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6. Chemical kinetics > Collision theory and rates of reaction > Flashcards

Collision theory and rates of reaction Flashcards

1
Q

What is the collision theory?

A
  • In order for a chemical reaction to occur between two or more particles, they must collide
  • Not all collisions result in a chemical reaction
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2
Q

What two factors must exist for a collision to result in a chemical reaction?

A
  • Reactant particles must collide with the correct orientation, must collide head on
  • Reactant particles must collide with sufficient energy (activation energy, Ea)
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3
Q

What happens when the factors are not met?

A
  • The particles will bounce apart without reacting, known as unsuccessful collision
  • Collisions can also be unsuccessful when the particles do not have equal or more than the activation energy needed in the reaction
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4
Q

What is activation energy (Ea) and why is it needed?

A
  • The minimum amount of energy that colliding particles must have for a chemical reaction to occur
  • This is needed to overcome the energy barrier between the reactants and products
  • Ea does not change with temperature
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5
Q

How do you calculate the activation energy of a energy level profile? How does the activation energy vary between reactions?

A
  • The difference in energy between the reactant and the transition state (peak in the profile)
  • In faster reactions, there is a lower activation energy, in slower reactions there is a higher activation energy
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6
Q

What is the transition state?

A
  • The highest energy state on a reaction coordinate, the point at which new bonds are formed at the same time as old bonds are broken
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7
Q

What is the Maxwell-Boltzmann distribution?

A
  • In an ideal gas, the kinetic energy of the molecule is spread over a range of values (many different kinetic energies)
  • The total area beneath the curve is equal to the total number of particles in the sample
  • The particles in a certain region below the curve have a wide range of kinetic energies, the most probable value is where the curve is highest
  • Essentially, at the same temperature, all gases have the same average kinetic energy
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8
Q

What is the trend with temperature of the Maxwell-Boltzmann distribution?

A
  • Temperature increase –> particles gain kinetic energy –> curve flattens out
  • Area under each curve is the same
  • Most probable value of kinetic energy is the highest point
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9
Q

How is the Maxwell-Boltzmann useful with catalysts?

A
  • It helps understand how changes in temperature and a catalyst will affect the rate of a reaction
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10
Q

How does the Maxwell-Boltzmann distribution show gases in relation to the molecular speed?

A
  • Comparing different types of gases, at the same temperature, lighter gaseous molecules, on average move faster than heavier gaseous molecules
  • The molecular speed depends on the molar mass
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11
Q

What is the rate of reaction?

A
  • A measure of the speed of a reaction

- The change in concentration of a reactant or product per unit of time

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12
Q

How do you calculate the rate of reaction using the decrease in concentration of the reactant?

A

Rate = decrease in reactant concentration / change in time

- Is always positive

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13
Q

How do you calculate the rate of reaction using the increase in concentration of the product?

A

Rate = increase in product concentration / change in time

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14
Q

What is the instantaneous rate of reaction?

A
  • The rate of reaction at any particular point in time
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15
Q

How is the instantaneous rate of reaction determined?

A
  • Seen graphically from a change in reactant/product concentration against time
  • On a graph the instantaneous rate at any given time is equal to the gradient of the line at that point in time
  • Drawing a tangent to the curve at that time and calculating the gradient = ΔY/ΔX
  • The steeper the gradient the faster the rate of reaction
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16
Q

How is the average rate of reaction calculated?

A
  • Dividing the change in concentration of a reactant/product by the time taken for the change to occur
  • It is the rate of reaction averaged over a certain time period (straight line on the graph), same gradient calculation
17
Q

What factors can affect the rates of reaction?

A
  • Temperature
  • Concentration
  • Pressure
  • Surface area
18
Q

How does temperature affect the rate of reaction?

A
  • Increased temperature makes particles gain kinetic energy, they have a higher velocity
  • Frequency of collisions increases
  • A greater proportion of reactant particles will collide with energy equal or greater than the Ea
19
Q

How is the affect of increased temperature represented on a Maxwell-Boltzmann distribution?

A
  • Using a Maxwell-Boltzmann distribution, only the particles in the area underneath the flat part of the curve, have sufficient energy to react
  • Increased temperature causes the peak of distribution curve shifts to the right, meaning at higher temperatures, the average kinetic energy increases
  • Also the curve flattens so the total area under the curve remains constant
  • The area under the curve increases to the right of the activation energy value
20
Q

How does concentration affect the rate of reaction?

A
  • Increasing concentration of a solution increases rate of reaction because of the increased frequency of collisions between the substances
  • Increased collisions results in a greater probability of successful collisions between the reactant particles, increase rate of reaction
21
Q

How does pressure affect the rate of reaction?

A
  • Increasing the pressure of gases means more particles in a given volume, increasing the concentration
  • Increase in the frequency of collisions between reactant particles and increase rate of reaction
22
Q

How does surface area affect the rate of reaction?

A
  • Breaking up a solid, increases SA:V ratio causing increase in rate, more space to react on
  • Smaller particles have larger surface area
23
Q

How can you measure a change in concentration during an experiment?

A
  • Monitoring the change in mass of the product or reactant, volume or any colour changes too
24
Q

What are catalysts?

A
  • They allow reactions to take place at lower temperatures and pressures
  • They essentially increase the rate of chemical reactions
  • Used in many industrial processes, they have a positive effect on the economics and safety of these processes
25
Q

How do catalysts increase the rate of reactions?

A
  • They provide an alternative reaction pathway that has a lower activation energy than the uncatalysed pathway
  • However the original activation energy for the reaction remains unchanged
  • The catalyst remains chemically unchanged at the end of the reaction
  • Be able to draw it on Maxwell-Boltzmann curve and in energy profiles

6. Chemical kinetics (1 decks)

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