Topic 6. Chemical kinetics
6.1 Collision theory and rates of reaction
Rate of reaction ( mol/dm³s ):
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an increase in the concentration of one of the products per unit of time
OR
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a decrease in the concentration of one of the reactants per unit of time
Concentration is for aqueous or liquid reactants/products
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so it can be replaced with mass (for solids) or volume (for gases)
Concentration is represented with square brackets
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for example, the concentration of A is [A]
From Ms. Fu's powerpoint
There are three types of ROR:
To find the average ROR, find the slope over a given time interval (plot two points on the graph)
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depending on whether you graph products or reactants, the sign will change
To find the instantaneous ROR, find the slope of the tangent at the given point. You can use limits.
The blue line represents the initial ROR, which is basically the instantaneous ROR when t = tₒ (usually 0).
Methods of measuring ROR:
Collision Theory
The three conditions of the collision theory:
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atoms must have frequent collisions
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atoms must collide with the appropriate geometry and coordination (A.K.A. the steric factor)
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atoms must collide with sufficient kinetic energy to overcome the activation energy
The activation energy (Ea) is the minimum energy that colliding molecules need in order to have successful collisions leading to a reaction.
* only if the frequency of effective/successful collisions increases, then the ROR can increase
Maxwell-Boltzmann Distribution
When temperature increases → kinetic energy of particles increases → frequency of collisions increases
From Chemsir on Twitter
The shaded areas represent the number of particles that have sufficient kinetic energy to overcome the activation energy → they are the ones that can successfully react.
Assuming the two curves represent different temperatures of the same reaction, the area under the two curves are equal → same number of particles.
T₂ is greater than T₁, which is why the curve leans more towards the right. The result is that there are more particles (for T₂) that have greater kinetic energy (more on this later).
From Ms. Fu's powerpoint
Factors determing ROR
{
factors affecting collision rate: concentration/pressure & surface area
factor affecting proportion with required Ea: temperature
factor affecting Ea: catalyst
1. concentration (for aqueous) / pressure (for gases) of reactants
* y-axis label is subject to change depending on the type of reaction given.
Both curves plateau / reach a maximum since there is a limited amount of reactants.
The ending points of the two curves are different for the CONCENTRATION graph, assuming the reactants are in excess. If they aren't, it will reach the same plateau as the original curve.
Explanation (use this for Paper 2):
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Concentration of reactants increases → frequency of overall collisions increases → frequency of effective collisions increases → ROR increases
For pressure, add that the # of gas particles in the given volume decreases → more space between them
2. surface area of reactants (usually for solids)
* y-axis label is subject to change depending on the type of reaction given.
Both curves plateau / reach a maximum since there is a limited amount of reactants.
The ending points of the two curves are the same.
Common examples include magnesium ribbons v.s. magnesium powder. Smaller particles (the powder) have a greater surface area → greater ROR
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think of trying to dissolve chunks of salt v.s. small salt grains
Explanation (use this for Paper 2):
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Surface area of reactants increases → contact area for particles increases → reacting particles are more exposed to each other → frequency of overall collisions increases → frequency of effective collisions increases → ROR increases
3. temperature of reaction
Pearson textbook pg. 282
Maxwell-Boltzmann curve/distribution
Changing the temperature CANNOT change the Ea, just the number of particles that have greater kinetic energy than the Ea.
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E > Ea
Average kinetic energy is proportional to temperature (in Kelvin).
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will display a linear relationship when graphed
Explanation (use this for Paper 2):
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Temperature of reaction increases → number of particles that will have sufficient kinetic energy to overcome the activation energy increases → frequency of effective collisions increases → ROR increases
4. catalysts
Pearson textbook pg. 284
This diagram depicts an exothermic reaction, but the same concept applies for endothermic reactions.
A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy and providing an alternative reaction pathway that requires less energy
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without itself undergoing permanent chemical change (this last bullet point is optional).
Unlike temperature, catalysts CAN change the activation energy.
Must know both diagrams.
Catalysts do not change the ∆H value.
Explanation (use this for Paper 2):
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Catalysts will create an alternative reaction pathway so the Ea of the reaction will decrease → number of particles with sufficient kinetic energy to overcome Ea increases → ROR increases