3.1.5 Kinetics Flashcards
What is collision theory?
Reactions can only occur when collisions take place between particles having sufficient energy. The energy is usually needed to break the relevant bonds in one or either of the reactant molecules.
What is activation energy (E(A))?
The minimum energy which particles need to collide to start a reaction.
What is Maxwell Boltzmann distribution?
The Maxwell-Boltzmann energy distribution shows the spread of energies that molecules of a gas or liquid have at a particular temperature.
Describe a Maxwell Boltzmann distribution curve.
Number of molecules on y axis.
Collision energy on x axis.
The energy distribution should go through the origin because there are no molecules with no energy.
A few have low energies because collisions cause some particles to slow down.
E(mp) is the most probable energy (not the same as mean energy).
The mean energy of the particles is not at the peak of the curve.
The area under the curve represents the total number of molecules present.
Molecules with energy > E(A) will collide with enough energy to react.
The energy distribution should never meet the x axis, as there is no maximum energy for molecules
Explain the effect of temperature on rate of reaction and Maxwell Boltzmann distribution.
Rate of reaction:
- Increasing temperature = particles have more kinetic energy so they move quicker
- This increases the frequency of collisions and the frequency of successful collisions
- Rate of reaction increases
Maxwell Boltzmann distribution:
- As the temperature increases the distribution shifts towards having more molecules with higher energies
- At higher temps, both the E(mp) and mean energy shift to higher energy values, although the number of molecules with those energies decrease.
- At higher temperatures the molecules have a wider range of energies than at lower temperatures.
Explain the effect of concentration and pressure on rate of reaction and Maxwell Boltzmann distribution.
Rate of reaction:
- Increasing concentration/pressure = more particles in a certain volume
- This increases the frequency of collisions and the frequency of successful collisions
- Rate of reaction increases
Note: If a question mentions a doubling of concentration then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions.
Maxwell Boltzmann distribution:
- If concentration increases, the shape of the energy distribution curves do not change (i.e. the peak is at the same energy) so the E(mp) 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 (doesn’t apply to increasing pressure)
- More molecules have energy > E(A) (although not a greater proportion)
Explain the effect of surface area on rate of reaction and Maxwell Boltzmann distribution.
Rate of reaction:
- More area exposed to react
- This increases the frequency of collisions and the frequency of successful collisions
- Rate of reaction increases
Maxwell Boltzmann distribution:
- No effect
Explain the effect of catalysts on rate of reaction and Maxwell Boltzmann distribution.
Rate of reaction:
- Increase the rate of reaction
- Provide an alternative route/mechanism for the reaction that has a lower activation energy
- Does not change the frequency of collisions, however, more collisions will be successful as more particles have sufficient energy to react (>= E(A)).
Maxwell Boltzmann distribution:
- E(A) labelled at a lower energy
How do you measure 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^-3s^-1
When a graph of concentration of reactant is plotted vs time, the gradient of the curve is the rate of reaction.
The initial rate is the rate at the start of the reaction where it is fastest.
Reaction rates can be calculated from graphs of concentration of reactants or products, by drawing a tangent to the curve (at different times) and calculating the gradient of the tangent.
How do you compare rate curves?
The higher the concentration/ temperature/ surface area the faster the rate (steeper the gradient).
Different volumes of the same initial concentrations-will have the same initial rate (if other conditions are the same) but will end at different amounts.
