9: Kinetics I Flashcards
What do particles need to react?
Minimum (activation) energy
Correct orientation relative to each other
What is an example of particles needing the correct orientation?
Bromoethane will only react with hydroxide ions if the hydroxide ion collides with the carbon bonded to the bromine
What happens when particles collide with less than activation energy?
No reaction
Will bounce off each other
What is the activation energy?
Minimum energy required before a reaction can occur
What does bond breaking and forming do?
Bond breaking - requires input of energy
Bond forming - releases energy
What is the equation for reaction rate?
Change in concentration / change in time
Gradient of tangent in a concentration vs time graph
How can the initial rate of reaction be calculated?
Gradient of tangent drawn at time = 0
What can increase the rate of a reaction?
Increase temperature Increase concentration of reactants Increase SA of solids Increase pressure of gases Addition of catalysts
How does increasing temperature increase the rate of reaction?
Particles have more kinetic energy on average
Collide more energetically
Greater proportions of particles collide with greater than activation energy
How does increasing concentration of reactants increase the rate of reaction?
Greater number of particles per unit volume
More collisions & successful ones per second
How does increasing surface area of solids increase the rate of reaction?
Greater number of particles exposed
Reactants collide more frequently
More collisions per second
How does increasing pressure of gases increase the rate of reaction?
Same number of gas particles in a smaller volume
More collisions & successful ones per second
How does adding a catalyst increase the rate of reaction?
Provides alternative pathway for reaction
Has lower activation energy
More collisions have more than activation energy
What occurs to a catalyst during the reaction?
It is not used up
What should be said when talking about rates of reaction?
State relative amounts - per unit volume, per second etc.
What is a Maxwell-Boltzman curve?
Distribution of energy among particles
x - energy
y - number of particles with that energy
What is temperature?
Average kinetic energy of its particles
What is the dotted line in the Maxwell-Boltzman curve?
Activation energy - particles with energy right of the line can collide successfully
Why does the Maxwell-Boltzman curve start at (0,0)
No particles with no energy
What are some notable features of the Maxwell-Boltzman curve?
Distribution doesn’t fall to 0 at highest energies
Area under curve proportional to number of gas particles present
Only particles right of dotted line will react when they react
How does the Maxwell-Boltzman curve change when the temperature increases?
Peak is lower and to the right of original peak
Area under curve should be the same
More particles with energy over activation energy
What are the two temperatures called in the Maxwell-Boltzman curve?
T1 - original temperature
T2 - higher/new temperature
How does using a catalyst change the Maxwell-Boltzman curve?
Shifts the activation energy to the left
More particles have energy above it
How does a catalyst change an energy-profile diagram?
Reduces height of the activation energy
How can rate of reaction be measured?
Monitoring loss of mass
Monitoring production of gas
What is a phase?
Phase is a physically distinctive form of matter
Liquid, solid, gas
What are the two types of catalysts?
Heterogeneous
Homogeneous
What is a heterogeneous catalyst?
Catalyst is in a different phase from the reactants
Usually solid catalyst with liquid/gas reactants
What is a homogeneous catalyst?
Catalyst is in the same phase as the reactants
How does a solid heterogeneous catalyst work?
Selectively causes one reactant to bond to but not react with it (adsorb)
Weakens the bonds making it more reactive
Products deadsorb and are released
Why is using an expensive catalyst still economically viable?
Product is produced at a faster rate
More money made by the company per (week/month/year)
Overall this causes an increase in profit
How is pressure related to concentration?
pV = nRT
p = (n/V) * RT
Therefore pressure is directly proportional to concentration
What does a heterogeneous catalyst do to the reaction profile?
Singular smaller peak
Therefore smaller activation energy
How does a homogeneous catalyst work?
Reactants combine with catalyst to make an intermediate
Intermediate then reacts to form products and reforms catalyst
What does a homogeneous catalyst do to the reaction profile?
First small peak (smaller than without) to form intermediate
Then even smaller peak after this
Why is there less energy when reacting with an intermediate species?
Intermediates are highly reactive and unstable so little energy required for them to react
Why are catalysts used?
Economic benefits in the long-term
Can change the properties and make a more useful product
