Calculate the average rate of appearance or disappearance.
Reactant (Disappearance)
Product (Appearance)
Use 1 if calculating rate of disappearance/appearance specifically.
Average Rate of Reaction
0.000 M/s
Rate = -Δ[A] / (n × Δt)
How to Calculate Rate of Reaction Equation
The rate of reaction is a measure of how quickly reactants are converted into products in a chemical reaction. It provides insight into the kinetics of the chemical process. The rate is generally expressed as the change in concentration (molarity) of a reactant or product per unit of time.
The Rate Equation Formula
The general formula for the average rate of reaction depends on whether you are measuring the disappearance of a reactant or the appearance of a product. To make the rate of reaction a positive value regardless of the substance measured, we use the following standard equation:
Δ[A]: Change in concentration of substance A (Final – Initial).
Δt: Change in time (Time Elapsed).
a, b, c, d: The stoichiometric coefficients from the balanced chemical equation.
Negative Sign: Used for reactants because their concentration decreases over time (making the delta negative), ensuring the final rate is positive.
Understanding the Variables
To use the calculator above effectively, you need three key pieces of data:
Concentration Change (Δ[C]): This is the difference between the final concentration and the initial concentration. If you are measuring a gas, this could also be change in volume or pressure. If measuring a solid, it could be change in mass.
Time Interval (Δt): The duration over which the change occurred. The standard unit is seconds (s), but minutes or hours can be used for slower reactions.
Stoichiometry (n): If you want the "Rate of Reaction" for the whole system, divide by the coefficient. If you only want the "Rate of Disappearance" of a specific molecule, use 1 as the coefficient.
Example Calculation
Let's say we have the decomposition of Nitrogen Dioxide: 2 NO₂ → 2 NO + O₂.
If the concentration of NO₂ drops from 0.0100 M to 0.0065 M over a period of 100 seconds, how do we calculate the rate?
Step 2: Apply the formula for reactants (Rate = -1/a × Δ[A]/Δt).
Rate = -1/2 × (-3.5 × 10⁻⁵) = 1.75 × 10⁻⁵ M/s.
Factors Affecting Rate of Reaction
While the equation calculates the speed, several physical factors influence this value:
Concentration: Higher concentrations usually lead to more frequent collisions.
Temperature: Higher temperatures increase kinetic energy, leading to more successful collisions.
Surface Area: For solids, a larger surface area (powder vs chunk) increases the reaction rate.
Catalysts: These lower the activation energy required for the reaction to proceed.
function updateLabels() {
var type = document.getElementById('substanceType').value;
var lblInit = document.getElementById('lblInit');
var lblFinal = document.getElementById('lblFinal');
if (type === 'reactant') {
lblInit.innerHTML = "Initial Concentration (mol/L) – [Start]";
lblFinal.innerHTML = "Final Concentration (mol/L) – [End]";
} else {
lblInit.innerHTML = "Initial Concentration (mol/L) – [Start]";
lblFinal.innerHTML = "Final Concentration (mol/L) – [End]";
}
}
function calculateRate() {
// 1. Get input values
var initVal = document.getElementById('initialValue').value;
var finalVal = document.getElementById('finalValue').value;
var timeVal = document.getElementById('timeElapsed').value;
var coeffVal = document.getElementById('coeff').value;
var type = document.getElementById('substanceType').value;
// 2. Validate inputs
if (initVal === "" || finalVal === "" || timeVal === "" || coeffVal === "") {
alert("Please fill in all fields correctly.");
return;
}
var c1 = parseFloat(initVal);
var c2 = parseFloat(finalVal);
var t = parseFloat(timeVal);
var n = parseFloat(coeffVal);
if (t <= 0) {
alert("Time elapsed must be greater than zero.");
return;
}
if (n <= 0) {
alert("Stoichiometric coefficient must be greater than zero.");
return;
}
// 3. Calculation Logic
// Calculate Delta (Final – Initial)
var delta = c2 – c1;
// Rate formula: Rate = (1/n) * (Delta / t)
// If reactant, Delta is usually negative, so we multiply by -1 to get positive rate
// If product, Delta is positive.
var rawRate = delta / t;
var reactionRate = 0;
// Logic check: Reactants decrease, Products increase
if (type === 'reactant') {
// Reaction Rate = – (1/n) * (d[A]/dt)
reactionRate = -1 * (1/n) * rawRate;
} else {
// Reaction Rate = (1/n) * (d[P]/dt)
reactionRate = (1/n) * rawRate;
}
// Handle cases where user input might physically imply wrong direction but we want magnitude
// E.g. User puts Reactant but Initial < Final (physically impossible for simple reaction, but let's handle math)
// Usually we just show the magnitude if the context is purely kinetic speed.
// However, let's strictly follow the sign convention logic but display absolute rate as the final result usually expected.
// Note: Ideally, rate of reaction is always positive.
// If the calculation results in negative (e.g., user selected 'Product' but concentration went down),
// we should flag it or just show the value. Let's fix to absolute for general "Speed" context but warn.
var displayRate = reactionRate;
var note = "";
if (reactionRate < 0) {
note = "Note: The concentration change is opposite to the expected direction for the selected substance type.";
displayRate = Math.abs(reactionRate); // Rate is a scalar speed
}
// 4. Display Results
var resultBox = document.getElementById('resultBox');
var resultDisplay = document.getElementById('resultDisplay');
var formulaDisplay = document.getElementById('formulaUsed');
var explanation = document.getElementById('explanation');
resultBox.style.display = "block";
// Format to scientific notation if very small, or fixed decimal if normal range
if (displayRate 1000) {
resultDisplay.innerHTML = displayRate.toExponential(4) + " M/s";
} else {
resultDisplay.innerHTML = displayRate.toFixed(6) + " M/s";
}
explanation.innerHTML = note;
// Update formula display based on selection
var sign = (type === 'reactant') ? "-" : "+";
formulaDisplay.innerHTML = "Rate = " + sign + "(1/" + n + ") × (" + (c2 – c1).toExponential(2) + " M / " + t + " s)";
}