CI - kinetics *02

Question 1

what is rate of reaction

Answer 1

the reaction rate is the change in the amount of reactants per unit time (normally per second)

Question 2

what is the rate constant - and what does it tell us

Answer 2

k

units vary so have to work them out

the bigger it is the faster the rate of reaction

Question 3

what are orders of reaction?

Answer 3

tells you how the reactant concentration affects the rate
rate is proportional to [A]

• need order of reaction with respect to reactant
• reaction also has an overall order which is the sum of the orders of the different reactants

Question 4

how do you find the order w.r.t a reactant?

Answer 4

look at how each reactant affects the rate one by one

if [A] changes and rate stays the same: 0 order w.r.t [A]

if [A] doubles and rate double (proportional): 1st order w.r.t [A]

if [A] double and the rate is 2^2 faster (triples is 3^2 faster): 2nd order w.r.t [A]

Question 5

order of reaction from concentration-time graphs

Answer 5

zero = straight line down

1st = slope down

2nd = steeper slope down

Question 6

order of reaction from rate-concentration graphs

Answer 6

zero = horizontal line

1st = straight line through the origin (proportional)

2nd = curve upwards

Question 7

rate equation

Answer 7

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

m+n = order w.r.t reactant

Question 8

does temperature affect the rate constant?

Answer 8

yes

Question 9

how does temperature affect the rate constant?

Answer 9

if you increase temperature you will increase the rate of reaction

• more kinetic energy
• particle speed up and are more likely to collide
• have the right activation energy to react so greater proportion will have enough energy to react

at a higher temperature the reaction will have a higher rate constant

Question 10

what is half life

Answer 10

the time taken for a reactant to halve in quanitiy

Question 11

how to you calculate half-life - from graphs

Answer 11

pick values and see how long it takes to half

eg.
0.1
0.05
0.25
and so on
if time taken to half similar then half life is constant

Question 12

orders of reaction and half-life graphs

Answer 12

zero - if rate doesn’t change as concentration falls (graph is a straight line)

1st - the rate decreases but t1/2 is constant (curved graph)

2nd - the half-life increases as reaction goes on (steeper curve)

Question 13

how can you find rate constant from the half-life of a 1st order reaction?

Answer 13

k = ln2 / t1/2

units = no units / s = s-1

find half-life from the graph

Question 14

how can you work out reaction rate from concentration-time graph?

Answer 14

draw a tangent

like the main curve bit

work out gradient = change in y / change in x

Question 15

how can you estimate the initial rate from a concentration-time graph

Answer 15

draw a tangent at the start!!

find gradient

Question 16

how do you work of the initial rate of a reaction?

Answer 16

time how long it takes to make a set amount of product to form at the beginning of the reaction

• conc. of other reactants isn’t changing
• temperature constant
• reaction hasn’t proceeded to far

initial rate = amount of reactant used or product form / time

Question 17

how can to follow the rate of reaction - experimental methods

Answer 17

pH measurement

gas volume

loss of mass

colour change

titration

Question 18

example of an initial rate experiment

Answer 18

clock reactions

Question 19

what are clock reactions

Answer 19

measure how much time takes for a set amount of product to form changes as you vary concentrations of one of the reactants

has an easily observable end point - a colour change - that tells you when desired product is formed

the quicker the clock reaction finishes the faster the initial rate

Question 20

clock reactions - iodine clock reaction

Answer 20

H2O2 + 2I- + 2H+ —–> 2H2O2 + I2

1. a small amount of sodium thiosulfate and starch are added to an excess of hydrogen peroxide and iodide ions in an acid solution
2. the sodium thiosulfate that is added to the reaction mixture reacts instantaneously with any iodine that forms: 2S2O3 2- + I2 —–> 2I- + S4O6 2-
3. to begin with all the iodine that forms in the first reaction is used up straight away in the second reaction. But once all the sodium thiosulfate is used up, any more iodine that forms will stay in solution so the starch indicator will suddenly turn the solution blue-black - this is the end point of the reaction
4. varying concentrations of the iodide ions or hydrogen peroxide will give different times for the colour change

Question 21

what does the arrhenius equation link?

Answer 21

an equation that links the rate constant and activation enthalpy

Question 22

that is the arrhenius equation

Answer 22

K = A e^-Ea/RT

K = rate constant 
Ea = activation enthalpy (Jmol-1)
T = temp. (k)
R = gas constant 
A = the pre-exponential factor (another constant) 

e = ex button on calculator

Question 23

arrhenius equation - as the activation energy gets bigger…..

Answer 23

…K gets smaller

Question 24

arrhenius equation - a large Ea means….

Answer 24

…a slow rate

not many particles have enough energy to react
so only a few collisions will result in a reaction

Question 25

arrhenius equation - as temperature rises….

Answer 25

…K increases

Question 26

arrhenius equation - logarithmic form

Answer 26

ln K = -Ea/R x 1/T + ln A

ln button on calculator

Question 27

arrhenius equation - gradient

Answer 27

-Ea/R

Question 28

arrhenius equation - y intercept

Answer 28

ln A

Question 29

arrhenius equation - arrhenius plot

Answer 29

plot ln K against 1/T

Question 30

arrhenius plot - finding activation enthalpy

Answer 30

find gradient of graph
gradient = -Ea/R
then -Ea = R x gradient
x -1 for Ea

Question 31

arrhenius plot - finding A (the exponential factor)

Answer 31

the y-intercept

then A = e^ln A

Question 32

what is the rate determining step?

Answer 32

the slowest step in a multistep reaction

the overall rate is determined by this

Question 33

what is a multistep reaction

Answer 33

reaction mechanisms can have a series of steps

Question 34

how can you use the rate equation to determine the rate determining step?

Answer 34

if a reactant appears in the equation, it must affect the rate
- so this reactant or something made in it must be in the step determining step

if a reactant doesn’t appear in the rate equation, then it isn’t involved in the rate determining step

Question 35

does that rate determining step need to be the first step in a mechanism

Answer 35

no

Question 36

can the reaction mechanisms be predicted from JUST the chemical equation

Answer 36

not usually

Question 37

what does the order w.r.t a reactant show about the rate determining step

Answer 37

shows the no. of molecules of that reactant that are involved in the rate-determining step