STPM Chemistry Form 6 Notes – Reaction Kinetics (Part 2)

by BerryBerryTeacher

in Berry Reference (Notes)

This post is a follow-up to the mini definition list on reaction kinetics from Berry Berry Easy. So here we present you Part 2 of STPM Form 6 Chapter on Reaction Kinetics. In this part, you’ll learn about nucleophile and electrophile. (For your information, ‘phile’ means tendency to ‘like’) But more importantly, you’ll be exposed to the all important reaction order and reaction mechanisms.

(Tips: Do test yourself with the example at the end of the post. Try not to look at the answer first. That is a typical question on reaction mechanism. If you can answer it, try to answer other similar questions from revision books.)

STPM Chemistry Form 6 Notes – Reaction Kinetics (Part 2)

Nucleophile – Lewis bases / a species that attacks a positively-charged (electron deficient) carbon atom by donating an electron-pair to form a dative covalent bond.

Electrophile – Lewis acids / a species that attacks a negatively-charged (electron rich) carbon atom by accepting an electron-pair to form a dative covalent bond.

Important Notes in Organic Chemistry:

  • If the concentration of the nucleophile influences the rate of the reaction which means the rate determining step involves the nucleophile attacking the electrophile to form the transition state that evolves into product (SN2 reaction mechanism = common in organic chemistry).
  • If the concentration of the nucleophile shows no effect on the rate of reaction, the reaction is SN1 reaction mechanism.
  • Both reaction mechanisms show a rate dependence on the electrophile concentration.

Reaction Order

  • There are 3 reaction orders: zero, first and second.
  • Zero order reaction proceeds at a constant rate, independent of reaction concentration.
  • First order reaction depends only on one reactant (SN1 reaction).
  • Second order reaction depends on two reactants (SN2 reaction).
Zero order First order Second order
Rate law rate = k rate = k[A] rate = k[A]2
Half-life t1/2 = [A]o / 2k t1/2 = ln 2 / k t1/2 = 1 / k[A]o
Unit of rate constant mol dm-3 s-1 s-1 mol dm3 s-1

Reaction Mechanisms

First order Second order Second order
The slowest step involves one molecule breaking apart (its rate determining step). The slowest step involves two molecules forming a bond (its rate determining step). The slowest step involves two molecules forming a bond (its rate determining step).
A –> B + C A + B –> C 2A –> B + C
One step reactionDissociative mechanism One step reactionAssociative mechanism One step reactionAssociative mechanism
Rate = k[A] Rate = k [A][B] Rate = k [A]2

Not all reactions are concerted. There are reactants that form intermediates (or activated complexes) before the reactants form products and give a multi-step reaction.

Example 1

Reaction:

NO2(g) + CO(g) –> NO(g) + CO2(g)

Mechanism:

NO2(g) + NO2(g) –> NO(g) + NO3(g) (slow)
NO3(g) + CO(g) –> NO2(g) + CO2(g) (fast)

Which of the following options is the rate law for this reaction, assuming that this reaction mechanism is correct?

A. k (PNO2) (PCO)
B. k (PNO2)2 (PCO)
C. k (PNO2)2
D. k (PNO2)

Answer: C

Solution: The slowest step in the reaction mechanism is the rate determining step. The rate depends on the two molecules of NO2 gas and CO has no effect on the rate of this reaction which means the addition of CO does not increase the rate of reaction.

In the next part (Part 3), you’ll learn all about collision theory, rate constant and rate law,first order reactions, second order reactions and zero order reactions. These are very important and would require knowledge learned from this post. So try and master this before moving on.

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