Studied by 1 person
define rate of reaction.
the change in concentration of a reactant or product per unit time.
the measure of the amount of product formed or the amount of reactant used over a given time.
what is the rate of reaction directly proportional to?
what is this factor determined by?
the frequency of successful collisions (collisions per second)
which is determined by changes to reaction conditions and changes to the energy that particles have.
what are 4 examples of the units of rate of reaction?
g/s
cm3/s
mol/s
mol dm-3/s
how do you calculate the mean rate of reaction?
change in conc. of reactant or product OR change in mass of reactant OR volume of gas produced
divided by time in s
OR:
time taken for solution to change colour/turn opaque
how can you calculate the rate of reaction at a specific time from a reaction profile?
what does the steepness of the curve tell you about the rate of reaction?
draw a tangent to the curve at the time specified.
calculate the gradient of this tangent (rise/run).
the steeper the curve, the faster the rate of reaction.
define activation energy (Ea).
the minimum amount of energy that particles must have in order to successfully collide and cause a reaction to occur.
what is collision theory?
give 5 points.
molecules must collide for a reaction to occur.
a reaction can only occur if collisions occur between molecules having sufficient energy (Ea).
not all collisions lead to a reaction; if particles have insufficient energy they will bounce off each other.
particle orientation must be correct; particles must approach each other in a certain relative way, and the collision must occur at the correct location within the molecule (steric factor).
when particles react, chemical bonds must be broken using energy equal to or greater than Ea (so that new chemical bonds can be formed).
during a reaction, why is the energy of each individual molecule constantly changing?
when molecules collide with each other, they gain or lose energy.
a molecule that doesn’t have enough energy to react may gain enough energy in a collision so that it can react in a further collision.
how does increasing temperature impact the rate of reaction?
thermal energy is transferred to molecules and is converted to KE.
so increase in the mean KE of particles.
particles move faster and further.
so successful collisions occur much more frequently and with greater energy.
increase in KE means many more molecules will have energy greater than Ea.
so a higher rate of successful collisions.
how does increasing concentration impact the rate of reaction?
reactant molecules are packed closer together.
so increase in the number of reactant molecules in the same volume.
so particles collide more often.
so a higher rate of successful collisions.
how does reducing the volume of a container containing gas impact the rate of reaction?
gas molecules are packed closer together, so increase in gas pressure.
so increase in the number of gas molecules in the same volume.
so particles collide more often.
so a higher rate of successful collisions.
increase in gas pressure increases the mean KE of gas molecules.
faster movement means successful collisions occur much more frequently and with greater energy.
how does increasing the surface area impact the rate of reaction?
more particles exposed on the surface to the other reactant.
so more particles are available to be collided with.
higher chances of particles colliding
higher rate of successful collisions.
which has a greater surface area to volume ratio?
magnesium lumps.
magnesium powder.
powder, more pieces broken up
define a catalyst.
a substance that increases the rate of reaction, without being changed in chemical composition or amount.
this means they can be reused.
how do catalysts increase the rate of reaction?
they provide an alternatie reaction route of a lower activation energy.
so many more molecules will have energy greater than Ea.
so higher proportion of successful collisions.
what are economic and environmental benefits of using a catalyst?
what is a disadvantage of using catalysts?
can be reused, which saves money.
reduces the need to burn fossil fuels to provide energy for reactions.
so saves money.
so reduces CO2 emissions, making the chemical industry more sustainable.
some catalysts are toxic.
fill in the gaps:
The Maxwell Boltzmann distribution of molecular energies in gases shows that at a (1) ________ ____________ , not all molecules in a substance have the same (2) _______ ___ _______ .
constant temperature
amount of energy
in the Maxwell-Boltzmann distribution curve, describe the locations of:
molecules with 0 energy.
the most probable amount of energy (Emp).
the mean amount amount of energy.
the activation energy (Ea)
molecules with high energies.
at the origin, 0 molecules have 0 energy.
the peak of the curve shows the most probable energy.
the mean energy is to the right of the most probably energy.
the activation energy is to the right of the mean energy.
high energy particles are on the tail (the right end) of the curve, and there are few of these.
what is meant by the ‘most probable energy’?
the energy that more molecules have than any other energy
why isn’t the mean energy equal to the most probable energy?
molecules at high temperatures increase the mean to a value above the most probable energy
why does the Maxwell Boltzmann distribution curve not touch the x-axis?
to show that there’s no maximum energy that the particles can have in the system
changing the temperature of a reaction alters the shape of the Maxwell Boltzmann distribution.
what must always remain constant, despite changes to the shape of the graph?
give a reason why.
the area under the graph must stay constant,
as it represents the total number of particles in the system, which doesn’t change when reaction temperature is changed.
how does the Maxwell Boltzmann distribution curve change when there’s an increase in temperature?
the peak of the curve is lower and displaced to the right.
the curve starts at the orgin and only crosses the original curve once.
use the Maxwell Boltzmann distrbution to explain why a small temperature increase can lead to a large increase in reaction rate.
many more molecules will have energy greater than Ea.
so there’s a higher rate of successful collisions.
how does the Maxwell distribution curve change when there’s an decrease in temperature?
the peak of the curve is higher and displaced to the left.
the curve starts at the orgin and only crosses the original curve once.
use the Maxwell Boltzmann distrbution to explain why a small temperature decrease can lead to a large decrease in reaction rate.
fewer molecules will have energy greater than Ea.
so there’s a lower rate of successful collisions.
how does the Maxwell distribution curve change when the volume of a container of gas is reduced?
no change in shape.
although the increase in pressure increases the reaction rate, there is no impact on the distribution of energy.
how does the Maxwell distribution curve change when a catalyst is added?
no change in shape.
activation energy will be lower so will shift to the left.
use the Maxwell Boltzmann distrbution to explain why most collisions do not result in a reaction.
lots of molecules have energy lower than Ea.
because many molecules have the most probably energy, which is lower than Ea.
give one reason for a slow reaction rate at room temperature.
few molecules have the required activation energy.
why is the rate of reaction fastest at the start and slower as the reaction continues?
initially, there’s a high concentration of reactant molecules.
so there’s a high frequency/high rate of successful collisions.
as reactant molecules are used up and form product, they decrease in concentration.
so there’s a lower frequency/high rate of successful collisions.
reaction rate reaches zero when all reactant molecules have reacted and there are no more successful collisions.
which of these 2 statements is true about molecules in a gas?
at a fixed temperature, they all move at the same speed.
at a fixed temperature, their average kinetic energy is constant.
at a fixed temperature, their average kinetic energy is constant.
Note 
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