Chemistry midterm 2: Kinetics

Question 1

A rate is a

Answer 1

measure of how some property varies with time

Question 2

The rate of reaction is

Answer 2

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

Question 3

What is thermodynamics

Answer 3

spontaneity & position of equilibrium

Question 4

What is kinetics

Answer 4

reaction rates & time to achieve equilibrium

Question 5

Reaction rates can be influenced by

Answer 5

• 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

Question 6

Before atoms react with each other, they must

Answer 6

Collide

Question 7

What is collision theory

Answer 7

Theory that explains many observations regarding chemical kinetics, including factors affecting reaction rates

Question 8

What are the 3 postulates of collision theory

Answer 8

1. The rate of a reaction is proportional to the rate of reactant collisions (#collisions/time)
2. The reacting species must collide in an orientation that allows contact between the atoms that will become bonded together in the product.
3. 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)

Question 9

In addition to a proper orientation, the collision must also occur with

Answer 9

sufficient energy to result in product formation

Question 10

what is an activated complex (transition state)

Answer 10

When reactant species collide with both proper orientation
and adequate energy, they combine to form an unstable species

Question 11

why do most reaction rates increase as concentrations increase

Answer 11

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

Question 12

What is the activation energy

Answer 12

The minimum energy necessary to form a product during a collision between reactants

Question 13

If the activation energy is much larger than the average kinetic energy of the molecules

Answer 13

the reaction will occur slowly since only a few fast-moving molecules will have enough energy to reac

Question 14

If the activation energy is much smaller than the average kinetic energy of the molecules

Answer 14

a large fraction of molecules will be adequately energetic and the reaction will proceed rapidly

Question 15

What is a rate mechanism

Answer 15

• 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

Question 16

A reaction mechanism cannot be proved, it can only be
validated by the available data if

Answer 16

1. The elementary steps in the mechanism add up to the overall reaction
2. The rate law predicted by the mechanism is consistent with the experimentally determined rate law.

Question 17

What is a catalyst

Answer 17

• 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

Question 18

What are homogenous catalysts

Answer 18

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)

Question 19

What is a heterogeneous catalyst

Answer 19

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

Question 20

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