Physical Unit 1.9: Rate Equations (A2 Kinetics)

Question 1

define rate of reaction

Answer 1

the change in concentration of a reactant or product per unit time

Question 2

finding rates using gradients

Answer 2

tangents

Question 3

by experiment, what can the rate of reaction be related to?

Answer 3

the concentration of reactants by a rate equation

Question 4

what is the form of a rate equation?

Answer 4

rate = k[A]^m[B]^n
where m & n are the orders of reaction with respect to the reactants A & B
k is the rate constant

Question 5

m & n are either 0, 1 or 2

Question 6

what factor affects the value of the rate constant k & what is the formula?

Answer 6

temperature
k = Ae^(-Ea/RT)
A is the Arrhenius constant
Ea is activation energy
T is temp. in K

Question 7

define order of reaction

Answer 7

order of reaction with respect to a given reactant = the power to which the concentration of the reactant is raised in the rate equation

overall order of reaction= the sum of powers of the concentration terms in the rate equation
m + n

Question 8

finding orders using initial rates

Answer 8

1- find 2 experiments where the conc. of one reactant stays the same
2- see relationship b/w changing conc. of other reactant & initial rate
3- find 2 experiments where the conc. of other reactant stays the same OR multiply the initial rate by the calculated order of the first reactant then see relationship b/w this value –> initial rate & change in conc. of other reactant

see booklet

Question 9

define rate constant

Answer 9

constant in rate equation & give example
when the reaction temperature is constant

Question 10

units of k

Answer 10

see booklet

Question 11

calculation using rate equation

Answer 11

see booklet

Question 12

what is the qualitative effect of changes in temp. on the rate constant k?

Answer 12

as temp. increases, rate constant k increases
increase of 10C roughly doubles k

Question 13

calculation using k = Ae^(-Ea/RT)

Answer 13

see booklet

Question 14

useful rearrangements of k = Ae^(-Ea/RT)

Answer 14

lnk = -Ea/RT + lnA

Ea = RT(lnA-lnk)

Question 15

use lnk = -Ea/RT + lnA with experimental data to plot straight line graph

Answer 15

graph 1/T vs lnk
gradient is -Ea/R - straight line downwards
y-intercept is lnA

Question 16

what do the concentration-time graphs look like for 0, 1st & 2nd order (w respect to a reagent)?

Answer 16

0:
straight line
constant rate & gradient

1st: curve
decreasing rate

2nd: curve
‘more curved than 1st’
more rapidly decreasing rate

see booklet

Question 17

what do the rate-concentration (w respect to a reagent) graphs look like for 0, 1st & 2nd order?

Answer 17

0: straight horizontal line

1st: straight line from origin upwards

2nd: upwards curve from origin

Question 18

how can rate-concentration graphs be obtained?

Answer 18

initial rates method
continuous rates method

Question 19

compare the principle of initial rates method to continuous method

Answer 19

see table in booklet

initial:
do several reactions starting with different concentrations of reactants & measure the rate of each
plot rate vs conc. (x)

continuous:
measure the rate at several points during one reaction
plot conc. (y) vs time (x)
plot rate (from gradients) vs conc. (x)

Question 20

compare the gas production of initial rates method to continuous method

Answer 20

initial:
measure volume/mass of gas produced v time & plot graph then find gradient at start, to which rate is proportional
or
measure time taken for specific vol./mass gas produced early on in the reaction - rate is proportional to vol. or mass / time

continuous:
measure the vol./mass of gas produced vs time & find gradient at several point throughout experiment, to which rate is proportional

Question 21

compare the colour change of initial rates method to continuous method

Answer 21

initial:
use colorimeter to measure time taken to reach specific point in reaction e.g. specific absorbance
rate = 1/t

continuous:
use colorimeter to measure absorbance at several times during the reaction - proportional to rate
plot absorbance vs time (x) & rate is proportional to gradient at different points
(conc. found by calibration curve)

Question 22

compare the quenching of initial rates method to continuous method

Answer 22

initial:
add large vol. of water (to decrease conc.) to stop reaction/rate = 0
calculate rate by change in conc./time

continuous:
at regular time intervals, remove sample from reaction mixture & add large vol. of water (to decrease conc.) to stop reaction/rate = 0
plot graph of conc. vs time & calculate rate by finding gradient at dif. points

Question 23

define rate-determining step

Answer 23

the slowest step of the reaction
determines the overall rate of reaction
name the specific species & the # of times they have ‘been used’ & link to rate equation
e.g. ‘step 1 includes __ & __ AND these are also in the rate equation’

Question 24

notes about rate-determining step

Answer 24

only the species involved in steps up to & including RDS appear in rate equation

species only involved after RDS do not appear

Question 25

how to find RDS

Answer 25

find rate equation by experiment then use rate equation to deduce possible mechanisms catalyst appears in rate equation but not in overall chemical equation RDS is step in which you have met the reactant(s) in the rate equation the required number of times & BE SPECIFIC TO Q IN ANSWER see booklet

Question 26

explain qualitatively why doubling the temp. has a greater effect on the rate of the reaction than doubling the concentration of E

Answer 26

reaction occurs when molecules have E > Ea doubling T by 10 °C causes many more molecules to have this E > Ea whereas doubling [E] only doubles the number with this E > Ea

Question 27

describe the graph of k vs T (temp.)

Answer 27

curve upwards exponential relationship

Question 28

when investigating the order of a reaction with respect to reagent A, why can the order with respect to B be ignored?

Answer 28

because B is in excess so the concentration of B is constant