Chemical Kinetics - Summarising the 3 rates of reaction
The rate of a chemical reaction measures how quickly the reactants are transformed into products
There are three different ways to express the rate of a reaction
Let’s summarize these three rates of reaction in this lecture
Average Rate of Reaction
The average rate of reaction is calculated by dividing the change in concentration of a reactant or product by the change in time
It gives the overall rate of reaction over a specific time interval
It is represented by the equation:
Average Rate = (Change in concentration of reactant or product) / (Change in time)
For example, if the concentration of reactant A decreases by 0.1 M over 10 seconds, the average rate of reaction would be 0.01 M/s
Instantaneous Rate of Reaction
The instantaneous rate of reaction is the rate of reaction at a specific point in time
It is determined by calculating the gradient of the tangent to the concentration-time graph at that point
It gives the rate of reaction at a particular moment
The equation for instantaneous rate of reaction can be written as:
Instantaneous Rate = -d[A] / dt = d[B] / dt
(where [A] is the concentration of reactant A, [B] is the concentration of reactant B, and dt is the change in time)
Initial Rate of Reaction
The initial rate of reaction is the rate of reaction at the start of the reaction
It is determined by measuring the change in concentration of a reactant or product over a very short time interval
It provides information about the rate at which the reaction begins
The equation for initial rate of reaction can be written as:
Initial Rate = -d[A] / dt = d[B] / dt
(where [A] is the concentration of reactant A, [B] is the concentration of reactant B, and dt is the time interval)
Example: Determining Rates of Reaction
Let’s consider the reaction: A + B → C
We can measure the changes in concentration of A, B, and C over different time intervals to determine the rates of reaction
By dividing the change in concentration by the corresponding time interval, we can calculate the average, instantaneous, and initial rates of reaction for each reactant or product
Example Calculation for Reactant A:
Time interval 1: Change in [A] = 0.1 M, Change in time = 10 s
Average Rate = (0.1 M) / (10 s) = 0.01 M/s
Instantaneous Rate at t = 5 s = -d[A] / dt = 0.05 M/s
Initial Rate = Instantaneous Rate = 0.05 M/s
(Similar calculations can be done for reactant B and product C)
Factors Affecting Rate of Reaction
There are several factors that can affect the rate of a chemical reaction:
Concentration of reactants: Increasing reactant concentration generally increases the rate of reaction
Temperature: Higher temperatures usually result in faster reaction rates
Surface area: Increasing the surface area of reactants can accelerate the reaction
Catalysts: Certain substances (catalysts) can speed up the rate of reaction without being consumed in the process
These factors influence the collision frequency and energy collisions, both of which affect the rate of reaction
Collision Theory
The collision theory explains how chemical reactions occur
According to this theory, for a reaction to occur, particles must collide with sufficient energy and proper orientation
The rate of reaction depends on the frequency of effective collisions
Effective collisions have sufficient energy and correct orientation to break bonds and form new ones
Activation energy is the minimum energy required for a reaction to occur
Only particles with energy greater than or equal to the activation energy can undergo a successful collision
Relationship between Rate and Concentration
In general, the rate of a reaction is directly proportional to the concentration of the reactants
The rate equation expresses this relationship mathematically
For a reaction: A + B → C, the rate equation can be written as:
Rate = k[A]^m[B]^n
(where k is the rate constant, [A] and [B] are the concentrations of reactants A and B respectively, and m and n are the reaction orders)
The coefficients m and n are determined experimentally and can be different from the stoichiometric coefficients
Reaction Orders
The reaction order determines how the concentration of a reactant affects the rate of reaction
It is determined experimentally and can be zero, positive, or negative
Zero Order:
The rate of reaction is independent of the concentration of the reactant
The rate equation is: Rate = k[A]^0 = k (constant)
Example: Decomposition of dinitrogen pentoxide
First Order:
The rate of reaction is directly proportional to the concentration of the reactant
The rate equation is: Rate = k[A]^1 = k[A]
Example: A → B
Second Order:
The rate of reaction is proportional to the square of the concentration of the reactant
The rate equation is: Rate = k[A]^2
Example: 2A → B
(Similar orders can be observed for reactions involving multiple reactants)
Summary
The average rate of reaction measures the overall change in concentration over a specific time interval.
The instantaneous rate of reaction is the rate at a specific point in time, determined by the gradient of the concentration-time graph.
The initial rate of reaction is the rate at the start of the reaction, measured over a short time interval.
Factors affecting the rate of reaction include reactant concentration, temperature, surface area, and catalysts.
The collision theory explains how reactions occur, emphasizing the importance of effective collisions.
The rate equation relates the rate of reaction to the concentrations of reactants, with reaction orders determining their impact on rate.
Relationship between Concentration and Rate
The rate of a chemical reaction is directly proportional to the concentration of the reactants
This relationship can be expressed using the rate law equation: Rate = k[A]^m[B]^n
The rate constant, k, depends on the temperature and the specific reaction
The exponents, m and n, are the reaction orders and can be determined experimentally
Example: Reaction Order Calculation
Consider the reaction: 2A + B → C
Experimental data gives the following initial rates at different concentrations:
[A]0 = 0.1 M, [B]0 = 0.2 M, Rate1 = 0.05 M/s
[A]0 = 0.2 M, [B]0 = 0.2 M, Rate2 = 0.2 M/s
Let’s calculate the reaction orders (m and n) using these data
Chemical Kinetics - Summarising the 3 rates of reaction The rate of a chemical reaction measures how quickly the reactants are transformed into products There are three different ways to express the rate of a reaction Let’s summarize these three rates of reaction in this lecture