5A - Rates

Question 1

Define Rate of reaction

Units = ?

Answer 1

Rate of reaction = The change in concentration of a reactant/ product in a given time

Units = moldm^-3S^-1

Question 2

Rate equation = ?

Answer 2

Question 3

Define Order of reaction

Answer 3

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

It’s always 0,1 or 2

Question 4

Define Rate constant (k)

Answer 4

The constant proportionality between the rate of the reaction and the cconcentration of reactants

Question 5

Define Overall order

Answer 5

Overall order = Sum of the individual orers of reactants in the rate equation

Question 6

Draw the graphs for 0, 1^st and 2^nd order:

Conc/ rate

Answer 6

Question 7

Name 3 methods of monitoring a chemical reaction continuously

Answer 7

• Gas collection
• Mass collection
• Colorimeter
  
  • Wavelength of light passing through a solution is controlled by a filter that’s the complementary colour to the solution
  • Amount of light absorbed by solution is measured

Question 8

Define Half life

Answer 8

Half life = The time taken for the concentration of a reactant to decrease by half

Question 9

For 0, 1^st and 2^nd reactions - Draw the graphs to show their change in half life over time

Answer 9

Zero order - Decreases

First order - Remains constant

Second - Increases

Question 10

Equation:

k = ?

When you have the half life?

What is particular this equation?

Answer 10

k = ln2 / t_1/2

Where t_1/2 is the half life

NOTE:

Only works if you have overall reaction of first order

Question 11

Define initial rate of reaction

Answer 11

The change in concentration of a reactant or product per unit time at t = 0

Question 12

Why do you calculate the initial rate at t = 0?

Answer 12

That’s the only time when you definitely know the [Reactants]

Question 13

Effect on the rate constant:

• Increase temperature
• Decrease temperature
• Adding a catalyst

Answer 13

Question 14

What is a clock reaction used for?

Answer 14

To measure the initial rate of a reaction - but the value measured is NOT the initial rate, it is the average rate of the first part of the reaction

Question 15

How the end of a clock reaction usually signalled?

Relatonship between rate and time measured here?

Answer 15

A colour change

Rate is proportional to 1/t (inverse proportion)

Question 16

Define Rate determining step

Answer 16

Rate determining step = Slowest step in a mechanism

Question 17

Name the 5 different types of mechanisms you need to know and what type of compound undergoes that mechanism for reactions?

Answer 17

Free radical substitution - Alkanes

Electrophilic addition - Alkenes

Electrophilic substitution - Benzene (aromatic compounds)

Nucleophilic substitution - Haloalkanes/ anything polar

Nucleophilic addition - Carbonyls (C=O)

Question 18

3 things a mechanism must satisfy in order to be plausible

Answer 18

1. Slowest step must be consistent with rate equation
2. When you add up theequations you must get the full equation
3. Can’t have more than 2 things colliding at once

Question 19

Writing rate equations and for one step mechanisms?

Give an example

Answer 19

If there’s 1 step in the mechanism then that must be the rate determining step ∴ all reactants must be present in the rate equation

E.g. Nucleophilic substitution

Question 20

Arrhenius equation = ?

1. Normal form (on data sheet)
2. Logarithmic form (LEARN)

Units = ?

Answer 20

k = Rate constant

A = Frequency factor

Ea = Activation energy/ KJmol^-1

R = Gas constant (on data sheet)

T = Temperature/ K

Question 21

What is used to measure the effect of temperature on the rate of reaction?

Answer 21

Arrhenius equation

Question 22

What does a Arrhenius equation graph look like?

How do you work out Ea from Arrhenius?

Answer 22

Question 23

Effect of Arrhenius on temperature on rate constant

• Increase temperature
• Increase in Ea
• Reduction in A

Answer 23

Increase temperature - Increase k

Increase Ea - Decrease k

Reduction in A - Decrease k