Overview
Two separate questions determine what a chemical reaction actually does: how fast does it happen, and does it happen at all. The first is kinetics โ rate, activation energy, half-life โ and the second is thermodynamics โ entropy and Gibbs free energy. This guide covers both, since a complete picture of any reaction needs answers to both questions.
Work through rate and activation energy first, then temperature sensitivity and half-life, then the thermodynamic questions that determine favorability independent of speed.
Step 1: Calculate Rate Constant and Activation Energy
Reaction rate depends on reactant concentration through the rate constant (k), a fixed value for a given reaction at a given temperature. Activation energy is the minimum energy threshold reactant molecules need to collide successfully โ reactions with high activation energy proceed slowly at room temperature without a catalyst to lower that threshold.
The Rate Constant Calculator solves for k from rate and concentration data, and the Activation Energy Calculator calculates the energy threshold from rate measurements at different temperatures.
Step 2: Apply the Arrhenius Equation and Temperature Sensitivity
The Arrhenius equation connects rate constant to temperature exponentially, which is why reaction rate is so sensitive to even modest temperature changes โ the rough rule that rate doubles every 10ยฐC (a Q10 of about 2) is a useful approximation, but it varies meaningfully by reaction.
The Arrhenius Equation Calculator calculates rate constant, activation energy, or temperature dependence directly, and the Q10 Calculator calculates the actual temperature coefficient for a specific reaction rather than assuming the rough approximation.
Step 3: Calculate Half-Life
For first-order reactions, half-life โ the time for reactant concentration to drop by half โ is directly related to rate constant (tยฝ = 0.693/k), so a faster reaction (larger k) has a shorter half-life. This same math applies beyond chemistry, to radioactive decay and pharmacokinetic drug elimination.
The Half-Life Calculator converts between half-life and rate constant for any first-order process.
Step 4: Check Thermodynamic Favorability
Rate and half-life describe how fast a reaction proceeds, but they say nothing about whether the reaction is thermodynamically favorable in the first place โ that's determined by entropy change (disorder) and Gibbs free energy (the combined effect of enthalpy and entropy at a given temperature).
The Entropy Calculator calculates entropy change from reactant and product values, and the Gibbs Free Energy Calculator combines entropy with enthalpy to determine whether a reaction is spontaneous โ independent of how fast it proceeds, and unaffected by any catalyst.
Key Terms
- Rate constant (k) โ a fixed value relating reaction rate to reactant concentration for a specific reaction at a specific temperature
- Activation energy โ the minimum energy threshold reactant molecules must reach in a collision for a reaction to occur
- Arrhenius equation โ the formula k = Ae^(-Ea/RT), describing how rate constant increases exponentially with temperature
- Half-life โ the time required for a reactant's concentration to decrease by half in a first-order process
- Entropy โ a measure of disorder or randomness in a system, which factors into reaction spontaneity
- Gibbs free energy โ a thermodynamic quantity combining enthalpy and entropy that determines whether a reaction is spontaneous at a given temperature
- Catalyst โ a substance that lowers a reaction's activation energy without being consumed, speeding up the rate without changing its thermodynamic favorability