Calculate the average rate of reaction based on concentration change over time.
What is Reaction Rate?
The reaction rate is the speed at which a chemical reaction proceeds. It is typically expressed as the change in the concentration of a reactant or product over a specific period of time. In most cases, we measure how many moles per liter (Molarity) change per second (mol/L·s).
Rate = |Δ[Concentration]| / (Coefficient × ΔTime)
Understanding the Calculation
When calculating the rate of reaction, it is important to consider the stoichiometry of the balanced chemical equation. If you are tracking a reactant, the concentration decreases; if you are tracking a product, it increases. The rate is always expressed as a positive value.
Initial Concentration: The molarity (M) at the start of the timing (t = 0).
Final Concentration: The molarity (M) at the end of the observed interval.
Time Elapsed: The duration (Δt) between the two measurements.
Stoichiometric Coefficient: The number in front of the chemical formula in a balanced equation.
Reaction Rate Calculation Examples
Scenario
Δ Concentration
Time
Result (Rate)
Decomposition of H₂O₂
0.50 M to 0.25 M
10 s
0.025 mol/L·s
Formation of NO₂
0.00 M to 1.20 M
60 s
0.020 mol/L·s
Reaction of Mg with HCl
2.00 M to 1.50 M
5 s
0.100 mol/L·s
Factors Influencing Reaction Rates
Several factors can increase or decrease the speed of a chemical reaction:
Concentration: Higher concentration leads to more frequent molecular collisions.
Temperature: Increasing heat gives particles more kinetic energy, leading to more effective collisions.
Surface Area: For solids, a finer powder reacts faster than a large chunk.
Catalysts: These substances lower the activation energy without being consumed in the reaction.
function calculateReactionRate() {
var c1 = parseFloat(document.getElementById("initialConc").value);
var c2 = parseFloat(document.getElementById("finalConc").value);
var time = parseFloat(document.getElementById("timeElapsed").value);
var coeff = parseFloat(document.getElementById("stoichCoeff").value);
var resultDiv = document.getElementById("rrResult");
var valueDiv = document.getElementById("rrValue");
var interpDiv = document.getElementById("rrInterpretation");
// Validation
if (isNaN(c1) || isNaN(c2) || isNaN(time) || isNaN(coeff)) {
alert("Please enter valid numerical values for all fields.");
return;
}
if (time <= 0) {
alert("Time elapsed must be greater than zero.");
return;
}
if (coeff <= 0) {
alert("Stoichiometric coefficient must be greater than zero.");
return;
}
// Calculation: Rate = |(C2 – C1)| / (coeff * time)
var deltaC = Math.abs(c2 – c1);
var rate = deltaC / (coeff * time);
// Formatting the result
var displayRate = rate.toFixed(6);
if (rate < 0.0001) {
displayRate = rate.toExponential(4);
}
resultDiv.style.display = "block";
valueDiv.innerHTML = "Average Rate: " + displayRate + " mol/L·s";
// Contextual message
if (c2 c1) {
interpDiv.innerHTML = "Calculation based on the formation of a product.";
} else {
interpDiv.innerHTML = "The concentration did not change; the rate is zero.";
}
}