Topic 6 & 16 Kinetics

Question 1

outline some of the benefits and drawbacks to using mass loss methods

Answer 1

• use a cotton wool bung to prevent solution from escaping due to spitting whilst still allowing gas to escape
• not suitable for H2 as mass loss is not significant enough, so it is only suitable for heavier gases

Question 1

which methods can be used to measure rate of reaction

Answer 1

1. gas collection (i.e. gas syringe)
2. mass loss methods
3. measuring pH
4. colorimetry
5. conductivity
6. titration

Question 2

when using a colorimeter, what will the absorbances be for a darker and lighter color

Answer 2

darker color will have high absorbance, lighter color will have lower absorbance

Question 3

how and why does rate change over time

Answer 3

rate decreases because the limiting reactant is used up, causing its concentration in solution to decrease which means the frequency of collisions decreases, and hence so does the rate

Question 4

what does the gradient of the tangent at t = 0 tell you

Answer 4

gives the initial rate

Question 5

how would you calculate average rate

Answer 5

quantity being measured / time

Question 6

how would you calculate the instantaneous rate

Answer 6

draw a tangent line then calculate the gradient

Question 7

outline the collision theory

Answer 7

for reactions to take place, reactants must collide with one another with 1. proper orientation and with 2. sufficient energy to react (i.e. energy must be at or greater than Ea)

Question 8

how does concentration and pressure affect the rate

Answer 8

increases the number of particles in a certain volume, so the frequency of collisions between particles increases, so the rate of rxn inc as well (**Note: pressure only applies to gases)

Question 9

how does surface area affect the rate

Answer 9

increase SA means inc # particles exposed at the surface, increases the frequency of collision between other reactant particles and particles on surface of solid, so rate of rxn increases.

*powdered reactants react the fastest

Question 10

how does temperature affect the rate

Answer 10

particles have greater average kinetic energy, more frequency collisions between particles, greater proportion of particles with energy greater than or equal to the Ea, so ROR increases

Question 11

describe some features of Maxwell Boltzmann for two curves whereby T2 > T1

Answer 11

the curve at temp 2 will have its peak at a higher energy, but the peak will be lower than the graph for T1 because the area under the two curves must be equal.

Question 12

what does the area under the curve of a maxwell Boltzmann graph tell you

Answer 12

area is proportional to the total number of molecules

Question 13

catalyst

Answer 13

substance that increases the rate of a chemical reaction by providing an alternative pathway of lower activation energy without being used up

Question 14

how does lower Ea affect the rate

Answer 14

more particles have energy at or greater than the Ea, so a greater proportion of collisions will be successful

Question 15

which factors would affect the leveling off point for a reaction

Answer 15

if the concentration of the limiting reactant is changed, if the volume or mass of the limiting reactant is changed (because the moles of the reactant are being changed)

Question 16

on a conc v time graph, outline what happens to the half lives for 0,1,2 order

Answer 16

zero order: half-life decreases
1st order: half life remains constant
2nd order: half life increases

Question 17

what is the half-life

Answer 17

time taken for concentration of reactant to half

Question 18

what should you always make sure to draw in on the graph of rate vs conc for first and second order

Answer 18

the point (0,0)

Question 19

outline the variables for the arrhenius equation

Answer 19

k = Ae^(-Ea/RT)

where k is the rate constant
A is the frequency factor
Ea is the activation energy
R is the gas constant (8.31 in data booklet too)
T is temperature

Question 20

which two factors is the rate constant affected by

Answer 20

T and Ea

Question 21

how does an increase in temperature affect the rate constant

Answer 21

as T increases, -Ea/RT decreases in magnitude, so e^(-Ea/RT) becomes bigger/more positive, so Ae(-Ea/RT) increases, so k also increases and hence the rate increases

Question 22

how does adding a catalyst affect the rate (explain using the Arrhenius equation)

Answer 22

Ea decreases, so Ea/RT decreases in magnitude, so -Ea/RT is less negative, so k increases

Question 23

what would you plot on the x and y axis to find the activation energy using arrhenius

Answer 23

1/T on the x-axis and lnk on the y-axis
and then find the gradient of the line (-Ea/R) and then solve for Ea using gas constant

Question 24

what sign should the activation energy always end up being

Answer 24

positive

Question 25

what does lnA act as on the graph of lnk against 1/T

Answer 25

y-intercept

Question 26

what does the “A” in the arrhenius equation tell you

Answer 26

pre-exponential/frequency factor; takes into account the frequency of collisions with proper orientation and is constant as temp changes

Question 27

why is the frequency factor smaller for more complicated molecules

Answer 27

the molecules can collide in many different ways, and only a small proportion of the collisions will be successful

Question 28

which reaction takes place in a single step with a transition state

Answer 28

SN2

Question 29

which reaction is a multi-step process with an intermediate

Answer 29

SN1

Question 30

what is the rate equation

Answer 30

rate = k[A]^x[B]^y

where k is the rate constant and the overall order of the reaction is x + y

x is the order with respect to A
y is the order with respect to B

Question 31

what is the unit of rate

Answer 31

moldm^-3 s^-1

Question 32

outline how titrations could be used to measure the rate of reaction

Answer 32

for a neutralization reaction, take aliquots of the solution and then quench the reaction by putting them in the freezer to reduce KE and slow down the reaction so the concentrations reflect the concs of when it was aliquoted. the titre volume of the NaOH will decrease over time because the concentration of the acid should also be decreasing over time