rate equations (physical)

Question 1

define the term rate of reaction

Answer 1

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

Question 2

at a given instant, how do you calculate rate of reaction

Answer 2

change in conc/change in time

Question 3

how could you measure the rate of reaction experimentally? (different methods)

Answer 3

use a colorimeter at different intervals if there is a colour change.
if gas is evolved, use a gas syringe to collect volume of gas evolved or measure change of mass in the reaction mixture

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 concentration
monitors reactions that occur between 10-12s

Question 5

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

Answer 5

only experimentally

Question 6

what factors effect the value of the rate reaction for a given reaction?

Answer 6

temperature, nothing else

Question 7

write a generic rate expression and explain what each term means

Answer 7

rate = k[X]x[Y]y;
k= rate constant for the reaction
[X] and [Y] = concentrations for X and Y
x and y = the orders of the reaction wrt 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 (catalysts) may be included

Question 9

define the term order of reaction with respect to a given product

Answer 9

the power to which a species’ concentration is raised in the rate equation

Question 10

define the term overall order of reaction

Answer 10

the sum of the orders of reaction of all species in the rate expression (e.g. total order = x+y)

Question 11

how would you calculate the units of the rate constant?

Answer 11

units of rate are moldm-3s-1 and units of conc are moldm-3
rearrange the equation to get k=
sub in units then cancel them out

Question 12

how would you draw a rate concentration graph?

Answer 12

plot [A] against time, draw tangents at different values -> draw a secondary graph of rate against [A]

Question 13

describe what a rate concentration graph would look like for a zero order reactant

Answer 13

straight horizontal line towards the top

Question 14

describe what a rate concentration graph would look like for a first order reaction

Answer 14

straight diagonal line from 0

Question 15

describe what a rate concentration graph would look like for a second order reactant

Answer 15

gradual curve upwards from 0
(you cant tell if it is 2nd order or an order greater than 2)

Question 16

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

Answer 16

do a series of experiments where you vary concentrations, so the concentration of 1 reactant changes each time.
plot a graph of concentration against time for each reactant and use a tangent t=0 to find the initial rate of the reaction.
compare rates and concentrations between each experiment to find order of reactants and overall rate equation

Question 17

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

Answer 17

known moles of sodium thiosulfate and a little starch; reacts with I₂ in 1:2 ratio
I₂+ 2S₂O₃²⁻ -> S₄O₆²⁻ + 2I⁻

Question 18

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

Answer 18

When all of the Na2S2O3 has been used up and so I2 is produced, which reacts with starch and causes a blue black colour

Question 19

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

Answer 19

record time taken for colour change to occur. se rate = 1/t . this effectively is the initial rate

Question 20

what is true of the half life of a first order reactant (conc against time graph)

Answer 20

half life is constant

Question 21

what is arrhenius equation and what does each term mean

Answer 21

k = Ae -ea/RT
k=rate constant for reaction
A=pre-exponential factor (number of collisions between reactant molecules)
e= mathematical quantity
R= gas constant
T= temperature
Ea = activation energy

Question 22

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

Answer 22

k=-Ea/RT + lnA
graph of lnk against 1/T is a straight line: gradient = -Ea/R and y-intercept is lnA

Question 23

what is the rate determining step

Answer 23

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

Question 24

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

Answer 24

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