kinetics 2 p2

Question 1

rate of reaction

Answer 1

change in concentration (of any reactant or product) per unit time
state what is being monitored (usually production of a product)

Question 2

rate of reaction equation

Answer 2

rate of reaction = change in concentration / change in time

Question 3

how could you measure the rate of reaction experimentally?

Answer 3

colorimeter at suitable intervals if there’s a colour change
use a gas syringe to collect volume of gas evolved, or measure change in mass
titration to time how long it takes for the end-point to occur

Question 4

how would you measure reaction rate for really fast reactions?

Answer 4

use a flash of light to break bonds, use probe flashes to record amount of light absorbed by a species; this can show its concentration
can monitor reactions that occur in times as fast as 10-12 seconds

Question 5

how can you determine the rate constant and rate expression for a reaction?

Answer 5

only experimentally

Question 6

what affects the value of the rate constant for a given reaction?

Answer 6

temperature, nothing else

Question 7

generic rate expression?

Answer 7

rate = k[X]x[Y]y
k = rate constant
[X] and [Y] are concs of species X and Y
x and y are the orders of reaction with respect to X and Y

Question 8

do species need to be in the chemical equation to be in the rate expression?

Answer 8

no - species in the chemical equation may be excluded and species not in the chemical equation e.g. catalysts, may be included

Question 9

overall order of reaction?

Answer 9

the sum of the orders of reaction of all species in the rate expression

Question 10

how do you calculate the units of the rate constant?

Answer 10

units of rate are mol dm-3s-1 and units of concentration are mol dm-3
rearrange rate equation to get k =
sub in units and cancel them out

Question 11

how would you draw a rate concentration graph?

Answer 11

plot [A] (y) against time (x), draw tangents at different values —> draw a secondary graph of rate (y) against [A] (x)

Question 12

what does a rate concentration graph look like for a zero order reactant?

Answer 12

straight horizontal line

Question 13

what does a rate concentration graph look like for a first order reactant?

Answer 13

x=y
directly proportionate

Question 14

what does a rate concentration graph look like for a second order reactant?

Answer 14

kind of like half a parabola starting from origin

Question 15

what does a concentration time graph look like for a zero order reactant?

Answer 15

negative gradient
directly proportionate

Question 16

what does a concentration time graph look like for a first order reactant?

Answer 16

1/x vibe (only + quadrant)
above x axis

Question 17

what does a concentration time graph look like for a second order reactant?

Answer 17

steeper version of the first order

tell the difference since the conc time graph for first order reactant has a constant half life

Question 18

how can you find the rate expression using the initial rate method?

Answer 18

series of experiments, during which you vary concentrations, so the conc of just one reactant changes each time

(usually a graph question - work out order based on results)

Question 19

what must you add to react with the I2 as it is produced for an iodine clock reaction?

Answer 19

known moles of sodium thiosulfate and a little starch
reacts with I2 in 1:2 ratio
I2 + 2S2O3 2- —> S4O6 2- + 2I-

Question 20

when does the starch turn a blue-black colour in an iodine clock reaction and why?

Answer 20

when all the Na2S2O3 has been used up and so I2 is produced which reacts with starch leading to a blue black colour

Question 21

how can you calculate the rate of reaction from the data from an iodine clock reaction?

Answer 21

record time taken for colour change to occur
use rate = 1/t
effectively the initial rate

Question 22

what is the effect of a 10K temperature increase on the rate of reaction, roughly?

Answer 22

doubles rate of reaction

Question 23

what is the Arrhenius equation? what does each term mean?

Answer 23

k = Ae ^ -Ea/RT
k - rate constant
A - number of collisions between reactant molecules
R - gas constant
Ea - activation energy in J

Question 24

how can you convert the Arrhenius equation into a useful form for plotting a graph?

Answer 24

ln k = -Ea/RT + ln A

graph of lnk (y) against 1/T (x) is a straight line
gradient = -Ea/R and y intercept is lnA

Question 25

what is the rate determining step?

Answer 25

the slowest step in a reaction mechanism, which determines the overall rate of reaction

Question 26

how does the rate determining step link to the species involved in the rate expression?

Answer 26

any species involved in the rate determining step appear in the rate expression
species only involved after the rate determining step do not appear in the rate expression

Question 27

what is an intermediate?

Answer 27

substance formed during a middle step of a chemical reaction
usually extremely reactive and short-lived