are the only products |
| Conversion | X = (FA0 − FA)/FA0 | Reactant A |
| Selectivity (reactant basis) | S(D,A) = a·Fd/(FA0 − FA) | a = mol A consumed per mol D |
| Yield | Y = a·Fd/FA0 = X·S(D,A) | Fraction of A fed that became D |
| Energy scale | ΔΔG‡ = RT·ln S | Competing pathways under kinetic control |
| Symbol | Quantity | SI Unit | Common Units | Meaning |
|---|---|---|---|---|
| S | Selectivity | — | D : U | Desired-to-undesired ratio |
| φ | Fractional selectivity | — | % | Share of D among D + U |
| Fd, Fu | Product flow | mol/s | kmol/h, lbmol/h | Molar flow of D or U |
| rD, rU | Reaction rate | mol/(L·s) | mol/(m³·s) | Formation rate of D or U |
| k | Rate constant | varies | varies with order | Sets the rate at unit concentration |
| α | Reaction order | — | — | Exponent on CA |
| CA | Concentration | mol/m³ | mol/L | Reactant concentration |
| A | Pre-exponential factor | same as k | — | Frequency factor |
| E | Activation energy | J/mol | kJ/mol, kcal/mol | Temperature sensitivity of k |
| R | Gas constant | 8.314 J/(mol·K) | — | Universal gas constant |
| T | Temperature | K | °C, °F | Absolute temperature in equations |
| X, Y | Conversion, yield | — | % | Reactant converted, product formed per A fed |
How to Use This Calculator
- Select Basic or Advanced mode, or pick a quick preset.
- In Basic mode, choose whether to solve for S, Fdesired, or Fundesired.
- In Advanced mode, choose a method: product flows, rate laws, or Arrhenius.
- Enter the values and choose a unit for each field.
- Click Calculate.
- Review the primary result, the product-distribution chart, and the sensitivity graph.
- Read the calculation steps to check the arithmetic.
- Use the Advanced Engineering Results tabs for target checks, operating-condition design, yield, and run comparison.
- Use the Conversion Tables to translate selectivity, flow units, energies, and temperatures.
What This Tool Offers
- Three-variable S = Fd / Fu solver
- Power-law rate-law selectivity with reaction orders
- Arrhenius temperature dependence of selectivity
- Fractional selectivity, log₁₀ S, and energy-scale equivalents
- Conversion, yield, and reactant-basis selectivity with balance check
- Target check and reverse design for concentration, temperature, or flows
- Side-by-side comparison of two runs
- Selectivity, flow, energy, and temperature conversion tables
- Product-distribution bar chart and split bar
- Sensitivity graph: S vs Fu, CA, or T with your operating point marked
- Automatically generated step-by-step calculations
- Six ready-made scenario presets
Engineering Theory, Assumptions & Limitations
S = Fdesired / Fundesired is the overall selectivity of a desired product relative to an undesired one. When rates replace flows, it becomes the instantaneous selectivity rD / rU. The calculator's models rest on these assumptions:
Real selectivity also depends on things this tool does not model: series reactions (A → D → U), where selectivity changes with conversion; reactor type, since in a plug-flow or batch reactor the overall selectivity is the integral of the instantaneous selectivity over the changing composition, whereas a CSTR operates at the exit composition; mass- and heat-transfer limits; catalyst deactivation; and reversible steps. Selectivity definitions also vary between textbooks, so check which one your course or plant uses — the Yield & Conversion tab gives the reactant-consumed form.
Common Mistakes
- Mixing flows (mol/s) and batch amounts (mol) in the same ratio
- Swapping desired and undesired, which inverts S
- Confusing selectivity (D/U) with fractional selectivity (D/(D+U)) or with yield
- Ignoring stoichiometry when converting product moles to reactant consumed
- Entering activation energy in kcal/mol while the field is set to kJ/mol
- Using °C in the exponent instead of absolute temperature (K)
- Comparing k values with different units when reaction orders differ
- Applying instantaneous selectivity as if it were the overall selectivity of a PFR or batch reactor
- Assuming higher temperature always improves selectivity — it depends on ED versus EU
References
- Fogler, H.S. — Elements of Chemical Reaction Engineering (multiple reactions, instantaneous and overall selectivity, and yield).
- Levenspiel, O. — Chemical Reaction Engineering (parallel and series reactions, and the effect of concentration and temperature on product distribution).
- Schmidt, L.D. — The Engineering of Chemical Reactions (selectivity in catalytic and multi-reaction systems).
- Atkins, P. & de Paula, J. — Physical Chemistry (Arrhenius equation and activation energy).
This tool is for engineering education and preliminary estimation. Preset values are illustrative, not measured plant data, and results do not replace experimental kinetics or a qualified engineer's design.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| S | Selectivity | |
| F_desired_product | Molar flow of desired product | mol/s |
| F_undesired_product | Molar flow of undesired byproduct | mol/s |
What it means
Selectivity is a measure of how effectively a reaction produces the desired product relative to undesired by‑products. It is defined as the ratio of the molar flow of the desired product to that of the undesired product (or to the total products). In complex reactions (e.g., parallel or consecutive), selectivity is critical because by‑products can reduce yield and increase separation costs. High selectivity is often more important than high conversion for economic viability. Selectivity depends on reaction kinetics, temperature, pressure, catalyst, and residence time. It is used to evaluate catalyst performance and to optimise operating conditions. In the chemical industry, selectivity is a key metric in process design and in assessing process intensification. Understanding selectivity helps engineers design reactors and separation trains that maximise profit and minimise waste.
Worked example
Reaction Selectivity – Two Examples
Real‑World| Parameter | Value |
|---|---|
| F_desired | 8 mol/s |
| F_undesired | 2 mol/s |
| Parameter | Value |
|---|---|
| F_desired | 15 |
| F_undesired | 5 |
Common mistakes
- Desired vs. undesired: F_desired and F_undesired are molar flow rates of the respective products.
- Selectivity > 1: Is possible if desired product is formed in greater amount than undesired.
- Based on molar flows: Ensure both flows are in the same units.
- Selectivity vs. yield: Selectivity compares products; yield compares product to reactant fed – they are different.
- Temperature and catalyst effect: Selectivity often depends strongly on operating conditions.
Applications
Reaction selectivity, S = F_desired_product / F_undesired_product, measures the preferential formation of the desired product over by‑products. High selectivity is critical for economic viability, as it reduces separation costs and waste disposal. Engineers use selectivity to evaluate catalysts, to optimise operating conditions, and to select reactor types. Selectivity can be influenced by temperature, pressure, concentration, and presence of inhibitors. In complex reaction networks, selectivity is a key design parameter. By understanding and optimising selectivity, professionals can maximise the value of the product, minimise environmental impact, and improve process economics. It is an essential metric in both petrochemical and fine chemical industries.
- Selection of catalysts and reaction conditions for desired product
- Process optimisation to minimise by‑product formation
- Reactor design for high‑selectivity applications (e.g., pharmaceutical synthesis)
- Environmental impact reduction by avoiding hazardous by‑products
- Economic evaluation of alternative process routes
Related calculators · Kinetics
Recently updated formulas
Suggest an improvement
Found a mistake? Have a better example? Let us know.
You may send up to 3 suggestions per 24 hours.