Reaction rates Flashcards

1
Q

What is the rate of a reaction?

A

The change in concentration of a reactant of product in a given time

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2
Q

What is the equation for the rate of a reaction?

A

rate = change in concentration / time

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3
Q

What is the units for the rate of a reaction?

A

mol dm^-3 s^-1

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4
Q

When is the rate of reaction fastest?

A

At the start of the reaction, as each reactant is at its highest concentration

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5
Q

When does the rate of reaction become zero?

A

Once of the reactants has been completely used up, the concentrations stop changing and the rate of the reaction is 0

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6
Q

What 4 factors can change the rate of a chemical reaction?

A
  • Concentration (or pressure when reactants are gases)
  • Temperature
  • Use of catalyst
  • Surface area of solid reactants
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7
Q

What does the collision theory state?

A

2 reacting particles must collide for a reaction to occur

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8
Q

What is an effective collision?

A

One that leads to a chemical reaction

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9
Q

What are the 2 conditions for a reaction to be effective?

A
  • The particles collide with the correct orientation
  • The particles have sufficient energy to overcome the activation energy barrier of the reaction
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10
Q

How does increasing the concentration affect the rate of reaction?

A
  • When the concentration of a reactant is increased, the rate of reaction generally increases
  • An increase in concentration increases the number of particles in the same volume
  • The particles are closer together and collide more frequently
  • In a given period there will be more effective collisions and therefore an increased rate of reaction
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11
Q

How does increasing the pressure of a gas affect the rate of reaction?

A
  • When a gas is compressed into a smaller volume the pressure of a gas in increased and the rate of reaction increases
  • The concentration of the gas molecules increases as the same number of gas molecules occupy a smaller volume
  • The gas molecules are closer together and collide more frequently, leading to more effective collisions
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12
Q

If a reaction produces a gas, what 2 methods can be used to determine the rate of reaction?

A
  • Monitoring the volume of gas produced at regular time intervals using gas collection
  • Monitoring the loss of mass of reactants using a balance

The volume of gas produced and mass loss are both proportional to the change in concentration of a reactant or a product; so the change in volume or mass with time both give a measure of the rate of reaction

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13
Q

If a reaction produces a gas, what 2 methods can be used to determine the rate of reaction?

A
  • Monitoring the volume of gas produced at regular time intervals using gas collection
  • Monitoring the loss of mass of reactants using a balance

The volume of gas produced and mass loss are both proportional to the change in concentration of a reactant or a product; so the change in volume or mass with time both give a measure of the rate of reaction

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14
Q

What is a catalyst?

A

A substance that changes the rate of a chemical reaction without undergoing any permanent change itself

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15
Q

What are the rules of a catalyst?

A
  • The catalyst is not used up in the chemical reaction
  • The catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
  • At the end of the reaction the catalyst is regenerated
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16
Q

What does a catalyst do to the rate of reaction?

A

It increases the rate of chemical reaction by providing an alternative reaction pathway of lower activation energy

17
Q

What is a homogeneous catalyst?

A

A catalyst that has the same physical state as the reactants

18
Q

How does a homogeneous catalyst work?

A

The catalysts reacts with the reactants to form an intermediate. The intermediate then break downs to give the product and regenerates the catalyst

19
Q

What are 2 examples of homogenous catalysts?

A
  • Making esters with sulfuric acid as the catalyst
  • Ozone depletion with Cl radicals as the catalyst
20
Q

What is a heterogeneous catalyst?

A

A catalyst that has a different physical state from the reactants

21
Q

How do heterogenous catalysts work?

A

They are usually solids in contact with gaseous reactants or reactants in solution. Reactant molecules are absorbed onto the surface of the catalyst, where the reaction takes place. After reaction, the product molecules leave the surface of the catalyst of desorption

22
Q

What are 3 examples of heterogeneous catalysts?

A
  • Iron catalyst when making ammonia - the Haber process
  • Nickel catalyst for the hydrogenation of alkenes
  • V2 O5 catalyst when making sulfur trioxide
23
Q

What is the environmental and economical importance of catalysts?

A

Using a catalyst means that less energy is required during a chemical process. This in turn means that less electricity or fossil fuel is required.

  • Making the product faster and using less energy can cut costs and increase profitability - the economic advantage of using a catalyst outweigh any costs associated with developing a catalytic process
  • Using less fossil fuel will cut emissions of carbon dioxide, a gas linked to global warming
24
Q

What is the Boltzmann distribution?

A

The spread of molecular energies ; number of molecules with a given energy against energy

25
Q

What are the features of the Boltzmann distribution?

A
  • No molecules have zero energy - the curve starts at the origin
  • The are under the curve is equal to the number of molecules
  • There is no maximum energy for a molecule - the curve does not meet the x-axis at high energy. The curve would need to reach infinite energy to meet the x-axis
26
Q

What is the effect of temperature on the Boltzmann distribution?

A

At higher temperature:
- More molecules have an energy greater than or equal to the activation energy
- Therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction
- Collisions will also be more frequent as the molecules are moving faster, but the increased energy of the molecules is much more important than the frequency of collisions

27
Q

What is the effect of a catalyst on the Boltzmann distribution?

A
  • There is reduced activation energy
  • Now a greater proportion of molecules have an energy equal to or greater than the lower activation energy
  • On collision more molecules will react to form products