Chem 122 Exam II

Reaction Rate

Reaction Rate

Change in concentration of reactants or products per unit of time

Four factors influence rate

-Physical state of reactants
-Reactant concentrations
-Reaction temperature
-Presence of catalyst

Physical State of the Reactants and Reaction Rates

-The more readily the reactants collide, the more rapidly they react.
-Homogenous reaction (all gases or liquids) are often faster

Reactant Concentrations and Reaction Rates

-More molecules means more collisions are possible
-If the surface area is increased, the reaction rate increase

Reaction Temperature and Reaction Rates

-At higher temperatures, molecules move more quickly, increasing the number of collisions and the energy the molecules possess during the collisions.
-More molecules cross the barrier to reaction (activation energy).

Presence of Catalyst and Reaction Rates

-They DO NOT appear in the overall balanced equation.
-Change the mechanism of the collisions.

Average Rate

The increase in product or decrease in reactant with respect to time.

Instantaneous Rate

-The slope of the curve at one point in time gives the instantaneous rate
-Rate at time zero is called the initial rate.

Reaction Order

The sum of the exponents in a rate law, where each exponent provides the reaction order with respect to its reactant.

Zero Order

M/time

1st Order

1/time

Second Order

1/(M*s)

Orientation Factor

molecules must be oriented in a certain way during collisions in order for a reaction to occur

Shortcomings of Orientation Factor

-Does not explain the minimum energy requirement
-No direct method to calculate the probability molecules will have the correct orientation except in simple systems

Activation Energy

the minimum amount of energy required to start a chemical reaction

Transition State

-The organization of the atoms at the highest energy state
-The energy difference between energy of the reactants and the highest energy along the reaction pathway.

Effect of Temperature on the Distribution of the energy of Molecules

-Molecules have an average temperature, but each individual molecule has its own energy.
-At higher energies, more molecules possess the energy needed for the reaction to occur.

Elementary Reaction

Reaction that occurs by a single step or through several discrete steps

Molueculairty

Tells how many molecules are involved in that step of an elementary reaction.

Unimolecular

Involves a single molecule

Bimolecular

Two molecules collide

Temolecular

Three molecules collide

Rate-determining step

The overall reaction cannot occur faster than the slowest reaction in the mechanism.

Intermediates

They occur between transition states, the trough of an upward parabola.

Catalysis

-The change in reaction speed without undergoing permanent chemical change by a catalyst.

Homogenous Catalysts

-The reactants and catalyst are in the same phase
-Many times, reactants and catalysts are dissolved in the same solvent.

Heterogenous Catalysts

-The catalyst is in a different phase than the reactants.
-Often, gases are passed over a solid catalyst.
-The adsorption of reactants is often the rate-determining step.

Enzymes

Biological catalysts

Active Site

The part of an enzyme where the chemical reaction occurs.

Substrates

The substances that react at the active site.

Lock-and-Key Model

The model of the enzyme that shows the substrate fitting perfectly into the active site

Chemical Equilibrium

When a reaction and its reverse reaction proceed at the same rate.

Evaluation K_c

The ratio of the two species remains constant no matter the initial concentrations.

If K>>>1

The reaction favors the products

If K<<<1

The reaction favors the reactants.

Homogeneous Equilibira

Occur when all reactants and products are in the same phase.

Heterogeneous Equilibria

Occur when a component in the equilibrium is in a different phase.

Pure Liquid or Solid in Heterogeneous Equilbirium

Its concentration is not included in the equilibrium-constant expression.

Comparing Q and K

Nature wants Q=K
If Q<K, nature will make the reaction proceed to products
If Q=K, the reaction is in equilibrium
IfQ>K, nature will make the reaction proceed to reactants

Le Chatelier's Principle

States that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.

Adding a Reaction Component and Le Chatelier's Principle

The equation will shift to use some of it up

Removing a Reaction Component and Le Chatelier's Principle

The equation will shit to produce some of it

Change in Volume or Pressure and Le Chatelier's Principle

Higher volume or lower pressure favors the side of the equation with more moles (and vice versa).

Change in Temperature and Le Chatelier's Principle

Endothermic: Heat acts like a reactant; adding heat drives a reaction toward products. K increases.

Exothermic: Heat acts like a product; adding heat drives a reaction toward reactants. K decreases.

Arrhenius Acids and Bases

Acids: When dissolved in water, increase [H+]
Bases: When dissolved in water, increases [OH-]

Bronsted-Lowry Acids and Bases

-An acid is a proton (H+) donor.
-A base is a proton (H) acceptor.
-Has at least one removable proton to donate and at least one non-binding electron pair to accept a proton.

Lewis Acids and Bases

-An acid is an electron-pair acceptor.
-A base is an electron-pair donor.
-All Bronsted-Lowry acids and bases are also called Lewis acids and bases.

Water as a Proton Acceptor

-When a hydrogen ion is formed in water, it interacts with other molecules.
-Hydrogen bonds form with other water molecules.

Amphiprotic Substances

-Substances that can act as both an acid and a base.
-Water is a Bronsted-Lowry acid and base.

Conjugate Acid-Base Pairs

-An acid and base that differ by the presence of H+.
-Reactions between acids and bases always yield their conjugate bases and acids, respectively.

Relative Strengths of Acids and Bases

-Stronger acids have weaker conjugate bases.
-Stronger bases have weaker conjugate acids.

Acid-Base Equilibrium

In acid-base reactions, equilibrium favors proton transfer to transform stronger acids and bases to from weaker acids and bases.

Autoionization of Water

In the presence of acid, water acts as a base.
In the presence of a base, it acts as an acid.

Aqueous Solution Acidity

If solution is neutral, [H+] = [OH-]
If solution is acidic, [H+] > [OH-]
If solution is basic, [H+] < [OH-]

The pH scale

Neutral pH is 7.00
Acidic pH is < 7.00
Basic pH is > 7.00
*Only the digits after the decimal point are significant figures in logs.

Comparing Strong and Weak Acids

Strong acids completely dissociate to ions
Weak acids only partially dissociate to ions.

Polyprotic Acids

Have more than one acidic proton
Easier to remove the first proton than successive protons
If K_a1 and K_a2 differ by a factor of >1000, the pH depends only on the first dissociation.

Types of Weak Bases

1) Neutral substances with an
atom that has a nonbonding
pair of electrons that can
accept H+ (like ammonia and
the amines)
2) Anions of weak acids, which
are their conjugate bases

Acid-Base Properties of Salt Solutions

Hydrolysis is water reacting with ions to from H+ and OH-.

Determining Whether a Salt is an Acid or Base

Look at the cation and anion separately.
-The cation can be acidic or neutral
-The anion can be acidic, basic, or neutral.

Anion Hydorlysis

-Anions of strong acids are neutral.
-Anions of weak acids are conjugate bases, creating OH- in water.
-Ka > Kb anion will be acidic
-Ka < Kb anion will be basic.

Cation Hydrolysis

-Group I or II metal cations are neutral.
-Polyatomic cations are typically the conjugate acids of a weak base, creating H+ to form acidic solutions.
-Transition and post-transition metal cations are acidic.

Select Hydrated Cations

-Transition and post-transition metals from hydrated cations.
-The water attached to the metal is more acidic than free water molecules, making the hydrated ions acidic.
-Higher charges result in stronger water-to-metal bonds, making them stronger acids.

Salt Solutions - Neutral

-If the salt contains both an anion and cation that does not react with water, pH is neutral. Ex: NaCl

Salt Solutions - Basic

-If the salt contains an anion that reacts with water to produce hydroxide ions and a cation that does not react with water, we expect the pH to be basic. Ex: NaClO, RbF, BaSO3

Salt Solutions - Acidic

If the salt contains a cation that reacts with water to produce hydronium ions, and an anion that does not react with water, expect the pH to be acidic. Ex: NH4NO3, AlCl3.

Sal Solutions - Both reactive

If the salt contains both an anion and a cation that reacts with water, the values of K(Ka, Kb) determine whether the solution is acidic or basic. The larger K value determines pH.

Factors that affect acid strength:

1. H—A bond must be polarized with δ+ on the H atom and δ- on the A atom
2. Bond strength: Weaker bonds can be broken more easily, making the acid stronger.
3. Stability of A-: The more stable the conjugate base (anion), the stronger the acid.

Binary Acids

Increase strength from left to right and down the periodic table.

Oxyacids

-Consists of H, O, and one other element, which is a nonmetal.
-As the electronegativity of the nonmetal increases, the acidity increases for acids with the same structure.

Oxyacids with Same "Other" Element

If an element can form more than one oxyacid, the oxyacid with more O atoms is more acidic.

Carboxylic Acids

Organic acids containing the -COOH group.
-Other O attached to C draws electron density form the O-H bond, increasing polarity.
-Its conjugate base (carboxylate anion) has resonance forms to stabilize the anion.

The Common-Ion Effect

If you have a solution of HA, and you add A-, then it will push the reaction toward HA. Follows Le Chatelier Principle.

Buffers

Solutions of a weak conjugate acid-base pair that resist drastic changes in pH are called buffers.

How Buffers are Made

1. Mix a weak acid and a salt of its conjugate base or a weak base and a salt of its conjugate acid.
2. Add strong acid and partially neutralize a weak base or add strong base and partially neutralize a weak acid.

Acid-Base Titrations

-An acid (or base) solution of a known concentration is slowly added to a base (or acid) solution of an unknown concentration.

Equivalence Point

the point at which the two solutions used in a titration are present in chemically equivalent amounts

Titration of a Strong Acid with a Strong Base

-From the start of the titration to near the equivalence point, the pH goes up slowly.
-Just before (and after) the equivalence point, the pH rises rapidly.
-At the equivalence point, pH = 7.
-As more base is added, the pH
again levels off.

Titration of a Strong Base with a Strong Acid

-It looks like you "flipped over" the strong acid being titrated by a strong base.
-Start with a high pH (basic solution); the pH = 7 at the equivalence point; low pH to end.

Titration of a Weak Acid with a Strong Base

-Use Ka to find the initial pH.
-Find the pH in the "buffer region" using stoichiometry followed by the Henderson-Hasselbalch equation.
-At the equivalence point the pH is >7. Use the conjugate base of the weak acid to determine the pH.
-As more base is added, the pH levels off. This is the same as for strong acids.

Titrations with an Acid-Base Indicator

Indicators are weak acids that have a different color than their conjugate base form

Titrations of Polyprotic Acids

-There are multiple equivalence points.
-Treat each step separately with their corresponding polyanions and Ka
-The halfway to each equivalence point gives the pKa
-The concentration of the dianion is always equal to Ka2