function calculateRate() {
var k = parseFloat(document.getElementById('rateConstant').value);
var concA = parseFloat(document.getElementById('concA').value);
var orderA = parseFloat(document.getElementById('orderA').value);
var concB = parseFloat(document.getElementById('concB').value);
var orderB = parseFloat(document.getElementById('orderB').value);
// Validation
if (isNaN(k)) {
document.getElementById('result').style.display = 'block';
document.getElementById('result').innerHTML = "Please enter a valid Rate Constant (k).";
return;
}
if (isNaN(concA) || isNaN(orderA)) {
document.getElementById('result').style.display = 'block';
document.getElementById('result').innerHTML = "Please enter valid Concentration and Order for Reactant A.";
return;
}
// Handle optional Reactant B
// If B is empty, we assume it's zero order or not present (concentration effect = 1)
var termB = 1;
if (!isNaN(concB) && !isNaN(orderB)) {
termB = Math.pow(concB, orderB);
} else if (!isNaN(concB) && isNaN(orderB)) {
// Provided concentration but no order, assume 0 order? Or error?
// Better to assume 0 order if undefined, so effect is 1.
termB = 1;
}
// Calculate term A
var termA = Math.pow(concA, orderA);
// Calculate Rate
var initialRate = k * termA * termB;
// Display Result
var displayRate = initialRate;
// Format for very small or very large numbers
if (initialRate 10000) {
displayRate = initialRate.toExponential(4);
} else {
displayRate = initialRate.toFixed(5);
}
document.getElementById('result').style.display = 'block';
document.getElementById('result').innerHTML = "Initial Rate: " + displayRate + " M/s (mol·L⁻¹·s⁻¹)";
}
How to Calculate the Initial Rate of Reaction
In chemical kinetics, the initial rate is the instantaneous speed of a chemical reaction at the very moment the reactants are mixed (time t=0). Calculating the initial rate is fundamental for determining the rate law, reaction orders, and the rate constant of a chemical process.
Unlike average rates, which change as reactant concentrations decrease over time, the initial rate represents the reaction velocity when concentrations are at their known initial values. This calculator uses the standard Rate Law equation to determine this value.
Rate = k [A]ᵐ [B]ⁿ
Understanding the Variables
Rate (v₀): The speed of the reaction, typically measured in Molarity per second (M/s or mol·L⁻¹·s⁻¹).
Rate Constant (k): A proportionality constant specific to the reaction at a given temperature. Its units vary depending on the overall order of reaction.
[A] & [B]: The initial molar concentrations of the reactants (mol/L).
Exponents (m & n): The reaction orders with respect to each reactant. These must be determined experimentally and are not necessarily equal to the stoichiometric coefficients.
Step-by-Step Calculation Example
Let's calculate the initial rate for a reaction between Nitrogen Dioxide (NO₂) and Carbon Monoxide (CO). Suppose we have the following experimental data:
Identify the Rate Law: Assume the rate law is determined to be second order for NO₂ and zero order for CO. Formula: Rate = k [NO₂]² [CO]⁰
Input the Rate Constant: Let's say k = 0.5 M⁻¹s⁻¹.
Input Concentrations: Initial [NO₂] = 0.2 M. Since CO is zero order, its concentration does not affect the rate mathematically (anything to the power of 0 is 1).
Perform the Math: Rate = 0.5 × (0.2)² × 1
Rate = 0.5 × 0.04
Initial Rate = 0.02 M/s
Why is Initial Rate Important?
Using the "Method of Initial Rates" allows chemists to isolate the effect of concentration on reaction speed without the interference of the reverse reaction or product inhibition. Since there are virtually no products present at t=0, the reverse reaction rate is negligible, making the math significantly cleaner and more accurate for determining reaction mechanisms.
Common Reaction Orders
The impact of changing concentration depends on the order:
Zero Order (0): Changing concentration has no effect on the initial rate.
First Order (1): Doubling concentration doubles the initial rate.
Second Order (2): Doubling concentration quadruples the initial rate.