2.2 rates of reaction Flashcards
what must occur for a collision to be successful?
the reactant particles must collide with:
- the correct orientation
- energy greater than or equal to the activation energy of the reaction
(chemical reactions occur when particles of reactants collide successfully)
can reaction conditions be changed to increase the likelihood of successful collisions in order to increase the rate of reaction?
yes
what factors can change the rate of reaction?
- surface area
- concentration
- temperature
- catalyst
activation energy definition
the minimum amount of energy needed to start a reaction
increasing the temperature of a reaction, ____ the rate of the reaction
increases
why does increasing the temperature of a reaction increase the rate?
- the reactant particles have greater kinetic energy
- they move a lot faster
- more of the particles have energy that is greater than the activation energy
- more successful collisions
increasing the concentration/pressure of a reaction, ____ the rate of the reaction
increases
why does increasing the concentration/pressure of a reaction increase the rate?
- there are more reactant particles in the same volume
- increase in the number of collisions per unit time
- greater chance of successful collisions
increasing the surface area of a reaction, ____ the rate of the reaction
increases
why does increasing the surface area of a reaction increase the rate?
- only particles on the surface of a solid can collide with other reactant particles
- so increasing surface area increases the number of particles available to collide
- which increases number of collisions per unit time
- so increases the chance of successful collisions occuring
rate of reaction definition
the change in quantity/concentration of a reactant or product per unit time
when is the rate of reaction greatest?
at the beginning of the reaction
why is the rate of reaction the greatest at the beginning of the reaction?
as reactant concentration are at their highest
- as the reactants are used up, the rate of reaction slows until one of the reactants is used up, at which point the reaction stops
how can the rate of reaction at a given time be found?
by calculating the gradient of a tangent to the curve at that time
what are energy profiles?
diagrams that represent the enthalpy changes in both exothermic and endothermic reactions
at a given temperature, not all the particles in a sample have the same energy.
the energy distribution of the particles in a system is shown by a graph known as what?
the Boltzmann energy distribution curve
what are some key features of the Boltzmann energy distribution curve?
- the highest peak of the graph shows the most probable energy at that temperature
- the mean energy of all particles is always slightly higher than the most probable energy, due to the greatest number of particles with an energy greater than the most probable energy
- there are no particles with an energy value of zero, at any temperature
- at higher energies, the number of particles approaches zero but never reaches zero. hence the curve never touches the x-axis
what does the area under the Boltzmann energy distribution curve represent?
the total number of particles in the system
(therefore, the area under the curve must remain constant in a closed system)
as the temperature of a reaction increases, what happens to the Boltzmann energy distribution curve?
- the peak shifts to the right
- the peak moves lower
- the area under the curve remains the same
- the number of particles with energies greater than or equal to the activation energy increases
as the temperature of a reaction increases, why does the peak of the Boltzmann energy distribution curve shift to the right?
- due to higher overall energy of the particles at higher temperature
as the temperature of a reaction increases, why does the peak of the Boltzmann energy distribution curve move lower?
as the range of particle energies becomes wider
as the temperature of a reaction increases, why does the area under the Boltzmann energy distribution curve remain the same?
as increasing temperature does not change the total number of particles
as the temperature of a reaction increases, the number of particles with energies greater than or equal to the activation energy increases.
what does this lead to?
a greater change of successful collisions
does adding a catalyst to a reaction change the shape of the Boltzmann energy distribution curve? why?
no
as the catalyst does not add energy to the reaction
what does adding a catalyst do to a Boltzmann energy distribution curve?
- the position of the activation energy shifts to the left
(showing that adding a catalyst lowers the activation energy)
- as the a.e has been lowered, there are now additional particles with energy greater than the new lower a.e, which increases the chance of successful collisions occuring
how do catalysts lower the activation energy?
by providing an alternative reaction pathway/route that has a lower activation energy
what are the two types of catalysts?
- homogeneous catalysts
- heterogeneous catalysts
what are homogeneous catalysts?
- they are in the same physical state as the reactants
- these catalysts take an active part in the reaction and are then regenerated as the reaction proceeds