18 - Rates Of Reactions

Question 1

Rate of reaction definition

Answer 1

The change in the amount of reactants and products per unit time

Question 2

Rate concentration equation

Answer 2

Rate (moldm-3s-1)= concentration (moldm-3) / time (s)

Question 3

Order definition

Answer 3

The power to which the concentration of a reactant is raised in the rate equation

Question 4

Overall order definition

Answer 4

The sum of the individual orders of reactants in the rate equation

Question 5

Rate constant definition

Answer 5

Links the rate of reaction with the concentration of reactants raised to powers of their orders in the rate equation

The bigger the constant the faster the reaction

Question 6

What does the order of a reactant tell you

Answer 6

The order of reaction with respect to a reactant tells you how the reactants concentration affects the rate

Question 7

What is the relationship between rate of reaction and concentration

Answer 7

The rate of reaction is proportional to the concentration of a reactant raised to a power

Question 8

When is a reaction zero order with respect to a reactant

Answer 8

When concentration of a reactant has no effect on the rate

The rate stays the same as concentration changes

Question 9

When is a reaction first order with respect to a reactant

Answer 9

When the rate depends on its concentration

And is directly proportional

Question 10

When is a reaction second order with respect to a reactant

Answer 10

When rate depends on its concentration raised by a power of 2

Question 11

Different methods of measuring rate of reaction

5

Answer 11

Measure volume of gas produced 
Measure mass lost 
Measure change in pH
Measure change in conductivity 
Use colorimetry to measure colour change

Question 12

What does the colorimeter measure

Answer 12

Measures the intensity of light passing through a sample
The filter is complementary to the absorbed colour
Absorbance is recorded - links to concentration

Question 13

Coloromitry practical

To get concentration time graph

Answer 13

1) prepare standard solutions = known concentrations of the coloured chemical
2) select a complimentary filter
3) zero the colorimeter
4) measure absorbante readings for each solution
5) plot a calibration curve = absorbance/concentration
6) carry out the reaction - take absorbance reading at timed intervals
7) use calibration curve to find concentration of iodine at each absorbance Reading
8) plot concentration time graph

Question 14

What can you tell from a concentration time graph

Answer 14

The orders of the reaction

Only if other reactants concentration remain constant

Question 15

Concentration time graph

Zero order details

Answer 15

Straight negative line

Gradient = rate constant

Question 16

Concentration time graph

first order details

Answer 16

Negative curve

Reaction slows

The time taken for concentration of reactant is halved
Half life is constant - used to find rate constant

Question 17

Concentration time graph

Second order details

Answer 17

Same pattern as first

Steeper at the start
And tails off slower

Question 18

Continuous monitoring definition

Answer 18

Repeatedly taking measurements during a reaction

Question 19

How to determine the rate constant from a first order concentration time graph

Answer 19

1) using a tangent, to find the rate, then using the rate = k() equation to rearrange and find k
2) use K= ln2 / half life

Question 20

Rate - Concentration graph

Zero order details

Answer 20

Straight horizontal line
Zero gradient

K= Y intercept

Question 21

Rate - Concentration graph

First order details

Answer 21

Straight line from origin
Rate is directly proportional to concentration

K = gradient

Question 22

Rate - Concentration graph

Second order details

Answer 22

Upward increasing curve

K = plotting second graph of rate against concentration2
Will be straight line through origin = gradient

Question 23

What is the initial rate definition

Answer 23

Instantaneous rate of the start of a reaction when t = 0

Question 24

How to measure initial rates practical

Answer 24

1) continuously monitor one reactant
2) plot a concentration time graph
3) repeat experiment using different initial concentrations of the reactants - keep the other reactants concentrations the same
4) use graphs to calculate initial rate for each experiment
5) repeat process for the other reactants

Question 25

Why is finding the clock reaction better than finding initial rates

Answer 25

More convenient
Only taking one measurement
Less chance of human error

Question 26

What are clock reactions

Answer 26

They estimate the initial rate of reactions by measuring the time taken for a set amount of product to form changes as you vary the concentration of the reactants. The end is usually indicated by a observational change.

Question 27

Clock reactions

What does the speed of the reaction to finish indicate

Answer 27

Faster end of reaction

Faster initial rate

Question 28

What are the assumptions you need to make for clock reactions
(4)

Answer 28

The temperature is constant
When the endpoint is seen the reaction has not proceeded too far
The concentration of each reactant does change significantly

-The rate of reaction stays constant during measurements

Question 29

Iodine clock reaction practical

Answer 29

1) sodium thiosulfate and starch added to excess of hydrogen peroxide and iodine ions in acid solution
2) sodium thiosulfate added to reaction reacts instantaneously with Iodine that forms
3) when thiosulfate is used up, solution turns blue black

Question 30

How accurate are clock reactions

Answer 30

Is accurate
Assuming rate of reaction is constant = initial rate

Providing that less than 15% of reaction has taken place

Question 31

What do clock reactions measure

Answer 31

Measures the average rate of a change in reactant during first part of reaction

Question 32

Reaction mechanism definition

Answer 32

The sequence of bond forming and bond breaking steps that shows the oath taken by electrons during a reaction

Question 33

Rate determining step definition

Answer 33

The slowest step in the reaction mechanism of a multi step reaction

Question 34

2 features used to predict reaction mechanisms

Answer 34

• the rate equation only includes the reacting species involved in the rate determining step
• the order of reaction with respect to a reactant match the number of species (molecules) involved in the rate determining step

Question 35

What is the effect of temperature on the rate of reaction and rate constant

Answer 35

An increase is temperature = increases in KE, increases speed of particles, leads to more frequent collisions

Also mean more particles would have an energy equal or greater than the Ea

As a result the rate of reaction would increase whilst the concentration of the reactants stayed the same

So the rate constant would increase exponentially

Question 36

What does the Arrhenius equation do

Answer 36

Links the rate constant and activation energy

Question 37

What is the Arrhenius equation

Answer 37

K = A e^ -Ea/ RT

Question 38

K = A e^ -Ea/ RT

What does each thing stand for and units

Answer 38

K= rate constant

A = pre exponential factor = frequency of collisions with correct orientation

Exponential factor = proportion of molecules that exceed Ea

Ea = activation energy (jmol-1)

R = gas constant = 8.314 (jk-1mol-1)

T = température (K)

Question 39

How to plot a suitable graph to calculate Ea and A

Answer 39

1) calculate rate - 1/temp
2) calculate ln k
3) plot lnk against rate
4) Gradient = - Ea / R
5) rearrange to find activation energy
6) find ln A = y intercept
7) use logs to find A

Question 40

Arrhenius equation sin Y=mx+c

Answer 40

lnK = lnA - Ea/R x 1/T