Calculate reaction order and rate constant from experimental table data.
Experiment 1 (Reference Row)
Experiment 2 (Comparison Row)
Used to determine Reaction Order. Concentration must differ from Exp 1.
If you already know the reaction order, enter it here to skip calculation based on Exp 2.
Reaction Order (m)
—
Rate Constant (k)
—
Rate Law Equation
Rate = k[A]m
How to Calculate Rate Constant from a Table
In chemical kinetics, calculating the rate constant ($k$) usually involves analyzing experimental data provided in a table. This data typically lists the initial concentrations of reactants and the observed initial rates for several experiments. The process involves two main steps: determining the reaction order and then solving for $k$.
Step 1: Determine the Reaction Order
To find the order of the reaction with respect to a reactant (let's call it A), you must compare two experiments where the concentration of A changes while other conditions remain constant. The relationship is governed by the rate law:
Rate = k[A]m
By taking the ratio of the rate laws for two experiments (Experiment 1 and Experiment 2), the rate constant $k$ cancels out, allowing you to solve for the order $m$:
(Rate2 / Rate1) = ([A]2 / [A]1)m
Mathematically, you can solve for $m$ using logarithms:
m = log(Rate2 / Rate1) / log([A]2 / [A]1)
Step 2: Calculate the Rate Constant (k)
Once the reaction order ($m$) is known, you can calculate the rate constant by rearranging the rate law equation and plugging in the data from any single experiment (usually Experiment 1):
k = Rate / [A]m
Understanding Rate Constant Units
The units of $k$ depend entirely on the overall order of the reaction. This is a common point of confusion for chemistry students. Since Rate is always measured in M/s (Molarity per second) and Concentration in M (Molarity), the units for $k$ adjust to make the equation balance.
Reaction Order
Units of k
Zero Order (m=0)
M · s-1 (M/s)
First Order (m=1)
s-1 (1/s)
Second Order (m=2)
M-1 · s-1 (1/(M·s))
Third Order (m=3)
M-2 · s-1
Example Calculation
Consider the data below:
Exp 1: [A] = 0.10 M, Rate = 0.005 M/s
Exp 2: [A] = 0.20 M, Rate = 0.020 M/s
1. Find Order: Concentration doubles (0.1 to 0.2). Rate quadruples (0.005 to 0.020). Since $2^2 = 4$, the order is 2.
function calculateRateConstant() {
// Get Input Values
var c1 = parseFloat(document.getElementById('conc1').value);
var r1 = parseFloat(document.getElementById('rate1').value);
var c2 = parseFloat(document.getElementById('conc2').value);
var r2 = parseFloat(document.getElementById('rate2').value);
var manualOrder = parseFloat(document.getElementById('manualOrder').value);
var resultBox = document.getElementById('results');
var orderDisplay = document.getElementById('orderResult');
var kDisplay = document.getElementById('kResult');
var kUnitDisplay = document.getElementById('kUnit');
var rateLawDisplay = document.getElementById('rateLawDisplay');
// Validation: Need at least Exp 1
if (isNaN(c1) || isNaN(r1) || c1 <= 0 || r1 <= 0) {
alert("Please enter valid positive numbers for Experiment 1 Concentration and Rate.");
return;
}
var order = 0;
var calculatedOrder = false;
// Logic: Determine Order
if (!isNaN(manualOrder)) {
// User provided order manually
order = manualOrder;
} else {
// Calculate order from Exp 1 and Exp 2
if (isNaN(c2) || isNaN(r2) || c2 <= 0 || r2 2)
// However, keep precision if it's actually fractional (like 1.5)
var displayOrder = Math.round(order * 100) / 100;
// If it's very close to an integer, snap it for calculation purposes
// (common in chemistry problems)
if (Math.abs(displayOrder – Math.round(displayOrder)) < 0.05) {
displayOrder = Math.round(displayOrder);
order = displayOrder; // use the integer for k calculation
}
// Logic: Calculate k using Exp 1
// k = Rate / [A]^m
var k = r1 / Math.pow(c1, order);
// Determine Units
// Unit generic form: M^(1-m) * s^-1
var unitString = "";
if (displayOrder === 0) {
unitString = "M · s⁻¹";
} else if (displayOrder === 1) {
unitString = "s⁻¹";
} else if (displayOrder === 2) {
unitString = "M⁻¹ · s⁻¹";
} else if (displayOrder === 3) {
unitString = "M⁻² · s⁻¹";
} else {
var power = 1 – displayOrder;
unitString = "M^(" + power + ") · s⁻¹";
}
// Display Results
resultBox.style.display = "block";
if (calculatedOrder) {
orderDisplay.innerHTML = displayOrder + " (Calculated)";
} else {
orderDisplay.innerHTML = displayOrder + " (Manual Input)";
}
// Format k to scientific notation if very small or very large, else fixed
if (k 10000) {
kDisplay.innerText = k.toExponential(4);
} else {
kDisplay.innerText = k.toPrecision(5);
}
kUnitDisplay.innerText = unitString;
rateLawDisplay.innerHTML = "Rate = " + (k 10000 ? k.toExponential(2) : k.toPrecision(3)) + " [A]" + displayOrder + "";
}