Calculadora de Constante de Taxa
- Created by
- Renato Passos, Eng. de Software
- Reviewed by
- Renato Passos, Eng. de Software
Last updated: Apr 18, 2026
Formula
k = ln(C₀/Ct) / t ; t½ = ln2 / k
About this calculator
The Rate Constant Calculator determines the rate constant (k) for first-order reactions using the formula k = ln(C₀/Ct) / t, where C₀ is the initial concentration, Ct is the concentration at time t. It also calculates the half-life (t½ = ln2 / k). Essential for chemical kinetics, it allows predicting how the concentration of a reactant changes over time.
The calculation is based on the integrated rate law for first-order reactions. Simply enter the initial concentration (C₀), the final concentration (Ct), and the elapsed time (t). The tool applies the natural logarithm and returns the value of k. Additionally, it automatically calculates the half-life, which is the time required for the concentration to decrease by half.
Use this calculator in chemistry labs, pharmaceutical industry, or environmental studies. For example, to determine the decomposition constant of a drug, monitor the degradation of pollutants, or study enzymatic reactions. It is useful for any situation where the reaction rate depends linearly on the concentration of a single reactant.
Caution: ensure the reaction is truly first-order. Concentration units must be consistent (e.g., mol/L) and time should be in the same unit (e.g., seconds). The formula assumes first-order kinetics; for other orders, the result will be inaccurate. Always verify that experimental conditions are ideal (constant temperature, no interferences).
Frequently asked questions
Can I use this calculator for second-order reactions?
No, the formula used is specific to first-order reactions. For second-order reactions, a different equation is required.
What does it mean if the k value is negative?
A negative k value indicates that concentration is increasing over time, which is not expected for a forward reaction. Check the entered data.
What units should I use for concentration and time?
Concentration units must be consistent (e.g., mol/L) and time should be in the same unit (e.g., seconds or minutes). The k value will have the inverse unit of time.
How do I interpret the calculated half-life?
The half-life is the time required for the reactant concentration to drop by half. For first-order reactions, it is constant and independent of initial concentration.
Does the calculator work for gas-phase reactions?
Yes, as long as concentrations are expressed in appropriate units (e.g., partial pressure in atm) and the reaction is first-order. Use the ideal gas law if needed.