Collision Theory And Catalysis Flashcards
What is a catalyst?
A substance that increases the rate of reaction without modifying the overall standard Gibbs energy change in the reaction
Undergoes no nit chemical change - reusable
Provides an alternative reaction pathway - most commonly lowering Ea
What assumptions are made in the kinetic gas model?
Molecules are hard spheres whose motion is predicted by classical mechanics
Size of molecules is negligible and distance travelled is»_space; molecule size
Only elastic collisions, molecules do not interact except during collisions
What equation defines the root mean square speed of molecules?
What is collision density (Z(AB))?
It is the number of collisions per unit time and volume
What is the relationship between reaction rate and speed of molecules in collision theory?
They are proportional to each other
What is Z(AB) equal to?
Z(AB) = ∂(AB) v(rel) Na^2 [A][B]
What is the collision cross section of two molecules?
∂(AB) = 3.14r^2 = 3.14( Ra + Rb)^2
Where Ra is the radius of molecule A, a hard sphere
What is the mean relative speed when calculating the collision density of two molecules?
What is the number density of two molecules in a reaction vessel?
Number density of a species is the concentration multiplied by Avogadro’s constant, Na
Number density = Na[A] x Na[B]
What does the Maxwell-Boltzmann distribution do?
It shows the probability of finding a particle with a given kinetic energy, depending on temperature and its molecular mass.
What does integrating the Maxwell-Boltzmann distribution give?
It gives the mean relative speed, i.e. the mean speed one molecule approaches another
= Vrel
What does collision density actually mean?
It is the number of collisions per unit time and volume, i.e. number of products forming per unit time and volume
What is the collision density divided by Avogadro’s, Z(AB)/NA = ?
The rate constant of a reaction, k
For products to form, what must happen?
Colliding reactants must have enough energy to overcome the energy barrier of the transition state.
Only some collisions lead to reactions
What’re the key points from the Maxwell-Boltzmann distribution?
A fraction of molecules with a very high speed is small as exp(-v^2) becomes very smaller with large velocity.
When M is large the exponential factor goes more rapidly to zero, heavy molecules are less likely to have high speed
As T increases, expect more molecules at higher speeds
What does f(react) = ?
f(react) = exp(-Ea/RT)
What does ‘f(react) =‘ mean?
f(react) gives the probability that a molecule will have a specific energy (Ea)
Not every collision = reaction
What’re the geometric and steric requirements for a reaction?
Molecules must collide in correct conformation, this is accounted for by P = experimental k / theoretical k
Why does K + Br2 have a steric factor, P of 4.8 (high)?
Because the number of collisions have been underestimated - ignoring that the molecule don’t interact
There is a “harpoon” effect that gives rise to K+ and Br2- due to a long distance electron transfer. This coulombic attraction draws the two charged species together.
What is the normal estimation of the steric factor P?
Normally P < 1, less reactions per second with correct alignment and energy than predicted value.
Why is P sometimes «_space;1?
This is due to a highly specific conformation required for the reaction to occur - usually for larger / more complicated reactions.
Why would P > 1?
Indicates that the molecules are interacting with each other which results in more collisions.
How does the Arrhenius equation and collision theory differ?
Arrhenius equation is an empirical relationship
Collision theory is derived from (mostly) fundamental parameters for gas phase I molecular reactions
What’re the disadvantages of using collision theory? What’s better?
Limited to gas phase, bimolecular reactions
Experimental value of P
More accuracy given by transition state theory
Does pre-exponential factor A of the Arrhenius equation rely on temperature? Why?
It depends on the square root of T. However, since a change in T has very little effect on A, we assume in the Arrhenius equation that A does not in fact change with T.
