Chapter 4: The Study Of Chemical Reactions

Front 1

Activation Energy

Back 1

- The energy difference between the reactants and the transition state

- The minimum energy the reactants must have for the reaction to occur

Front 2

Bond Dissociation Enthalpy

Back 2

- The amount of enthalpy required to break a particular bond homiletically to give radicals

Front 3

Carbanion

Back 3

- A strongly nucleophilic species with a negatively charged carbon atom having only three bonds

- The carbon atom has a nonbonding pair of electrons

Front 4

Carbene

Back 4

- A highly reactive species with only two bonds to an uncharged carbon atom with a nonbonding pair of electrons

- The simplest carbon is methane, :CH2

Front 5

Carbocation

Back 5

- Carbonic ion

- A strongly electrophilic species with a positively charged carbon atom having only three bonds

Front 6

Catalyst

Back 6

- A substance that increase the rate of a reaction (by lowering the activation energy) without being consumed in the reaction

Front 7

Chain Reaction

Back 7

- A multistep reaction where a reactive intermediate formed in one step brings about a second step that generates the intermediate needed for the following step

Front 8

Initiation Step

Back 8

- The preliminary step in a chain reaction, where the reactive intermediate is first formed

Front 9

Propagation Steps

Back 9

- The steps in a chain reaction that are repeated over and over to form the product.

- The sum of the propagation steps should give the net reaction

Front 10

Termination Steps

Back 10

- Any steps where a reactive intermediate is consumed without another one being generated

Front 11

Enthalpy

Back 11

- Heat content; H

- A measure of the heat energy in a system

- In a reaction, the heat absorbed or evolved is called the heat of reaction, delta H

- A decrease in enthalpy (negative delta H) is favorable for a reaction

Front 12

Endothermic

Back 12

- Consuming heat (having a positive delta H)

Front 13

Exothermic

Back 13

- Giving off heat (having a negative delta H)

Front 14

Entropy

Back 14

- Symbolized by S

- A measure of disorder or freedom of motion.

- An increase in entropy (positive delta S) is favorable for a reaction

Front 15

Equilibrium

Back 15

- A state of a system such that no more net change is taking place; the rate of the forward reaction equals the rate of the reverse reaction

Front 16

Equilibrium Constant

Back 16

- A quantity calculated from the relative amounts of the products and reactants presented at equilibrium

Front 17

Free Energy

Back 17

- Gibbs free energy; G

- A measure of a reaction's tendency to go in the direction written.

- A decrease in free energy (negative delta G) is favorable for a reaction

Front 18

Standard Gibbs Free Energy Change

Back 18

- Delta G

- The free-energy change corresponding to reactants and products in their standard states (pure substances in their most stable states) at 25 degrees celsius at 1 atm pressure

Front 19

Endergonic

Back 19

- Having a positive delta G

- Unfavorable

Front 20

Exergonic

Back 20

- Having a negative delta G

- Favorable

Front 21

Halogenation

Back 21

- The reaction of a halogen or halogen-containing reagent that incorporates one or more halogen atoms into a molecule

- Free-radical halogenation of alkane is an important industrial synthesis, but it is rare used in a laboratory setting.

- We study there reaction primarily because it serves as an uncomplicated example for studying its thermodynamics and k inetics

Front 22

Hammond Postulate

Back 22

Related species ( on a reaction-energy diagram) that are closer in energy are also closer in structure

- In an exothermic reaction, the transition state is closer to the reactnats in energy an din structure

- In an endothermic reaction, the transition state is closer to the products in energy and in structure

Front 23

Heterolytic Cleavage

Back 23

- Ionic cleavage

- The breaking of a bond in such a way that one of the toms retains both of the bond's electrons

- A heterolytic cleavage forms two ions

Front 24

Homolytic Cleavage

Back 24

- Radial Cleavage

- The breaking of a bond in such a way that each atom retains one of the bond's two electrons

- A homolytic cleagae produces two radicals

Front 25

Inductive Effect

Back 25

- A donation (or withdrawal) of electron density through sigma bonds

Front 26

Intermediate

Back 26

- A molecule or a fragment of a molecule that is formed in a reaction and exists for a finite length of time before it reacts in the next step.

- An intermeidiate corresponds to a relative minimum (a low point) in the reaction-energy diagram

Front 27

Reactive Intermediate

Back 27

- A short-lived species that is never present in high concentrations because it reacts as quickly as it is formed

Front 28

Kinetics

Back 28

- The study of reaction rates

Front 29

Mechanism

Back 29

The step-by-step pathway from reactants to products, showing which bonds break and which bonds form in what order.

- The mechanism should include the structures of all intermediates and curved arrows show the movement of electrons

Front 30

Radical

Back 30

- Free radical

- A highly reactive species in which one of the atoms has an odd number of electrons.

- Most commonly, a radical contains a carbon atom with three bonds and an "odd" (unpaired) electron

Front 31

Radical Inhibitor

Back 31

- A compound added to prevent the propagation of free-radical chain reactions

- In most cases, the inhibitor reacts to form a radical that is too stable to propagate the chain

Front 32

Rate Equation

Back 32

- Rate law

- The relationship between the concentrations of the reagents and the observed reaction rate

Front 33

Kinetic Order

Back 33

- The power of a concentration term in the rate equation

Front 34

Rate Constant

Back 34

- The proportionality constant in the rate equation

Front 35

Rate-limiting Step

Back 35

- Rate determining step

- The slowest step in a multistep sequence of reactions

- In general, the rate-limiting step is the step with the highest energy transition state

Front 36

Rate of a Reaction

Back 36

- The amount of product formed or reactant consumed per unit of time

Front 37

Reaction-energy Diagram

Back 37

- Potential-energy diagram

- A plot of potential-energy changes as the reactants are converted to products.

- The vertical axis is potential energy (usually free energy, but occasionally enthalpy).

- The horizontal axis is the reaction coordinate

Front 38

Reaction Coordinate

Back 38

- A measure of the progress of the reaction

Front 39

Resonance Stabilization

Back 39

- Stabilization that takes place by delocalization of electrons in a pi bonded system

- Cations, radicals, and anions are often stabilized by resonance delocalization

Front 40

Substitution

Back 40

- A reaction in which one atom replaces another, usually as a substituent on a carbon atom

Front 41

Termination Reaction

Back 41

- A step that produces fewer reactive intermediates (usually free radicals) than it consumes

Front 42

Thermodynamics

Back 42

- The study of the energy changes accompanying chemical transformations.

- Thermodynamics is generally concerned with systems at equilibirum

Front 43

Transition State

Back 43

- Activated complex

- The state of highest energy between reactants and products

- A relative maximum (high point) on the reaction-energy diagram