3.2.2 Reaction Rates

Question 1

what is the rate of a reaction

Answer 1

a fast a product is produced or a reactant is used up
- the change in concentration of a reactant/product over time

Question 2

what is the equation of rate of reaction, with units

Answer 2

change in concentration/time
- mol dm-3 s-1

Question 3

how does a rate of reaction change over the course of a reaction

Answer 3

1) the ROR is fastest at the start, as each reactant is at its highest concentration
2) the ROR slows as the reaction proceeds, as the reactants are being used up and their concentration decrease
3) once one of the reactants has been completely used up, the concentration stops changing and ROR=0

Question 4

which factors affect the rate of a reaction

Answer 4

• concentration (or pressure for gases)
• temperature
• use of a catalyst
• SA of solid reactant

Question 5

what happens in most collisions of particles

Answer 5

• the particles will bounce off each other
• will remain chemically unchanged

Question 6

what is collision theory

Answer 6

• 2 reacting particles must collide for a reaction to occur, but for an EFFECTIVE COLLISION:
  1) particles must collide with correct orientation
  2) particles must have enough energy to overcome the activation energy barrier of the reaction

Question 7

how does increasing concentration increase the rate of a reaction

Answer 7

• increasing concentration increases the number of particles in the same volume
• particles are closer and collide more frequently
• so in a given time, more effective collisions
• increasing rate of reaction

Question 8

how does increasing pressure affect the rate of reaction

Answer 8

• increasing pressure increases the concentration of gas molecules
• as same number of gas molecules occupy a smaller volume
• closer together
• so more effective collisions in a given time

Question 9

how can you measure the rate of a reaction

Answer 9

• monitor the decrease in concentration of reactants
• monitor the increase in concentration of products

Question 10

what are two ways to monitor a rate of a reaction that produces gases

Answer 10

• monitor the volume of gas PRODUCED at regularly times intervals using gas syringe
• monitor LOSS of mass of reactants using balance
• both are proportional to changes in concentration, so give ROR

Question 11

PAG: how do you measure the rate of reaction using a gas syringe

Answer 11

1) add initial reactant and set up with bung, capillary tube and gas syringe, measure initial volume of gas if present
2) add second reactant, replacing the bung back on immediately
3) start a stop clock
4) record the volume of gas produced at regular intervals until the reaction is complete (no more gas is produced)
5) plot graph of volume against time
6) initial rate can be found by drawing a tangent at 0 and extending it (the WHOLE WAY)

Question 12

PAG: how can you measure the rate of a reaction using a balance

Answer 12

1) record the initial mass on the balance
2) then record at regular intervals
3) reaction is complete when no more gas is produced, so mass no longer changes
4) graph of mass against time

Question 13

what is a catalyst

Answer 13

a substance which increases the rate of a reaction via providing an alternative reaction pathway with lower activation energy, without undergoing any permanent changes itself

Question 14

why does a catalyst not get used up

Answer 14

• may react with a reactant to form an intermediate
• or provide a surface on which the reactant can take place
• but at the end of a reaction, is regenerated

Question 15

what happens on an enthalpy change diagram with a catalyst present

Answer 15

• lowers the activation energy
• so curve doesn’t go as high

Question 16

what are the two types of catalysts

Answer 16

• homogeneous
• heterogeneous

Question 17

what is a homogeneous catalyst

Answer 17

a catalyst in the same physical state as the reactants

Question 18

how does a homogeneous catalyst act

Answer 18

• where it reacts with the reactants
• and forms an intermediate
• which breaks down to give products
• and the catalyst regenerates
• e.g. ozone making esters with H2SO4

Question 19

what is a heterogeneous catalyst

Answer 19

a catalyst in a different physical state from the reactants

Question 20

how does a heterogeneous catalyst act

Answer 20

• usually solids
• where the reactant molecules are absorbed onto the surface
• and is where the reaction takes place
• product molecules leave via desorption
• e.g. used in many industrial settings

Question 21

why are catalysts sustainable in terms of profitability

Answer 21

• catalysts increase rate by lowering activation energy
• so reduce the temperature needed for the process
• and reduce the energy requirements
• if less energy is required, less electricity and fossil fuels are used
• a faster reaction requiring less energy can cut costs
• increase profitability
• economic advantage outweighs any costs associated with developing the catalytic process (weighed out with potential toxicity of catalysts too)

Question 22

why are catalysts sustainable in terms of global warming

Answer 22

• less fossil fuels used
• so less CO2 emissions

Question 23

do all molecules in a gas have the same energy

Answer 23

NO

Question 24

explain the spread of molecular energies in gases

Answer 24

• some molecules move slowly with low energy
• some molecules move fast with high energy
• most molecules move close to the average speed

Question 25

what does the Boltzmann distribution show

Answer 25

the spread of molecular energies in gases

Question 26

what does the Boltzmann distribution look like

Answer 26

• energy on the x-axis
• number of molecules with given energy on the y-axis
• a curve which slopes on the downward

Question 27

what are the characteristics of the Boltzmann distribution

Answer 27

• no molecules have 0 energy, so the curve starts at the origin
• area under the curve equals the total number of molecules
• there is no maximum energy for a molecule, so the curve never meets the x-axis at high energy, as would need to reach infinity
• the Ea line is towards the end, as only small proportion of molecules have more energy than needed to react

Question 28

how does temperature affect the Boltzmann distribution

Answer 28

• as temperature increases
• average energy of the molecules also increases
• a small proportion will have a lower energy
• but most will have a higher energy

Question 29

how does the Boltzmann distribution graph change at a higher temperature

Answer 29

• the number of molecules is the same, so the area must stay the same
• so the graph peaks lower than at lower temperature
• but gets pulled upward at the higher energy level above T1
• and lower at the start

Question 30

what does the Ea line show at a higher temperature on the Boltzmann distribution graph

Answer 30

• more molecules have an energy greater than and equal to the Ea
• so a greater proportion of collisions will lead to a reaction
• increases the ROR
• (also more frequent collisions as the molecules are moving faster, but increased energy is more important

Question 31

how does a catalyst affect the Boltzmann distribution

Answer 31

• a catalyst provides an alternative reaction route with lower Ea
• so the Ec line will be drawn further to the left
• this means a greater proportion of molecules will now have energy which is greater than or equal to this new lower Ea
• so more successful collisions
• more products, increasing the ROR

Question 32

what is important to remember when comparing temperature and catalysts on a Boltzmann distribution

Answer 32

• original particles can be shown with dashes
• and new ones can be shown with 2 dashes