Chemistry midterm 2: Kinetics Flashcards
A rate is a
measure of how some property varies with time
The rate of reaction is
the change in the amount of a reactant or product per unit time. Reaction rates are therefore determined by measuring the time dependence of some property that can be related to reactant or product amounts
What is thermodynamics
spontaneity & position of equilibrium
What is kinetics
reaction rates & time to achieve equilibrium
Reaction rates can be influenced by
- The chemical nature of the reactants
- The physical state of the reactants
- The concentration of the reactants
- The temperature of the reactants
- The presence of catalysts
Before atoms react with each other, they must
Collide
What is collision theory
Theory that explains many observations regarding chemical kinetics, including factors affecting reaction rates
What are the 3 postulates of collision theory
- The rate of a reaction is proportional to the rate of reactant collisions (#collisions/time)
- The reacting species must collide in an orientation that allows contact between the atoms that will become bonded together in the product.
- The collision must occur with adequate energy to permit mutual penetration of the reacting species’ valence shells so that the electrons can rearrange and form new bonds (and new chemical species)
In addition to a proper orientation, the collision must also occur with
sufficient energy to result in product formation
what is an activated complex (transition state)
When reactant species collide with both proper orientation
and adequate energy, they combine to form an unstable species
why do most reaction rates increase as concentrations increase
With an increase in the concentration of any reacting substance, the chances for collisions between molecules are increased because there are more molecules per unit of volume. More collisions mean a faster reaction rate, assuming the energy of the collisions is adequate
What is the activation energy
The minimum energy necessary to form a product during a collision between reactants
If the activation energy is much larger than the average kinetic energy of the molecules
the reaction will occur slowly since only a few fast-moving molecules will have enough energy to reac
If the activation energy is much smaller than the average kinetic energy of the molecules
a large fraction of molecules will be adequately energetic and the reaction will proceed rapidly
What is a rate mechanism
- a series of individual chemical steps (called “elementary steps”) by which an overall reaction occurs.
- the slowest step in a reaction is called the “rate-
determining step” (like the narrowest portion of highway
401) and determines the rate law for the overall reaction
A reaction mechanism cannot be proved, it can only be
validated by the available data if
- The elementary steps in the mechanism add up to the overall reaction
- The rate law predicted by the mechanism is consistent with the experimentally determined rate law.
What is a catalyst
- a substance that can increase the reaction rate without being consumed in the reaction
- Does not appear in stoichiometric equation
- Ea with the catalyst is less than that of uncatalysed reaction
- A catalyzed reaction usually occurs in more elementary steps than an uncatalyzed reaction
What are homogenous catalysts
A homogeneous catalyst is present in the same phase as the reactants. It interacts with a reactant to form an intermediate substance, which then decomposes or reacts with another reactant in one or more steps to regenerate the original catalyst and form product
- Catalyst in same phase as reactants (dissolved)
- Usually small molecules or ions
- Can be protein (enzyme)
What is a heterogeneous catalyst
A heterogeneous catalyst is a catalyst that is present in a different phase (usually a solid) than the reactants. Such catalysts generally function by furnishing an active surface upon which a reaction can occur. Gas and liquid phase reactions catalyzed by heterogeneous catalysts occur on the surface of the catalyst rather than within the gas or liquid phase
- The catalyst and reactants in different phases
- Catalyst is usually solid with larger surface area-to volume ratios (noble metal or metal oxide)
- Reactants usually gas or liquid solution and bind (adsorb) to catalyst surface
- Poisons (e.g. S) deactivate active sites
A heterogenous catalyst involves the following processes 1. Adsorption of the reactant(s) onto the surface of the catalyst
2. Activation of the adsorbed reactant(s)
3. Reaction of the adsorbed reactant(s)
4. Desorption of product(s) from the surface of the catalyst
