Calculate the average rate of reaction based on concentration changes.
Reactant (Disappearing)
Product (Appearing)
Change in Concentration (Δ[A]):
Time Interval (Δt):
Average Reaction Rate:
How to Calculate Reaction Rate
In chemical kinetics, the reaction rate is the speed at which a chemical reaction proceeds. It is defined as the change in concentration of a reactant or product per unit of time. Understanding reaction rates is crucial for controlling industrial processes, biological systems, and environmental changes.
The Reaction Rate Formula
The average rate of reaction can be calculated by observing the change in concentration of a substance over a specific time interval. The general formula depends on whether you are measuring the disappearance of a reactant or the appearance of a product.
Mathematically, reaction rates are expressed as positive values.
Reactants: Since reactants are consumed, their concentration decreases over time, resulting in a negative Δ[Concentration]. To make the rate positive, we multiply by -1.
Rate = – ( [A]₂ – [A]₁ ) / ( t₂ – t₁ )
Products: Products are formed, so their concentration increases, resulting in a positive Δ[Concentration].
Rate = ( [P]₂ – [P]₁ ) / ( t₂ – t₁ )
Example Calculation
Imagine a reaction where the concentration of a reactant decreases from 0.50 M to 0.20 M over a period of 60 seconds.
Initial Concentration ([A]₁): 0.50 M
Final Concentration ([A]₂): 0.20 M
Time Interval (Δt): 60 s
Change in Concentration (Δ[A]): 0.20 – 0.50 = -0.30 M
Rate: -(-0.30 M) / 60 s = 0.005 M/s
Factors Affecting Reaction Rate
Several factors influence how fast a reaction occurs:
Concentration: Higher concentrations of reactants lead to more frequent collisions, increasing the rate.
Temperature: Higher temperatures increase the kinetic energy of particles, leading to more forceful and frequent collisions.
Surface Area: For solid reactants, a larger surface area (smaller particles) allows more collisions to occur simultaneously.
Catalysts: Substances that lower the activation energy required for the reaction, speeding it up without being consumed.
Units of Measurement
Reaction rates are typically expressed in units of molarity per second (M/s or mol·L⁻¹·s⁻¹), though for slower reactions, units like M/min or M/hr might be used. Gas phase reactions may use pressure units (atm/s or Pa/s) instead of concentration.
function calculateReactionRate() {
// Get input elements
var initialConcInput = document.getElementById('initialConc');
var finalConcInput = document.getElementById('finalConc');
var initialTimeInput = document.getElementById('initialTime');
var finalTimeInput = document.getElementById('finalTime');
var substanceTypeInput = document.getElementById('substanceType');
var resultBox = document.getElementById('resultBox');
var errorMsg = document.getElementById('errorMsg');
// Parse values
var c1 = parseFloat(initialConcInput.value);
var c2 = parseFloat(finalConcInput.value);
var t1 = parseFloat(initialTimeInput.value);
var t2 = parseFloat(finalTimeInput.value);
var type = substanceTypeInput.value;
// Reset display
errorMsg.style.display = 'none';
resultBox.style.display = 'none';
// Validation
if (isNaN(c1) || isNaN(c2) || isNaN(t1) || isNaN(t2)) {
errorMsg.innerText = "Please enter valid numbers for all fields.";
errorMsg.style.display = 'block';
return;
}
if (t2 <= t1) {
errorMsg.innerText = "Final time must be greater than initial time.";
errorMsg.style.display = 'block';
return;
}
if (c1 < 0 || c2 < 0) {
errorMsg.innerText = "Concentration cannot be negative.";
errorMsg.style.display = 'block';
return;
}
// Calculation
var deltaConc = c2 – c1;
var deltaTime = t2 – t1;
var rate = deltaConc / deltaTime;
// Apply sign convention based on substance type
// If reactant, rate = -d[A]/dt (to make rate positive when conc decreases)
// If product, rate = +d[P]/dt
var finalRate = 0;
if (type === 'reactant') {
// Reactants usually decrease, so deltaConc is negative.
// We invert it to get a positive rate representing speed of disappearance.
// If the user enters data showing an increase for a reactant, the rate will be negative (physically weird but mathematically correct).
finalRate = -rate;
} else {
// Products usually increase, so deltaConc is positive.
finalRate = rate;
}
// Formatting results
document.getElementById('deltaConc').innerText = deltaConc.toFixed(4) + " M";
document.getElementById('deltaTime').innerText = deltaTime.toFixed(2) + " s";
document.getElementById('finalRate').innerText = finalRate.toFixed(5) + " M/s";
// Show result
resultBox.style.display = 'block';
}