Chemical Industry-5: Rate Determining Step

Question 1

Describe a mechanism in terms of steps

Answer 1

A mechanism for a reaction may only involve one step, or could actually involve a series of steps

Question 2

When a reaction has a series of steps what is the overall rate dependent on

Answer 2

The rate of the slowest step

think only as strong as your weakest link

Question 3

What is the slowest step of a reaction mechanism called

Answer 3

Rate Determing step ( RDS)

Question 4

What does RDS stand for and define it

Answer 4

Rate Determing Step = slowest step in a reaction mechanism

Question 5

What are the 2 rules that use RDS for determing which species appear in the rate equation

Answer 5

1. Species that appear in steps up to and including the RDS = IN rate equation
2. Species that are only involved after the RDS = NOT in rate equation ( anything after RDS = 0 order)

Question 6

What is the general rule for the number of molecules/ atoms in each step

Answer 6

• usually only 2 molecules/ atoms are involved in each step

- 3 or more molecules reacting with the right orientation and activation energy is highly unlikely

Question 7

What is the rule for Catalysts and rate equation/ RDS

Answer 7

A catalyst will appear in the rate equation but not in the overall chemical equation for the reaction ( i.e a catalyst will always cancel out in the steps)

Question 8

How do you check whether a proposed mechanism is correct

Answer 8

-overall reaction after cancelling out the same species on both sides results in starting equation

( reactions add to overall equation)

Question 9

Be able to determine which step is the RDS in a given mechanism and which mechanisms match the rate equation

Answer 9

Look at notes ( 1st should always include at least one of the species in the rate equation)