Kinetics Flashcards
(21 cards)
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
• This minimum energy is called the activation energy
The activation energy, EA…
defined as the minimum energy which particles need to collide to start a reaction.
The Maxwell-Boltzmann energy distribution shows..
the spread of energies that molecules of a gas or liquid have at a particular temperature
Emp is..
the most probable energy (not the same as mean energy)
A few particles have low energies because..
collisions cause some particles to slow down
The energy distribution should go through..
the origin because there are no molecules with no 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 particles present
Most molecules have energies..
between the two extremes but the distribution is not symmetrical (normal)
Only a few particles have..
energy greater than the EA
The energy distribution should never meet..
the x axis, as there is no maximum energy for molecules
As the temperature increases the distribution shifts..
towards having more molecules with higher energies
How can a reaction go to completion if few particles have energy greater than EA?
Particles can gain energy through collisions
Increasing temperature
• As the temperature increases the distribution shifts towards having more molecules with higher energies
• The total area under the curve should remain constant because the total number of particles is constant
• At higher temps both the Emp 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
Effect of increasing concentration and increasing pressure
• At higher concentrations(and pressures) there are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions
• Note: If a question mentions a doubling of concentration/rate then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions
• If concentration increases, the shape of the energy distribution curves do 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
• More molecules have energy > EA (although not a greater proportion)
Effect of increasing temperature
• At higher temperatures the energy of the particles increases. The particles collide more frequently and more often with energy greater than the activation energy. More collisions result in a reaction
• 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.
Effect of increasing surface area
Increasing surface area will cause successful collisions to occur more frequently between the reactant particles and this increases the rate of the reaction
Effect of catalysts
• Catalysts increase reaction rates without getting used up by providing an alternative route or mechanism with a lower activation energy
• If the activation energy is lower, more particles will have energy > EA, so there will be a higher frequency of effective collisions - the reaction will be faster
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
