function calculateOrder() {
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 errorDiv = document.getElementById("error-message");
var resultDiv = document.getElementById("result-box");
// Reset display
errorDiv.style.display = "none";
resultDiv.style.display = "none";
// Validation
if (isNaN(c1) || isNaN(r1) || isNaN(c2) || isNaN(r2)) {
errorDiv.innerHTML = "Please enter valid numerical values for all fields.";
errorDiv.style.display = "block";
return;
}
if (c1 <= 0 || c2 <= 0 || r1 <= 0 || r2 <= 0) {
errorDiv.innerHTML = "Concentration and Rate must be positive values.";
errorDiv.style.display = "block";
return;
}
if (c1 === c2) {
errorDiv.innerHTML = "Concentrations for Experiment 1 and 2 must differ to calculate the order.";
errorDiv.style.display = "block";
return;
}
// Calculation: n = log(rate2/rate1) / log(conc2/conc1)
var rateRatio = r2 / r1;
var concRatio = c2 / c1;
var n = Math.log(rateRatio) / Math.log(concRatio);
// Rounding for display
var nRounded = n.toFixed(3);
var nInt = Math.round(n);
// Calculate Rate Constant k = Rate / [A]^n
// Using the rounded integer order for k calculation usually makes more sense physically,
// but we will use the calculated float for precision.
var k = r1 / Math.pow(c1, n);
// Determine units of k based on order (M^(1-n) s^-1)
var kUnits = "";
if (nInt === 0) kUnits = "mol L⁻¹ s⁻¹";
else if (nInt === 1) kUnits = "s⁻¹";
else if (nInt === 2) kUnits = "L mol⁻¹ s⁻¹";
else kUnits = "(units vary)";
// Update DOM
document.getElementById("order-result").innerText = nRounded;
document.getElementById("integer-order").innerText = nInt;
document.getElementById("k-constant").innerText = k.toExponential(3) + " " + kUnits;
document.getElementById("rate-law").innerText = "Rate = " + k.toExponential(2) + " [A]^" + nInt;
resultDiv.style.display = "block";
}
How to Calculate Order of Reaction from Concentration and Rate
Understanding chemical kinetics is fundamental to predicting how fast a reaction will occur under specific conditions. The Order of Reaction describes the relationship between the concentration of reactants and the actual rate of the reaction. This calculator uses the "Method of Initial Rates" to determine the reaction order with respect to a specific reactant.
The Rate Law Equation
For a general reaction involving reactant A, the differential rate law is expressed as:
Rate = k [A]n
Where:
Rate is the speed of the reaction (usually in M/s or mol L⁻¹ s⁻¹).
k is the rate constant, which depends on temperature.
[A] is the molar concentration of the reactant.
n is the reaction order.
Understanding Reaction Orders
The variable n (order) determines how sensitive the rate is to changes in concentration:
Zero Order (n=0): The rate is constant and independent of the concentration of A. Doubling [A] has no effect on the rate.
First Order (n=1): The rate is directly proportional to the concentration. Doubling [A] doubles the rate.
Second Order (n=2): The rate is proportional to the square of the concentration. Doubling [A] quadruples the rate ($2^2 = 4$).
The Calculation Method
To find the order n experimentally, chemists usually run two experiments at the same temperature but with different initial concentrations of the reactant.
By comparing the ratios of the rates and the concentrations, we can isolate n using the following derivation:
(Rate₂ / Rate₁) = ([A]₂ / [A]₁)n
To solve for the exponent n, we take the natural logarithm (ln) or log base 10 of both sides:
n = log(Rate₂ / Rate₁) / log([A]₂ / [A]₁)
Example Calculation
Let's assume we have the following experimental data:
Experiment 1: [A] = 0.10 M, Rate = 0.005 M/s
Experiment 2: [A] = 0.20 M, Rate = 0.020 M/s
Step 1: Calculate the ratios.
Concentration Ratio = 0.20 / 0.10 = 2
Rate Ratio = 0.020 / 0.005 = 4
Step 2: Apply the log formula.
We are looking for n where $2^n = 4$.
Using logs: $n = \log(4) / \log(2)$
$n = 0.602 / 0.301$ n = 2