Collision theory and rates of reaction Flashcards
What is the collision theory?
- In order for a chemical reaction to occur between two or more particles, they must collide
- Not all collisions result in a chemical reaction
What two factors must exist for a collision to result in a chemical reaction?
- Reactant particles must collide with the correct orientation, must collide head on
- Reactant particles must collide with sufficient energy (activation energy, Ea)
What happens when the factors are not met?
- The particles will bounce apart without reacting, known as unsuccessful collision
- Collisions can also be unsuccessful when the particles do not have equal or more than the activation energy needed in the reaction
What is activation energy (Ea) and why is it needed?
- The minimum amount of energy that colliding particles must have for a chemical reaction to occur
- This is needed to overcome the energy barrier between the reactants and products
- Ea does not change with temperature
How do you calculate the activation energy of a energy level profile? How does the activation energy vary between reactions?
- The difference in energy between the reactant and the transition state (peak in the profile)
- In faster reactions, there is a lower activation energy, in slower reactions there is a higher activation energy
What is the transition state?
- The highest energy state on a reaction coordinate, the point at which new bonds are formed at the same time as old bonds are broken
What is the Maxwell-Boltzmann distribution?
- In an ideal gas, the kinetic energy of the molecule is spread over a range of values (many different kinetic energies)
- The total area beneath the curve is equal to the total number of particles in the sample
- The particles in a certain region below the curve have a wide range of kinetic energies, the most probable value is where the curve is highest
- Essentially, at the same temperature, all gases have the same average kinetic energy
What is the trend with temperature of the Maxwell-Boltzmann distribution?
- Temperature increase –> particles gain kinetic energy –> curve flattens out
- Area under each curve is the same
- Most probable value of kinetic energy is the highest point
How is the Maxwell-Boltzmann useful with catalysts?
- It helps understand how changes in temperature and a catalyst will affect the rate of a reaction
How does the Maxwell-Boltzmann distribution show gases in relation to the molecular speed?
- Comparing different types of gases, at the same temperature, lighter gaseous molecules, on average move faster than heavier gaseous molecules
- The molecular speed depends on the molar mass
What is the rate of reaction?
- A measure of the speed of a reaction
- The change in concentration of a reactant or product per unit of time
How do you calculate the rate of reaction using the decrease in concentration of the reactant?
Rate = decrease in reactant concentration / change in time
- Is always positive
How do you calculate the rate of reaction using the increase in concentration of the product?
Rate = increase in product concentration / change in time
What is the instantaneous rate of reaction?
- The rate of reaction at any particular point in time
How is the instantaneous rate of reaction determined?
- Seen graphically from a change in reactant/product concentration against time
- On a graph the instantaneous rate at any given time is equal to the gradient of the line at that point in time
- Drawing a tangent to the curve at that time and calculating the gradient = ΔY/ΔX
- The steeper the gradient the faster the rate of reaction
How is the average rate of reaction calculated?
- Dividing the change in concentration of a reactant/product by the time taken for the change to occur
- It is the rate of reaction averaged over a certain time period (straight line on the graph), same gradient calculation
What factors can affect the rates of reaction?
- Temperature
- Concentration
- Pressure
- Surface area
How does temperature affect the rate of reaction?
- Increased temperature makes particles gain kinetic energy, they have a higher velocity
- Frequency of collisions increases
- A greater proportion of reactant particles will collide with energy equal or greater than the Ea
How is the affect of increased temperature represented on a Maxwell-Boltzmann distribution?
- Using a Maxwell-Boltzmann distribution, only the particles in the area underneath the flat part of the curve, have sufficient energy to react
- Increased temperature causes the peak of distribution curve shifts to the right, meaning at higher temperatures, the average kinetic energy increases
- Also the curve flattens so the total area under the curve remains constant
- The area under the curve increases to the right of the activation energy value
How does concentration affect the rate of reaction?
- Increasing concentration of a solution increases rate of reaction because of the increased frequency of collisions between the substances
- Increased collisions results in a greater probability of successful collisions between the reactant particles, increase rate of reaction
How does pressure affect the rate of reaction?
- Increasing the pressure of gases means more particles in a given volume, increasing the concentration
- Increase in the frequency of collisions between reactant particles and increase rate of reaction
How does surface area affect the rate of reaction?
- Breaking up a solid, increases SA:V ratio causing increase in rate, more space to react on
- Smaller particles have larger surface area
How can you measure a change in concentration during an experiment?
- Monitoring the change in mass of the product or reactant, volume or any colour changes too
What are catalysts?
- They allow reactions to take place at lower temperatures and pressures
- They essentially increase the rate of chemical reactions
- Used in many industrial processes, they have a positive effect on the economics and safety of these processes
How do catalysts increase the rate of reactions?
- They provide an alternative reaction pathway that has a lower activation energy than the uncatalysed pathway
- However the original activation energy for the reaction remains unchanged
- The catalyst remains chemically unchanged at the end of the reaction
- Be able to draw it on Maxwell-Boltzmann curve and in energy profiles
