Core Concepts – Kinetics and Rate of Reaction
In this article, we will learn about chemical kinetics and rate of reactions, including what properties affect them, conditions needed for reactions to occur, and how to calculate reaction rates.
Topics Covered in Other Articles
- Collision Theory
- Reaction Rates and How To Determine Rate Laws
- Activation Energy
- Understanding Kinetic Molecular Theory
- Solving Stoichiometric Problems
- Introduction to Energy
What is the Rate of a Reaction?
The rate of a reaction measures the speed at which a chemical reaction takes place. This is done by dividing the change in concentration of reactants by the change in time. So it measures either the rate of disappearance of the reactants or the rate of appearance of the products. To calculate the rate let’s first look at the general formula:
A few notes:
- The rate for reactants A and B are negative because they are being consumed. This means that C and D are positive for the opposite reason—they are being produced.
- The change in concentration over the change in time for each species (reactants and products) is divided by its stoichiometric equivalent.
- The concentration of each species is measured in molarity as denoted by the brackets [ ].
What is Kinetics?
The general definition of kinetics is the study of the effects of forces on mechanisms. In terms of chemistry (chemical kinetics), it is the study of rates of reaction. Using our knowledge of reaction rates now, we can say it is the study of how fast chemical reactions go from reactants to products or how fast reactants are consumed and products are produced. A useful measure in studying the kinetics of a reaction is kinetic energy, the energy a particle has due to its motion.
What is Collision Theory?
For us to discuss reaction rates and kinetics more in depth, we must first understand collision theory. Reacting particles must collide for a reaction to occur, but not every collision results in a reaction. Collision theory outlines the necessary conditions for a reaction to take place. There are three conditions that satisfy collision theory:
- Particles must collide
- Collisions must have sufficient energy
- Collisions must have proper orientation
Particles Must Collide
The first condition of collision theory—particles must collide—is the basis for reactions. Without colliding, particles do not interact, and therefore reactions do not occur.
Collisions Must Have Sufficient Energy
As stated before, the kinetic energy of a particle is the energy due to its motion. So a particle’s kinetic energy is proportional to its speed. The kinetic energy of reacting particles is important because reactions have specific activation energies, aka the energy needed for a reaction to occur (like a threshold). The kinetic energy of colliding particles must match the activation energy for the reaction to occur.
Collisions Must Have Proper Orientation
If particles collide and with sufficient energy, a reaction still isn’t guaranteed to occur. The direction at which the reactants collide will determine if the reaction occurs. Even further, a reaction may occur but the orientation of the collision can determine what products are formed. This is because different individual atoms will interact depending on which angle two molecules collide.
Effects on Kinetics and the Rate of Reactions
Knowing this about collision theory, we can now discuss properties that affect reaction rates. There are four properties than can affect the rate of a reaction:
Concentration and reaction rate are positively related, meaning that if the concentration of particles increases, the rate of the reaction will increase. The opposite is also true. Using collision theory we can explain this. The higher concentration of particles increases the amount of collisions and therefore increases the reaction rate.
Temperature and reaction rate are also positively related. This is because temperature is a measure of the average kinetic energy of particles. The higher the temperature, the higher the kinetic energy the particles have. This not only increases the amount of collisions (because particles are moving faster) but it also increases the likelihood that collisions are occurring at sufficient energy which will increase the reaction rate.
The physical state of a reactant can affect the rate of a reaction because of surface area. The particles in solids can not move freely like that of liquids and gasses which lowers the reactants surface area. The smaller the surface area, the less opportunity for reacting particle interaction which will slow the reaction rate. Conversely, gas particles move most freely opposed to other physical states which can also slow reaction rates. Unlike the other physical states, gas particles fill up the space of their containers, but imagine we are comparing the same amount of particles for each physical state. In the gaseous state there would be the same amount of particles taking up more space. This can make collisions more of a chance occurrence and slow the reaction rate.
Presence of a Catalyst or Inhibitor
Catalysts increase reaction rates while inhibitors decrease reaction rates. Catalysts lower the activation energy of a reaction which allows more collisions to have sufficient energy for a reaction to occur. Inhibitors act oppositely. They force a reaction to take a path with a higher activation energy. So, less collisions meet the sufficient energy and the rate of the reaction slows.
Chemical Kinetics and Rate of Reaction Practice Problems
Consider the combustion of butadiene:
How does the rate of butadiene consumption relate to the production of CO2? By what factor is butadiene consumption faster or slower than CO2 production?
Between the four properties affecting reaction rate (reactant concentration, temperature, reactant physical state, and use of catalyst/inhibitor), which properties increase or decrease the proportion of a given amount of reactant capable of performing the reactant?
Kinetics and Rate of Reaction Practice Problem Solutions
1: CO2 production is four times as fast as butadiene consumption.
2: Temperature (increases or decreases proportion of reactants above activation energy) and Physical State (increases or decreases proportion of available reactant based on surface area)