5.1.1 - How fast

Question 1

Order of a reaction

Answer 1

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

Question 2

1st order graphs

Answer 2

Slightly downward slope (conc vs. time graph)
Drops w/ a constant half life
Rate=conc; diagonal line in rate vs. conc graph

Question 3

2nd order graphs

Answer 3

Decline steeply then level off

Parabola in rate vs. conc graph

Question 4

2nd order

Answer 4

Rate is proportional to the square of the con. (upward slope)

Question 5

Zero order graphs

Answer 5

Constant decline in conc. (conc vs. time)
Rate is independent of []
Rate stays constant in rate vs. conc graph

Question 6

Overall order

Answer 6

Obtained by adding all the orders of the reactants

Question 7

Rate eqn.

Answer 7

Rate = k [A]^m [B]^n
Includes all reactants up to and including the rate determining step
May include catalysts

Question 8

Rate constant

Answer 8

A number that allows the equation to work.

It is unchanged by changes to concentration but will increase at higher temperatures

Question 9

Rate of reaction

Answer 9

Speed at which a reactant is destroyed
Speed at which a product is formed
conc/dm^3 s^1

Question 10

Half life of a reaction

Answer 10

Time it takes for the concentration of a reactant to halve

Question 11

Rate determining step

Answer 11

Reaction that occurs at a much slower rate and requires a much higher activation energy

Question 12

Finding k using [conc] vs time graph

Answer 12

k = ln (2)/ half life

Question 13

Units of k

Answer 13

s^-1

Increases w/ temp

Question 14

Finding k using rate vs conc graphs

Answer 14

Gradient = m = k

Question 15

Initial rates method

Answer 15

We always take the initial rate as that’s the maximum rate

Allows us to make a comparison of the effect of any changes of conditions as the rate changes during the reaction

Question 16

Colorimetry

Answer 16

Measures absorbancy (and loss of colour) and you can see how this changes with conc

Question 17

Units for zero order reaction

Answer 17

mol dm-3 s-1

Question 18

Units for first-order reaction

Answer 18

s-1

Question 19

Units for second order reaction

Answer 19

mol -1 dm3 s-1

Question 20

Units for third order reaction

Answer 20

dm6 mol-2 s-1

Question 21

Why is it better to measure a physical property than to do a chemical analysis of the reaction

Answer 21

Chemical requires reaction to stop

Physical does not interfere w/ progress

Question 22

Why do we use a little ‘clock reactant’ (PAG 10.1)

Answer 22

So that the colour change happens as close to the inital rate as possible as the colour change begins when all he S2O8 2- ions have been used up. It is only then the I2 can react w/ the starch

Question 23

Finding reaction mechanism

Answer 23

Mechanism adds up to overall
If species is both on reactants and products side, cancel out
Reactants in rate eqn are reactants of RDS

Question 24

Linking k and T

Answer 24

As T increases as does k and therefore so does the rate
If Ea is reduced, k increases
For every 10 degree rise in T, k is doubled as is the rate
T and k have a +ve exponential relationship

Question 25

Arrhenius eqn.

Answer 25

k = Ae^ -Ea/RT

A = pre exponential factor
Ea = activation enrgy (always +ve)
R = gas constant (8.314)
T = temperature (K)

Question 26

Finding Ea and A graphically

Answer 26

Plot 1/Temp against ln k
y - intercept = ln A
Gradient = -Ea/R

High Ea = steeper slope
Low Ea = shallower slope

Question 27

How to perform colorimetry w/ a clock reactant

Answer 27

Carry out titration w/ A in the burette and constant vol of E in the conical flask w/ C
Zero colorimeter
Select a suitable filter
As A is being added measure the absorbance at set intervals

Question 28

How to prove absorbance is proportional to the conc of E

Answer 28

Prepare standard sol of diff conc of E by serial dilutions

Measure the absorbance and plot abs vs [E]

Question 29

How would you know how many steps the reaction mechanism takes

Answer 29

Stoichiometry of the rate eqn doesn’t match stoichiometry of the overall eqn
Must happen in more than one step

Question 30

When to use colorimetry to determine rate

Answer 30

When you produce a coloured compound or using a clock reactant

Question 31

Finding rate from clock reactions

Answer 31

C has a colour change and E uses this up
Rate = dE/dt
Rate is proportional to 1/t
Repeat w/ diff conc of A/B

Question 32

Assumptions in clock reactions

Answer 32

E is used up before rate changes

Conc of A and B in such an excess that their conc doesn’t change significantly

Question 33

Why does absorbance decrease during clock experiments

Answer 33

C is coloured and [C] decreases