6.1 Rates of Reactions Flashcards
Chemical kinetics
the branch of chemistry that studies rates of reaction
Reaction Rate -
the change in concentration of a reactant or product over a period of time
The rate of appearance
the rate in which the concentration of product that appears over unit time
mol/L . s
The rate of disappearance
the amount of reactants consumed, or that disappears over unit time
factors affecting rate: temperature of a system
increasing temperature - increasing rate of reaction
factors affecting rate: concentration of reactants
Increasing the concentration of reactants will increase the opportunity for collisions to occur, thus increasing the rate of reaction
factors affecting rate: Increasing the pressure of the system
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
factors affecting rate of reaction: Presence of Catalysts
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
Biological catalysts
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
Heterogeneous Catalysts
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)
Homogeneous Catalysts
Catalyst is in the same phase, favouring more collisions (basically it will make reaction go faster), harder to reclaim
factors that affect reaction rates: Nature of the Reactants
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
factors that affect rates of reactions: Surface area
Larger surface area provides a higher likelihood of collisions and therefore a faster reaction
particle theory of matter
chemical systems consist of particles that are in constant and random motion
activation energy
minimum amount of energy that the reactants must have to start the reaction
How do Catalysts work
increase the rate of reaction by lowering the threshold of activation energy required
Mechanisms in a reaction
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
Molecularity
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)
Intermediates
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
Rate Determining Step -
the elementary step
that determines the rate of the overall
reaction; the slowest step in the
mechanism (discovered experimentally),
collision theory
chemical reactions can occur only if reactants collide with proper orientation and with enough kinetic energy to break reactant bonds and form product bonds
activated complex
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
Rate =
frequency of collisions x fraction of collisions that are effective
thing about reaction mechanisms
must agree with the experimentally determined rate law
elementary steps must add up to a balaned chemical equation similar to HESS’s law
