';
return;
}
// Calculation
var deltaC = c2 – c1;
// Rate is typically expressed as a positive value regardless of whether it's consumption or formation
// Rate = |(C2 – C1) / (t2 – t1)|
var rawRate = deltaC / deltaT;
var absoluteRate = Math.abs(rawRate);
// Determine if it is a reactant (consumption) or product (formation)
var reactionType = "";
var reactionColor = "";
if (deltaC 0) {
reactionType = "Product Formation (Concentration Increased)";
reactionColor = "#27ae60"; // Greenish
} else {
reactionType = "Equilibrium (No Net Change)";
reactionColor = "#f39c12"; // Orange
}
resultDiv.style.display = "block";
resultDiv.innerHTML =
'
Reaction Type:' + reactionType + '
' +
'
Change in Concentration (Δ[A]):' + deltaC.toFixed(4) + ' M
' +
'
Change in Time (Δt):' + deltaT.toFixed(2) + ' s
' +
'' +
'
Average Rate:' + absoluteRate.toFixed(6) + ' M/s
' +
'
(moles per liter per second)
';
}
How to Calculate Rate of Reaction with Concentration and Time
Understanding chemical kinetics is essential for controlling industrial processes, analyzing biological systems, and conducting laboratory experiments. The rate of reaction defines how quickly reactants are transformed into products. This guide focuses on calculating the average rate of reaction using the change in concentration over a specific time interval.
The Reaction Rate Formula
The average rate of reaction is calculated by determining the change in concentration of a substance (either a reactant or a product) divided by the time interval over which that change occurred.
The general formula is:
Rate = Δ[Concentration] / ΔTime
Where:
Δ[Concentration] (Delta C) = Final Concentration ($C_2$) – Initial Concentration ($C_1$)
ΔTime (Delta t) = Final Time ($t_2$) – Initial Time ($t_1$)
Units: Typically expressed in Molarity per second (M/s) or mol/(L·s).
Reactants vs. Products
When measuring the rate based on a reactant, the concentration decreases over time. This results in a negative Δ[Concentration]. However, reaction rates are conventionally expressed as positive values. Therefore, when calculating based on reactant disappearance, we often apply a negative sign to the formula:
Rate = – (Δ[Reactant] / Δt)
When measuring based on a product, the concentration increases, resulting in a positive value naturally.
Step-by-Step Calculation Example
Let's look at a realistic scenario in a chemistry lab involving the decomposition of Nitrogen Dioxide ($NO_2$).
Identify Initial Values: At the start of the stopwatch ($t_1 = 0$ s), the concentration of $NO_2$ is $0.0100$ M.
Identify Final Values: After 50 seconds ($t_2 = 50$ s), the concentration drops to $0.0079$ M.
Calculate Δ[Concentration]: $0.0079 – 0.0100 = -0.0021$ M.
This result indicates that, on average, $4.2 \times 10^{-5}$ moles of $NO_2$ are consumed per liter every second during this interval.
Factors Influencing Reaction Rates
While the calculator above helps you quantify the rate, several physical factors determine how fast a reaction actually proceeds:
Concentration: Higher concentrations usually lead to more frequent collisions between molecules, increasing the rate.
Temperature: Increased heat provides kinetic energy to molecules, resulting in harder and more frequent collisions.
Surface Area: For solid reactants, greater surface area (e.g., powder vs. block) increases the reaction rate.
Catalysts: Substances that lower the activation energy required for the reaction to occur, speeding up the process without being consumed.
Why Use Molarity (M)?
In chemical kinetics, concentration is almost always measured in Molarity (M), which stands for moles of solute per liter of solution (mol/L). This standardizes calculations allowing chemists to compare the kinetics of different reactions regardless of the total volume of the solution used.