3.2.2 Reaction rates

Question 1

activation energy definition

Answer 1

minimum energy particles need to collide with for a reaction to proceed

Question 2

effect of concentration on rate of reaction

Answer 2

higher conc
more particles per unit volume
particles collide with greater frequency
higher frequency of successful collisions

Question 3

rate of reaction definition

Answer 3

change in concentration of a substance in unit time

Question 4

what does the gradient of a line represent on a conc time graph

Answer 4

rate of reaction

Question 5

initial rate

Answer 5

where reaction is fastest

Question 6

how do you calculate the rate from a graph

Answer 6

draw a tangent to the curve then change in y/ change in x

Question 7

catalyst definition

Answer 7

substance that increases the rate of reaction without getting used up, provide an alternative route or mechanism with an alternative pathway with a lower activation energy so more molecules can collide successfully

Question 8

2 types of catalysts

Answer 8

heterogeneous

homogeneous

Question 9

heterogeneous catalysis

Answer 9

in a different phase to the reactants

usually solids, reaction occurs at the surface of the catalyst

Question 10

homogeneous catalysis

Answer 10

where the reactants are in the same phase as the catalyst, reaction proceeds through an intermediate

Question 11

benefits of catalysts

Answer 11

speed up rate without getting used up
can lower temperatures and pressures used which can save energy costs and less CO2 burned
can enable different reactions to be used with better atom economy and reduced waste/less undesired products
often enzymes and generate very specific products

Question 12

what are the different ways to measure the rate of a reaction

Answer 12

change in volume of gas
change in mass 
titrate sample mixtures with acid, alkali sodium thiosulphate
colorimetry 
change in electrical conductivity

Question 13

measuring change in volume of gas

Answer 13

this works if there is a change in the number of moles of gas in an equation
can use a gas syringe to measure

Question 14

measuring change in mass

Answer 14

works if there is a gas that is allowed to escape

works better with heavy gases such as CO2

Question 15

titrating samples of reaction mixture with acid,alkali, sodium thiosulphate etc

Answer 15

1. small samples are removed from the reaction mixture
2. quench to stop reaction by diluting with water/cooling/adding a reagent that reacts with one of the reactants
3. titrate with a suitable reagent

Question 16

colorimetry

Answer 16

if one of the reactants or products is coloured then colorimetry can be used to measure the change in colours in the reaction mixtures

Question 17

measuring change in electrical conductivity

Answer 17

can be used if there is a change in the number of ions in an equation

Question 18

describe the Maxwell-boltzmann distribution curve

Answer 18

x axis: collision energy then has Ea drawn as a line from the curve down to the x-axis
y axis: fraction of molecules with energy
curve goes from 0 up to a peak then gradually flattens back down
NEVER TOUCHES 0 ON THE X AXIS

Question 19

what doesthe area under the maxwell-botlzmann curve represent

Answer 19

the number of particles present

Question 20

what must the maxwell-boltzmann curve never do

Answer 20

touch the x axis other than at the beginning

Question 21

how can a reaction go to completion if only few particles have energy greater than Ea

Answer 21

particles gain energy through collisions

Question 22

increasing temperature effect on the maxwell boltzmann curve

Answer 22

distribution shifts to have more molecules with higher energies
molecules have a wider range of energies
peak is lower
end bit is higher
area under the curve should remain constant

Question 23

explain the increasing temperature effect on the maxwell boltzmann curve

Answer 23

at higher temperatures the energy of the particles increases
collide more frequently and more often with energy greater than the activation energy, more collisions results in a reaction

as temperature increases the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy so the frequency of collisions increases

Question 24

effect of increasing surface area

Answer 24

increasing surface area will cause collisions to occur more frequently between the reactant particles and this increases the rate of reaction

Question 25

effect of catalysts

Answer 25

if the activation energy is lowered then more particles will have greater energy than the activation energy so there will be a higher frequency of effective collisions, reaction will be faster
need to draw the Ea line further up the curve