[3.1.5] Kinetics Flashcards
Collision Theory, Maxwell-Boltzmann Distribution, Effect of Temperature, Pressure & Concentration on Reaction Rate and Catalysts.
What is activation energy?
Activation energy is defined as the minimum energy which particles need to collide to start a reaction.
What is the Maxwell-Blotzmann distribution?
The Maxwell-Boltzmann energy distribution shows the spread of energies that molecules of a gas or liquid have at a particular temperature.
What should the y-axis and the x-axis of a Maxwell-Boltzmann distribution curve be labelled as?
- Y-AXIS = Fraction of molecules with energy.
- X-AXIS = Energy.
Label this diagram of a Maxwell-Boltzmann distribution curve.
What is the effect of increasing temperature on the Maxwell-Boltzmann distribution curve?
- As the temperature increases, the distribution shifts towards having more molecules with higher energies (i.e. shifts left).
- Both the most probable energy (Eₘₚ) and mean energy shift to higher energy values.
- The total area under the curve should remain constant because the total number of particles is constant.
- At higher temperatures, the molecules have a wider range of energies than at lower temperatures.
What is the effect of decreasing temperature on the Maxwell-Boltzmann distribution curve?
- As the temperature decreases, the distribution shifts towards having more molecules with lower energies (i.e. shifts right).
- Both the most probable energy (Eₘₚ) and mean energy shift to lower energy values.
- The total area under the curve should remain constant because the total number of particles is constant.
- At lower temperatures, the molecules have a smaller range of energies than at higher temperatures.
How can a reaction go to completion if few particles have energy greater than the activation energy?
Particles can gain energy through collisions.
Define the term rate of reaction.
- The rate of reaction is defined as the change in concentration of a substance in unit time.
- The usual unit is mol dm⁻³ s⁻¹.
How can we calculate the rate of reaction from a graph?
- Reaction rates can be calculated from graphs of concentration of reactants or products.
- This can be done by drawing a tangent to the curve at the time you wish to know the rate of reaction and calculating the gradient of the tangent.
- Gradient = change in y ÷ change in x
What is the effect of increasing temperature on rate of reaction?
- At higher temperatures, the kinetic energy of the particles increases.
- The particles collide more frequently and more often with energy greater than the activation energy.
- Therefore, there is a higher proportion of successful collisions which results in an increased rate of reaction.
Use the Maxwell-Boltzmann distribution to explain why a small temperature increase can lead to a large increase in rate.
- As the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy.
- So the frequency of successful collisions increases and thus the rate of reaction increases.
What is the effect of increasing concentration and pressure on rate of reaction?
- At higher concentrations and pressures, there are more particles per unit volume.
- So the particles collide with a greater frequency and there will be more particles with energy greater than the activation energy so higher frequency of effective collisions.
- Therefore, rate of reaction increases.
What is the effect of increasing concentration on the Maxwell-Boltzmann distribution curve?
- If concentration increases, the shape of the energy distribution curves does not change (i.e. the peak is at the same energy) so the Eₘₚ and mean energy do not change.
- The curves will be higher and the area under the curves will be greater because there are more particles.
If a question mentions doubling concentration, what should you do?
- Double the number of particles per unit volume.
- Double the frequency of effective collisions.
What is the effect of increasing surface area on reaction rate?
- Increasing surface area will cause successful collisions to occur more frequently between the reactant particles.
- This increases the rate of reaction.
Use a Maxwell-Boltzmann distribution to help explain how a catalyst increases the rate of reaction.
- If the activation energy is lower, more particles will have energy greater than the activation energy.
- So there will be a higher frequency of successful collisions.
- Therefore, the rate of reaction increases.
Use this diagram and annotate it to explain how you would compare these rate curves.
