Rates of Reactions

Question 1

For each 10 degrees rise in temp in temp the k constant…… and the rate of reaction………

Answer 1

Doubles

Doubles

Question 2

Two factors that contribute to increased rate and rate constant

Answer 2

Increase temp,shifts botzmann to the right increasing proportion of particles that exceed Ea
As temp increases particles move faster and collide more frequently
Increased temp, increased frequency of collisions is comparatively small to increased in proportion of molecules that exceed Ea from shift in Boltzmann
So change in rate determined mainly by Ea

Question 3

How can rate constant be calculated at different temps

Answer 3

Carrying out same experiment at different temperatures

Question 4

The Arrhenius equation

Answer 4

Exponential relationship between rate constant k and temperature T

See card 4

Question 5

What does exponential factor represent

Answer 5

Proportion of molecules that exceed Ea and have sufficient energy to react

Question 6

Pre exponential term (frequency factor) A takes into account…

Answer 6

Frequency of collisions with correct orientation
This term slightly increases with temp as freq of collisions increases but it is constant over a small temp range
Freq factor gives rate if there were no Ea

Question 7

Logarithmic form of Arrhenius equation

Answer 7

See card 5
Useful because it enables both Ea and A to be determined graphically
A plot of lnK against 1/T gives straight line graph of type y=Mx+c

Question 8

Plot of lnK against 1/T gives….

Answer 8

See card 6
Downward straight line with
M:-Ea/R
C: lnA on y axis

Question 9

How to use Arrhenius equation

Answer 9

1. Calculate values of lnK and 1/T from data
2. Plot graph of lnK against 1/T
3. Calculate Ea from gradient of straight line
4. Calculate lnA from intercept

Question 10

How does colorimeter work?

Answer 10

Form of continuous monitoring
Wavelength of light passing through coloured solution is controlled using a filter
The amount of light absorbed by solution is measured

Question 11

Example of analysing colorimètre

Answer 11

Prepare standard solutions of known concentration of coloured chemical
Select filter with complimentary colour
Zero colorimeter with water
measure absorbance readings 
plot calibration curve of absorbance against concentration 
take readings at time intervals 
plot second graph of conc against time 
Can find order of reaction

Question 12

Conc time graphs

Answer 12

See card 1

Question 13

Calculating rate constant from half Life

Answer 13

Make use of exponential relationship for a constant half life
See card 2

Question 14

Rate concentration graphs

Answer 14

See card 3

Question 15

Clock reaction?

Answer 15

Used to obtain initial rate
Time is measured from beginning of experiment for visual change to be observed
Provided no significant change in rate in this time assumed that avg rate of reaction = initial rate
Initial rate proportional to 1/t
Clock reaction repeated several times with diff concs
Graph of 1/t (proportional to rate) against conc plotted
Example of reaction: iodine +starch

Question 16

Accuracy of clock reactions

Answer 16

The shorter the period of time over which an avg rate is measured the less the rate changes over that time period
Still reasonably accurate provided less than 15 % of reaction has taken place

Question 17

Stoichiometry

Answer 17

Relative amounts of species in reaction

Question 18

How do u know reaction mechanism likely to be correct

Answer 18

Rate equation only includes reacting species involved in the rate determining step
The orders in rate equation match the number of species involved in rate determining step

Question 19

How to work out the mechanism

Answer 19

Establish the slow rate determining step and which molecules have no effect on the reaction rate so must be part of the fast step
Identify the intermediate and work out the 2 step mechanism
Then check the slow step matches the rate equation and the sum of steps gives the overall equation