Collision Theory and Rates of Reaction Flashcards
What are the 4 methods of measuring rates?
- Changes in pH
- Changes in conductivity
- Changes in mass or gas volume
- Changes in colour
What is the Collision Theory?
Kinetic Molecular Theory states that all particles move randomly due to Kinetic E. This means not all particles share the same Kinetic E, and because of this, we use averages related to absolute temperatures (increased temp means increased Kinetic E).
What is a Maxwell - Boltzmann Distribution Diagram?
The range of Kinetic E is shown in graph. The area under the curve is the total number of particles. Increased temp will stretch the graph horizontally.
Collision theory in reactants
Kinetic E causes reactants to collide breaking and forming bonds. The reaction depends on the number of successful collisions. Depends on E and the geometry of the collision.
E of collision
Collison requires sufficient energy to overcome the electron repulsion and break bonds (endo). When Ea is reached, reactants enter a transition and then move to products as new bonds form. The rate depends on % of particles with enough Kinetic E (more E = higher rate)
Geometry of collision
Particles must collide in the correct orientation in 3D space.
Factors affecting the rate of the reaction
- Temperature: high temp means more particles with the required Ea
- Addition of a catalyst: will not be consumed, provides rxn a pathway using a lower Ea to increase rate
- Increased concentration of particles: more particles = more collisions = higher rate
- Particle size: increased surface area = more particles for rxn
Heterogenous catalyst
Different state from reactants
Homogenous catalyst
Same state as reactants
How does the concentration of reactants affect the rate?
More particles mean a higher collision rate.
What is the rate expression?
rate = k [A] [B] [C]
k - rate constant, varries
- exponents above concentrations are not coefficients and can only be determined through experiment
What is a rate?
Speed of change per unit time. (how fast [] of product or reactant changes). Measured as:
r = delta[] / delta T
What is the rate of reaction?
rate of rxn (+ve) = -1/A = -1/B = 1/C = 1/D
- products are always +ve
- reactants are always -ve
How to determine the instantaneous rate?
Find the tangent of a [] v t graph.
How to determine rate expression?
- Compare [] of a reactant b/w two experiments
- Use form A^m = r to find exponent (m)
- Repeat for all reactants
- Sub into rate expression (r= k [a]…) and simplify
How to determine K in the rate expression?
Rearrange for K and sub in values from one experiment
Define the different orders
Zero: to the exponent of 0, eliminates the term
First: to the exponent of 1, have a constant half-life
Second: to the exponent of 2
- Possibly third
What is a half-life?
T (subscript)1/2. The time for the [] of the reactant to decrease by half. lower half-life = faster reaction
What is a reaction mechanism?
A series of elementary steps involving a small number of particles. The sum of steps = overall rxn. Intermediate steps appear in elementary steps but not in overall reaction
What is the molecularity of an elementary step?
Number of reactants in that step
What is the RDS?
The slowest step. Overall reaction can only appear as fast as the slowest step. This will be given. If RDS is not in overall rxn, find the elementary step where it is produced and sub.
What is the rate of expression for elementary steps?
r = k[A]^m …
- [] is raised to the power of their coefficient
Mechanism w.r.t the RDS
- Zero order reactant does not appear in the mechanism until after RDS
- First or second order is in the RDS
- called ‘possible mechanism’ as it can’t be proven only supported by data.
Units of K
K = mol dm^-3 s^-1
Zero order = mol dm^-3 s^-1
1st order = s^-1
2nd order = mol^1 dm^3 s^-1
3rd order = mol^-2 dm^6 s^-1
Activation Energy and Temperature
Temp affects rate. Large Ea = increased temp has a big affect. Small Ea = increased temp has a lower affect
Arrhenius Formula
K = A e^-Ea/RT - exponential factor
- R = gas constant
- A = Arrhenius constant/frequency factor
- takes into account the frequency that successful collsisons occur based on geometry and E.
- Can remove e-factor through natural logs
