3.5 reaction rates

Question 1

rate of reaction

Answer 1

the change of concentration/amount of a reactant or product per unit time

Question 2

rate of reaction formula

Answer 2

rate = amount of reactant used(or)product made/time

Question 3

collision theory

Answer 3

for a reaction to take occur, the particles must collide in the right direction and they must have the minimum amount of kinetic energy

Question 4

activation energy

Answer 4

the minimum amount of energy required for a reaction to occur (always between reactants and top of profile)

Question 5

what does low activation energy mean?

Answer 5

reactions that have low activation energy need less energy to break/make bonds

Question 6

Maxwell-Boltzmann distribution

Answer 6

shows the energy in gas particles - particles of a gas in a sample move at different speeds because they have different kinetic energy

Question 7

what is the area under the curve of Maxwell-Boltzmann distribution graph

Answer 7

equal to the total number of molecules

Question 8

increased temperature effect on rate of reaction

Answer 8

particles have on average more kinetic energy when they are heated therefore a greater proportion of the molecules will have greater than the activation energy
(curve shifts to right and peak is lower - area under curve beyond activation energy increases)

Question 9

decreased temperature effect on rate of reaction

Answer 9

particles will on average have less kinetic energy so a smaller proportion of molecules will have energy greater than the activation energy
(curve shifts to the left and peak is higher - area under curve beyond activation energy decreases)

Question 10

WHY do we get a faster rate of reaction when temperature is increased

Answer 10

particles move around more at higher temperatures as they have more kinetic energy so they collide more frequently
combination of more collisions and more energetic collisions means a small increase in temperature leads to a large increase in rate

Question 11

increased pressure effect on rate of reaction

Answer 11

increasing pressure will increase rate of reaction
particles closer together, collide more often
more frequent collisions therefore higher chance of reaction

Question 12

increased concentration effect on rate of reaction

Answer 12

increasing concentration will increase the rate of reaction
particles closer together, collide more often
more frequent collisions therefore higher chance of reaction

Question 13

catalyst

Answer 13

a substance that increases the rate of reaction by providing an alternative pathway with a lower activation energy
the catalyst is chemically unchanged at the end of the reaction

Question 14

advantages of a catalyst

Answer 14

speed up a reaction
used to make product faster
used to lower the temperature required for a reaction
saves energy and money

Question 15

heterogeneous catalyst

Answer 15

catalyst that is in a different phase to the reactants
catalyst is in a different state to the reactants
Haber process:
N2(g)+3H2(g) -> 2NH3(g)
solid iron catalyst is used which is a different state to gaseous reactants

Question 16

increasing surface area if heterogeneous catalyst

Answer 16

increases rate of reaction because more particles can react with the catalyst at the same time

Question 17

homogeneous catalyst

Answer 17

catalyst that is in the same phase as reactants
generally aqueous in aqueous reactants
sulphuric acid used to make esters

Question 18

how do homogeneous catalysts from intermediate species?

Answer 18

homogeneous catalysts from intermediate species by reactants combining with catalyst which react to form products - the catalyst is reformed again

Question 19

catalyst effect on rate of reaction

Answer 19

catalyst lowers activation energy so there is a greater proportion of particles have greater than the activation energy
(reduced activation energy, curve is the same)

Question 20

why are catalysts used in industry?

Answer 20

1. lower the temperature needed for reaction, less money spent and less CO2 produced
2. speed up reaction by providing alternative pathway
3. change properties of product

Question 21

environmental benefits of catalysts

Answer 21

1. lower temperatures and pressures are required, reduces energy and reduces CO2 produced
2. less waste produced as catalysts can allow scientists to use reactions with better atom economies

Question 22

catalytic converters

Answer 22

used in cars to reduce levels of pollution
made from rhodium, platinum and palladium alloy:
2CO+2NO -> 2CO2+N2

Question 23

measuring rate:
1. how long it takes for precipitate to form
2. amount of mass lost
3. volume of gas produced

Answer 23

1. place cross on paper and time how long it takes for cross to disappear (ppt to form)
   problem - difficult to know when cross disappears due to perception
   solution - use same observer to reduce errors
2. place reaction on balance and measure the mass loss of gas - use fume cupboard if toxic gas is produced
3. measure amount of gas produced using gas syringe - measure over specified time

Question 24

calculating rate of reaction from graph

Answer 24

draw gradient
gradient = change in y / change in x
bigger section of graph, the better