16. Rate expression & reaction mechanisms

Question 1

What is the rate equation and the reason it is like that

Answer 1

The equation => rate expression or rate law

k is experimentally determined constant - can only be changed if T is chnaged

Question 2

What is reaction order/overal order and how it is determined?

Answer 2

Reaction order to respect of a specific reactant - shows the influence of the reactant’s conc on the rate of the reaction

Overall order of a reaction - sum of reactant orders

If conc doubled:

• no change - 0 order
• double - 1st order
• 4 times - 2nd order

Question 3

Units of constant k in different reaction orders

Answer 3

Question 4

Explain zero order reactions

Answer 4

Change in conc of the reactant - no effect on the reaction rate

rate = k [A]⁰ = k

Reaction proceeds - first graph not stagnant but the concentration does not influence th rate

Question 5

Explain first order reactions

Answer 5

Rate is directly proportional to conc of reactant

rate = k [A]¹ = k [A]

The reaction procedes in a curved line because the decrease is constant and ends up being a higher proportion: decrease by 1mol so 1mol from 5mol is less proportionally than 1mol from 2mol

Question 6

Explain second order reactions

Answer 6

Rate isproportional to square of concentration of reactant

rate = k [A]²

Time-conc graph is curved because the conc decreases twice each time - 10mol to 5mol, 5mol to 2.5mol

Question 7

Distinguish zero, first and second order reaction graphs

Answer 7

Question 8

Which reaction order follows the half-life concept?

Answer 8

First order reactions follow half time (t_1/2) concept

How much time needed for the reactants to decrease by half? - it is independent of the starting concentration

Example: radioactive decay

Question 10

What is the reaction mechanism

Answer 10

Reaction mechanism - a series of steps which the reactants undergo to form products

Individual steps are elementary steps

Products of elementary steps are used in further steps - intermediates are formed

Question 11

What is molecularity? Types

Answer 11

Molecularity is the meausre which shows how many reactants are involved in an elementary step (individual step in reaction mechanism)

Unimolecular: single reactant

Bimolecular: two reactants

Termolecular: more than two reactants BUT rare because of chance of collision

Question 12

What is rate determining step

Answer 12

RDS is the slowest elementary reaction in the reaction mechanism - determines the overall rate of teh reaction

Rate law for the RDS is determined by reaction coeficients NOT orders

Question 13

When is reaction order and when equation coefficient should be used?