6.1 Rates of Reactions

Question 1

Chemical kinetics

Answer 1

the branch of chemistry that studies rates of reaction

Question 2

Reaction Rate -

Answer 2

the change in concentration of a reactant or product over a period of time

Question 3

The rate of appearance

Answer 3

the rate in which the concentration of product that appears over unit time
mol/L . s

Question 4

The rate of disappearance

Answer 4

the amount of reactants consumed, or that disappears over unit time

Question 5

factors affecting rate: temperature of a system

Answer 5

increasing temperature - increasing rate of reaction

Question 6

factors affecting rate: concentration of reactants

Answer 6

Increasing the concentration of reactants will increase the opportunity for collisions to occur, thus increasing the rate of reaction

Question 7

factors affecting rate: Increasing the pressure of the system

Answer 7

Related to Ideal Gas Law, at constant temperature the pressure is proportional to n/V
Therefore increasing the pressure is equivalent to increasing the concentration
When we increase the pressure with a constant amount, we are decreasing the volume or space that the molecules occupy
This increases the opportunity for collisions in the same way that concentration increasing will

Question 8

factors affecting rate of reaction: Presence of Catalysts

Answer 8

Catalysts are substances that increase the rate of reaction without being consumed
This helps to overcome mechanisms of reaction and can be regenerated in a future step
Reactions can occur without a catalyst but will much slower
Catalysts are not essential, and will do nothing to a reaction that was not already occurring

Question 9

Biological catalysts

Answer 9

are made by a living system
Best represented by enzymes that are produced by organisms to facilitate timely reactions
ex) digestion would occur much slower without enzymes to help breakdown complex molecules

Question 10

Heterogeneous Catalysts

Answer 10

Catalyst is in a different phase than the reactants
Favoured when you want to easily recover the catalyst from the reaction (eg reactants in liquid form but catalyst in solid form)

Question 11

Homogeneous Catalysts

Answer 11

Catalyst is in the same phase, favouring more collisions (basically it will make reaction go faster), harder to reclaim

Question 12

factors that affect reaction rates: Nature of the Reactants

Answer 12

Simple ions will have faster reactions than complex molecules that require multiple steps

Periodic trends also denote different reactivities resulting in faster or more vigorous reactions
ie) Alkali metals are more reactive down the group as atomic radii increases

Question 13

factors that affect rates of reactions: Surface area

Answer 13

Larger surface area provides a higher likelihood of collisions and therefore a faster reaction

Question 14

particle theory of matter

Answer 14

chemical systems consist of particles that are in constant and random motion

Question 15

activation energy

Answer 15

minimum amount of energy that the reactants must have to start the reaction

Question 16

How do Catalysts work

Answer 16

increase the rate of reaction by lowering the threshold of activation energy required

Question 17

Mechanisms in a reaction

Answer 17

step by step breakdowns of what is occurring
over the course of the reaction at a molecular level, each of these steps are called elementary steps or elementary reactions
➢ These are the steps that involve 1, 2, or 3 entity collisions that cannot be
explained by a simpler reaction

Question 18

Molecularity

Answer 18

The number of reactant particles involved in an elementary
reaction
➢ Unimolecular A → product (collides with the container)
➢ Bimolecular A + B → product (collides with other entities)
➢ Termolecular 2A+ B →product (Rare)

Question 19

Intermediates

Answer 19

the short-lived species that are produced and subsequently
consumed during the reaction sequence,
i.e. Step 1: A + B → Int
Step 2: Int + C → D + B

Question 20

Rate Determining Step -

Answer 20

the elementary step
that determines the rate of the overall
reaction; the slowest step in the
mechanism (discovered experimentally),

Question 21

collision theory

Answer 21

chemical reactions can occur only if reactants collide with proper orientation and with enough kinetic energy to break reactant bonds and form product bonds

Question 22

activated complex

Answer 22

an unstable arrangement of atoms containing partially formed and unformed binds that represents the max potential energy point in the change; also called the transition state

Question 23

Rate =

Answer 23

frequency of collisions x fraction of collisions that are effective

Question 24

thing about reaction mechanisms

Answer 24

must agree with the experimentally determined rate law
elementary steps must add up to a balaned chemical equation similar to HESS’s law