Exam 2

Dynamic equilibrium

Occurs in a solution when rates of dissolution and recrystallization are equal, resulting in a saturated solution.

Solubility

For most solids in water, it increases with rising temperature.

Solubility of gases

Generally decreases in water with increasing temperature but rises with increasing pressure.

Concentration Units

Include molarity (M), molality (m), mole fraction, mole percent, percent by mass or volume, parts per million (ppm), and parts per billion (ppb).

Vapor Pressure Lowering

Occurs when a nonvolatile solute in a liquid leads to a lower vapor pressure compared to the pure liquid, as predicted by Raoult's law.

Freezing Point Depression

Adding a nonvolatile solute to a liquid results in a solution with a lower freezing point than the pure solvent.

Boiling Point Elevation

Adding a nonvolatile solute to a liquid results in a solution with a higher boiling point than the pure solvent.

Osmosis

The flow of solvent from a lower concentration solution to a higher concentration solution.

Colligative Properties

Phenomena such as vapor pressure lowering, freezing point depression, boiling point elevation, and osmosis, dependent only on the number of solute particles added.

Electrolyte solutes

Have a greater effect on colligative properties than an equivalent amount of a nonelectrolyte solute, as indicated by the van't Hoff factor.

Reaction Rate

Measure of how fast a reaction occurs, reported in M/s

Reaction Rate

reflects the change in the concentration of a reactant or product per unit time

First-Order Reaction

Rate directly proportional to reactant concentration

Second-Order Reaction

Rate proportional to square of reactant concentration

Zero-Order Reaction

Rate independent of reactant concentration

Rate Law

Shows relationship between rate and reactant concentrations

rate law for zero-order reaction

rate = k[A]°[B]° = k (in M/s)

rate law for first-order reaction

rate = k[A]¹ or k[B]¹ (in s⁻¹)

rate law for second-order reaction

rate = k[A]¹[B]¹ or k[A]² or k[B]² (in M⁻¹s⁻¹)

Integrated Rate Law

Describes relationship between reactant concentration and time

integrated rate law for a zero-order reaction

shows that the concentration of the reactant varies linearly with time.

integrated rate law for a first-order reaction

the natural log of the concentration of the reactant varies linearly with time

integrated rate law for a second-order reaction

the inverse of the concentration of the reactant varies linearly with time

Half-Life

Time for reactant concentration to halve; varies with reaction order

The half-life of a first-order reaction

is independent of initial concentration of the reactant.

The half-life of a zero-order or second-order reaction

depends on the initial concentration of reactant.

The frequency factor

represents the number of times that the reactants approach the activation barrier per unit time

The exponential factor

is the fraction of approaches that are successful in surmounting the activation barrier and forming products

Arrhenius Equation

Relates rate constant to temperature, with frequency and exponential factors

Activation Energy

Energy barrier for reactants to form products

Arrhenius Plot

Used to determine frequency factor and activation energy

Collision Model

Describes gas-phase reactions based on energetic collisions

p

which represents the fraction of collisions that have the proper orientation

z

which represents the number of collisions per unit time.

The frequency factor

contains two terms: p and z

In order for a proposed reaction mechanism to be valid, it must fulfill two conditions

(a) the steps must sum to the overall reaction, and (b) the mechanism must predict the experimentally observed rate law.

For mechanisms with a slow initial step

we derive the rate law from the slow step

For mechanisms with a fast initial step

we first write the rate law based on the slow step but then assume that the fast steps reach equilibrium, so we can write concentrations of intermediates in terms of the reactants.

Reaction Mechanism

Series of steps by which a reaction occurs

Catalyst

Substance increasing reaction rate by lowering activation energy

Homogeneous Catalyst

Same phase as reactants, forms homogeneous mixture

Heterogeneous Catalyst

Different phase from reactants

Enzymes

Biological catalysts increasing reaction rates in biochemical reactions

Equilibrium constant (K)

Expresses the relative concentrations of reactants and products at equilibrium;

a large K (>1)

indicates high product concentration,

a small K (<1)

indicates low product concentration.

Dynamic equilibrium

State where the rate of the forward reaction equals the rate of the reverse reaction, maintaining constant net concentrations of reactants and products.

Equilibrium constant expression

Derived from the law of mass action,

Equilibrium constant expression

it is the ratio of product concentrations raised to their stoichiometric coefficients to reactant concentrations raised to their stoichiometric coefficients.

Reaction quotient (Q)

Ratio of product concentrations to reactant concentrations at any point in a reaction;

If Q<K,

the reaction moves in the direction of the products;

if Q>K,

the reaction moves in the reverse direction.

at equilibrium,

Q equals K.

Law of mass action

Principle that the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants.

ICE table

A table used to organize initial (I), change (C), and equilibrium (E) concentrations when solving for equilibrium concentrations.