GCII Midterm II Study Guide – Chemistry Concepts

activation energy

The minimum energy with which molecules must collide to react.

active site

The region of an enzyme formed by specific amino acid side chains at which catalysis occurs.

Arrhenius equation

An equation that expresses the exponential relationship between temperature and the rate constant: k = Ae^-Ea/RT.

bimolecular reaction

An elementary reaction involving the collision of two reactant species.

catalyst

A substance or mixture that increases the rate of a reaction without being used up in the process.

collision theory

A model that explains reaction rate as based on the number, energy, and orientation of colliding particles.

effective collision

A collision in which the particles meet with sufficient energy and an orientation that allows them to react.

elementary reaction (step)

A simple reaction that describe a single molecular event in a proposed reaction mechanism.

enzyme

A biological macromolecule (usually a protein) that acts as a catalyst.

enzyme-substrate complex (ES)

The intermediate in an enzyme-catalyzed reaction, which consists of an enzyme and substrate(s) and whose concentration determines the rate of product formation.

frequency factor (A)

The product of the collision frequency Z and an orientation probability factor p that is specific for a reaction.

half-life (t1/2)

In chemical processes, the time required for the reaction concentration to reach half of its initial value. In nuclear processes, the time required for half the initial number of nuclei in a sample to decay.

heterogeneous catalyst

A catalyst that occurs in a different phase from the reactants, usually a solid interacting with gaseous or liquid reactants.

homogeneous catalyst

A catalyst (gas, liquid, or soluble solid) that exists in the same phase as the reactants.

hydrogenation

The addition of hydrogen to a carbon-carbon multiple bond to form a carbon-carbon single bond.

induced-fit model

A model of enzyme action that pictures the binding of the substrates inducing the active site to change its shape and become catalytically active.

integrated rate law

A mathematical expression for reactant concentration as a function of time.

lock-and-key model

A model of enzyme function that pictures the enzyme active site and the substrate as rigid shapes that fit together as a lock and key, respectively.

molecularity

The number of reactant particles involved in an elementary step.

rate-determining (rate-limiting) step

The slowest step in a reaction mechanism and therefore the step that limits the overall rate.

reaction energy diagram

A graph that shows the potential energy of a reacting system as it progresses from reactants to products.

reaction intermediate

A substance that is formed and used up during the overall reaction and therefore does not appear in the overall reaction.

reaction mechanism

A series of elementary steps that sum to the overall reaction and is consistent with the rate law.

substrate

A reactant that binds to the active site in an enzyme-catalyzed reaction.

transition state (activated complex)

An unstable species formed in an effective collision of reactants that exists momentarily when the system is highest in energy and that can either form products or reform reactants.

transition state theory

A model that explains how the energy of reactant collisions is used to form a high-energy transitional species that can change to reactant or product.

unimolecular reaction

An elementary reaction that involves the decomposition or rearrangement of a single particle.

equilibrium constant (K)

The value obtained when equilibrium concentrations are substituted into the reaction quotient.

Haber process

An industrial process used to form ammonia from its elements.

law of chemical equilibrium (mass action)

The law stating that, when a system reaches equilibrium at a given temperature, the ratio of quantities that make up the reaction quotient has a constant numerical value.

Le Châtelier's principle

A principle stating that, if a system in a state of equilibrium is disturbed, it will undergo a change that shifts its equilibrium position in a direction that reduces the effect of the disturbance.

reaction quotient (Q)

A ratio of terms for a given reaction consisting of product concentrations multiplied together and divided by reactant concentrations multiplied together, with each concentration raised to the power of its balancing coefficient.

van't Hoff equation

An equation for calculating the change in equilibrium constant that occurs with a change in temperature: ln(K2/K1) = (Hrxn/R)(1/T1 - 1/T2).

acid-base indicator

An organic molecular whose color is different in acid and in base; the color is used to monitor the equivalence point of a titration or the pH of a solution.

acid-dissociation (acid-ionization) constant (Ka)

An equilibrium constant for the dissociation of an acid (HA) in water to yield the conjugate base (A-) and H3O+.

adduct

The product of a Lewis acid-base reaction, a species that contains a new covalent bond.

amphiprotic

A substance that can either donate or accept a proton.

Arrhenius acid

A substance that yields hydrogen ions when dissolved in water.

Arrhenius base

A substance that yields hydroxide ions when dissolved in water.

autoionization

A reaction in which two molecules of a substance react to give ions.

base-dissociation (base-ionization) constant (Kb)

An equilibrium constant for the reaction of a base (B) with water to yield the conjugate acid (BH+ and OH-).

Brønsted-Lowry acid-base definition

A model of acid-base behavior based on proton transfer, in which an acid and a base are defined, respectively, as a species that donates a proton and one that accepts a proton.

conjugate acid-base pair

Two species related to each other through the gain or loss of a proton; the acid has one more proton than its conjugate base.

hydronium ion (H3O+)

A proton covalently bonded to a water molecule.

ion-product constant for water (Kw)

The equilibrium constant for the auto ionization of water; equal to 1.0 x 10^-14 at 298 K.

leveling effect

The inability of a solvent to distinguish the strength of an acid (or base) that is stronger than the conjugate acid (or conjugate base) of the solvent.

Lewis acid-base definition

A model of acid-base behavior in which acids and bases are defined, respectively, as species that accept and donate an electron pair.

neutralization

The process that occurs when an H+ ion from an acid combines with an OH- ion from a base to form water.

pH

The negative of the common logarithm of [H3O+].

polyprotic acid

An acid with more than one ionizable proton.

proton acceptor

A species that accepts an H+ ion; a Brønsted-Lowry base.

proton donor

A species that donates an H+ ion; a Brønsted-Lowry acid.