5.1.1 How fast?

Question 1

rate of reaction

Answer 1

change in the concentration of reactants or products per unit time (moldm-3s-1)

Question 2

half-life

Answer 2

the time taken for half of a reactant to be used up

Question 3

rate-determining step

Answer 3

the slowest step in a multi-step mechanism

Question 4

rate equation

Answer 4

rate=k[A]^m[B]^n

Question 5

zero order means

Answer 5

concentration of reactant has no effect on rate

Question 6

first order means

Answer 6

concentration of reactant is proportional to rate

Question 7

second order means

Answer 7

concentration of reactant squared is proportional to rate

Question 8

what kind of graph do you use to determine half life?

Answer 8

concentration time

Question 9

for a first order reaction, what are the two methods of calculating rate constant (k)?

Answer 9

• from the gradient of a tangent

- from the half life, using k=ln2/t 1/2

Question 10

what does a conc/time graph of zero order look like?

what’s the half life?

Answer 10

straight line with negative gradient

half life is decreasing

Question 11

what does a conc/time graph of first order look like?

what’s the half life?

Answer 11

exponential decay line

constant half life

Question 12

what does a conc/time graph of second order look like?

what’s the half life?

Answer 12

even steeper than first order exponential decay line

increasing half life

Question 13

what does a rate/conc graph of zero order look like?

how do you determine k from the graph?

Answer 13

a horizontal line

k=rate

Question 14

what does a rate/conc graph of first order look like?

how do you determine k from the graph?

Answer 14

a straight line through the origin

k=gradient

Question 15

what does a rate/conc graph of second order look like?

how do you determine k from the graph?

Answer 15

upward curve with increasing gradient

k can’t be determined straight from graph, but if you draw rate^2/conc, k=gradient

Question 16

3 methods of continuous monitoring

Answer 16

• gas collection
• mass loss
• colorimetry

Question 17

what does a colorimeter measure

Answer 17

the amount of light absorbed by a coloured solution.
Absorbance is recorded, and is directly proportional to concentration of solution. from this you can draw conc/time graphs

Question 18

how do you do an iodine clock procedure? (initial rates method)

Answer 18

• measure the time t from the start of experiment for the blue-black colour of iodine and starch to form
• it can be assumed that the average rate of reaction over this time is the same as the initial ROR
• initial rate ∝ 1/t
• plot a graph of 1/t / concentration (rate/conc) to determine order, then write rate eq. and calculate k.
• repeat for many different concentrations of reactant

Question 19

how does increasing the temperature affect the rate and therefore rate constant?

Answer 19

increasing temp increases rate constant exponentially

• Boltzmann distribution shifts to the right so a greater proportion of particles exceed Ea
• More KE so particles move faster and collide more frequently
• -change in rate is mostly determined by Ea
• -The Arrhenius equation is used to calculate how k changes with T

Question 20

What is the Arrhenius equation used to calculate?

Answer 20

how k changes with T

Question 21

Arrhenius equation and logarithmic form

Answer 21

k = Ae^-Ea/RT
lnk = -Ea/RT + lnA

Question 22

What do you plot to determine Ea and A graphically (arrhenius)

Answer 22

Plot lnk/ 1/T
gradient = -Ea/R
intercept = lnA