Rates

Question 1

Rate of reaction

Answer 1

The change in concentration per unit of time

Question 2

Activation energy

Answer 2

Minimum energy required for a reaction to occur

Question 3

Why does a higher temperature increase the rate of reaction

Answer 3

Increase kinetic energy, more successful collisions
Greater frequency of successful collisions

Question 4

Why does higher conc/pressure increase the rate of reaction

Answer 4

More particles within the space
More collisions
Frequency of successful collisions

Question 5

Why does breaking a solid into smaller particles increase the rate?

Answer 5

Increase surface area
More collisions
Greater frequency of successful collisions

Question 6

What is a catalyst

Answer 6

Substance with increases the rate of reaction without being used up

Question 7

How does a catalyst work

Answer 7

Provides an alternative route with a lower activation energy

Question 8

Rate equation

Answer 8

Rate = change in concentration/time

Question 9

What is zero order

Answer 9

Changing the concentration of a reactant has no effect on the rate of reaction

Question 10

What is first order

Answer 10

Concentration of reactant is directly proportional to the rate
[A] x2 = rate x 2

Question 11

What is second order

Answer 11

Rate proportional to [reactant]^2
[A] x2 = rate x 4

Question 12

Describe conc vs time graphs

Answer 12

Initially there is a high conc of reactants so a large frequency of collisions
As reactants start to get used up successful collisions become less frequent
Once one or more reactants are used up there can be zero successful collisions

Question 13

What are the two types of experiments that can be done to determine the order with respect to different reactants

Answer 13

Continuous monitoring method (following course of a single reaction)
Initial rates method (doing multiple experiments)

Question 14

What are the two ways to measure the progress of a reaction in continuous monitoring method

Answer 14

1 - by taking samples at regular intervals
2 - by using a visible indicator/physical property such as gas volume

Question 15

Continuous monitoring method enables you to plot a…

Answer 15

…concentration time and tangent graph CTT

Question 16

From the concentration time tangent graph how do you work out initial rate

Answer 16

Draw a tangent at t=0
Work out the gradient using change in y over change in x

Question 17

How do you measure the rate after a certain time

Answer 17

Draw tangent at t = 600s (example)
Work out gradient using change in y over change in x

Question 18

How do you work out orders from CTT graph

Answer 18

Take 2 tangents at different concentrations
Compare factor of change in concentration then factor of change in rates (gradients that you calculated)

Question 19

How to carry out the iodine clock initial rates practical

Answer 19

1) measure known volumes of reactants. Keep them separate so reactant doesn’t start
2) measure known volume of sodium thiosulphate
3) start the stop clock when the last of reactants is added to conical flask. Do not add sodium thiosulphate last
4) stop timer when mixture turns blue/black and record
5) repeat the experiment using same volume and altering volume of KI
6) Rate = 1/time
7) plot a graph of 1/t against volume of potassium iodide

Question 20

How to work out orders using initial rates experiment data

Answer 20

Compare the difference in concentrations to the difference in the rates

Question 21

What is the Arrhenius equation

Answer 21

K = Ae ^(-Ea/RT)

K ~ rate constant
A ~ Arrhenius constant
e ~ e button on calculator
Ea ~ activation energy
R ~ molar gas constant
T ~ temperature

Question 22

What can you rearrange the Arrhenius equation to

Answer 22

lnk = lnA - Ea / RT

Question 23

How can you use Arrhenius equation and compare it to equation for straight line graph

Answer 23

When written as lnk = -Ea/RT + lnA
It has same form as Y = mx + c
Y = lnk
M = -Ea/R
X= 1/t
C= lnA

Ink = -(ᵉᵃ⁄ᵣ) ¹⁄ₜ + lnA

Question 24

How do you work out the activation energy from the gradient

Answer 24

Calculate the gradient using ∆y/ ∆x
then x R and divide by 1000

Question 25

What is meant by rate determining step

Answer 25

When a chemical reaction takes place there is always one step which is slower than the others

Question 26

The species that take part in the rate determining step must…

Answer 26

….match the species in the rate equation

Question 27

Example for rate determining:

(CH3)3CBr + OH- ——> (CH3)3COH + Br-

step 1: (CH3)3CBr ——> (CH3)3C+ + Br-
step 2: (CH3)3C+ + OH- ———> (CH3)3COH

what is the rate determining step

Answer 27

step 1 depends on (CH3)3CBr whereas step 2 depends on (CH3)3C+ and OH-.
As step 1 is dependent on the same species as the rate equation step 1 must be the rate determining species