Chemistry Test 3

Energetics

Thermodynamics and Kinetics

Thermodynamics

• The study of energy and its transformation

• Are reactants or products energetically favored?

  • Gibbs Free Energy (Change of G)

  • Need enthalpy (H) and entropy (S) to answer thermodynamic questions

• Interested only in E_initial and E_final (starting point and ending point)

• Can predict whether a reaction is favorable or not

Kinetics

• Study of the process by which reactants form products

• How fast will the reaction happen, and by what mechanism?

• Interested in rates and mechanism (how fast and by which route)

1st Law of Thermodynamics

• Energy cannot be created or destroyed, but energy can be converted from one form to another

• (Change of energy)_universe = 0

• Universe = system + surroundings

  • System: what is being studied

  • Surroundings: everything else

• (Change of energy)_universe = (Change of energy)_system + (Change of energy)_surroundings = 0

  • (Change of energy)_system = -(Change of energy)_surroundings

    • If the system gains heat, the surroundings release heat

  • (Change of energy)_surroundings = -(Change of energy)_system

    • If the surroundings gain heat, the system releases heat

Enthalpy (H)

• The heat content of the reaction when the pressure is constant

• Exothermic: (Change of H) < 0

• Endothermic: (Change of H) > 0

Endothermic reaction

• Heat energy absorbed by the system, causing the surroundings to cool

• (Change of H) > 0

• Enthalpically unfavorable

  • low to high

  • bad for system

Exothermic reaction

• Heat energy released by system (causing the system to warm)

• (Change of H) < 0

• Enthalpically favorable

  • High to low (Preferred by nature)

  • Good for system

When a species is higher in energy, that means its

• more reactive

• less stable

When a species is lower in energy, that means its

• Less reactive

• more stable

How to predict endothermic reactions

• Reactants: Strong bonds — less reactive; more stable

• Products: Weak bonds — more reactive; less stable

How to predict exothermic reactions

• Reactants: Weak bonds — more reactive; less stable

• Products: Strong bonds — less reactive; more stable

Bond Energy / Bond Dissociation Energy (BDE)

• Energy input required to break 1 mole of a bond in the gas phase (fully separate the 2 bound atoms)

• Used to find Change of H

• Weak bonds – more reactive; require less energy to break

• Strong bonds – more stable; require more energy to break

Entropy (S)

• Related to the amount of disorder present in a system

• Related to the number of ways energy can be distributed in a system

• Low entropy system:

  • ordered system

  • energy is concentrated

• High entropy system

  • Disordered system

  • energy can be spread out

• (Change of S) > 0

  • increase disorder

  • entropically favorable

  • Solid → Liquid → Gas

  • Products have more gas molecules

• (Change of S) < 0

  • Increase order

  • entropically unfavorable

  • Gas → Liquid → Solid

  • Products have fewer gas molecules

Gibbs Free Energy (G)

• Equation:

  • T= Temp. (T in K)

• (Change of G) < 0; reaction is spontaneous (products lower in energy than reactants; reaction proceeds)

• (Change of G) > 0; reaction is non-spontaneous (Products higher in energy than reactants; reaction does not proceed)

Gibbs Free Energy Reaction Coordinate

• E_{a reverse} = E_{a forward} + Change of G

• The larger the E_a, the slower the reaction

• The smaller the E_a, the faster the reaction

How to determine transition state

• All bonds that form

• No bonds broken yet

• Consider atom charges when forming bonds

How can we speed up a reaction?

1. Increase temperature

• More frequent collisions

• Gives collisions more energy to overcome Ea

2. Increase the concentration of reactants

• More frequent collisions

3. Use a catalyst

• Brings reactant molecules together to facilitate their collisions

• Lowers the Ea

Enzyme catalysis

Reaction Coordinate diagram of an uncatalyzed reaction vs. a catalyzed reaction

Equilibrium

point at which forward rate = reverse rate and the concentrations of reactants and products remains constant

Equilibrium Expression

Reaction: aA + bB ←→ dD + eE

• Lower-case letters are meant to represent coefficients

• Upper-case letters are meant to represent molecules

K_eq = [D]^d [E] ^e / [A] ^a [B] ^b (Products/Reactants)

• For solutions (aq), use molar concentration (mol/L)

• For gases (g), use molar concentration or the partial pressure in atm

• Solids (s) and liquids (l) are omitted from the equation

K_eq ~ 1

• There is a significant amount of both reactants and products present at equilibrium

• Same amount of reactants and products being transferred

K_eq >>1 (generally greater than 1.0x10³)

• A very large K means an “extensive” reaction, or the reaction goes “completely” to products

• Large amount of product

K_eq << 1 (generally smaller than 1.0x10^-3)

• A very small K_eq means that products are not favored. The solution contains mainly reactants

• A large amount of reactants

Comparing and Contrasting the Kinetics of Various Reactions

Comparing and Contrasting the Thermodynamics of Various Reactions

Le Chatelier’s Principle

If a system at equilibrium is disturbed by changing the conditions, the system will respond to counteract that change and reestablish equilibrium

• (Examples of “disturbances” include changing [reactant], [product], and/or temperature)

• Adding reactants OR after removing products: “Shift equilibrium to the right.”

  • forms more products

  • Decreases reactants

• Removing reactants OR after adding products: “Shift equilibrium to the left”

  • Forms more reactants

  • Decreases products

• Endothermic (Reactant + heat ←→ product)

  • After heating: “Shift equilibrium to the right”

  • After cooling: “Shift equilibrium to the left”

• Exothermic (Reactant ←→ product + heat)

  • After heating: “Shift equilibrium to the left”

  • After cooling: “Shift equilibrium to the right”

Brønsted acid

• H+/proton donor

• Partial positive atom

Brønsted base

• H+/proton acceptor

• Lone pair on negative or partial negative atom that can be used to form new covalent bond

Brønsted acid-base reaction

• The transfer of a proton

  • Brønsted bases and conjugate acids differ by 1 proton

  • Brønsted acids and conjugate bases differ by 1 proton

Conjugate Base

Accepts H⁺ in the reverse reaction

• forms after Brønsted acid donates proton

Conjugate acid

Donates H⁺ in the reverse reaction

• Forms after Brønsted base accepts H⁺

The 7 “Strong Acids”

The acids that dissociate completely in water and undergo Brønsted reactions

• Take into account: The nomenclature of conjugate acid and conjugate base is still the same, even if the parent Brønsted acid is weak

HCl + H₂O → H₃O⁺ + Cl^-

HBr + H₂O → H₃O⁺ + Br^-

HI + H₂O → H₃O⁺ + I^-

HNO₃ + H₂O → H₃O⁺ + NO₃^-

HClO₃ + H₂O → H₃O⁺ + ClO₃^-

HClO₄ + H₂O → H₃O⁺ + ClO₄^-

H₂SO₄ + H₂O → H₃O⁺ + HSO₄^-

Different Definitions of Acids and Bases

Lewis definition:

• More broad

• Lewis acid: electron pair acceptor

• Lewis base: electron pair donor

Brønsted defintion:

• Brønsted acid: H+/proton donor

• Brønsted base: H+/proton acceptor

Note: Brønsted reactions always meet the definition for Lewis reactions, but lewis reaction are not always Brønsted reactions

Mechanism arrows and how to determine where they are

• Mechanism arrows represent how electrons (not atoms) are moved around

1) Identify bonds broken and bonds formed to determine if the reaction meets Lewis definition

2) Start with a molecule that is donating a pair of electrons

• Start arrow at the source of electrons: lone pair

• A bond to which atom? That’s what the arrow points to

• How to know where the bond is being formed:

  • Lone pairs attach to a partial positive Hydrogen atom

3) Go to the molecule that takes in lone pairs

• Start arrow at the source of electrons: bond breaking

• When a bond breaks, lone pairs go to the atom that is not forming a bond

  • What atom? That’s what the arrow points to

• How to know where to find where the lone pair is being formed

  • The atoms that are partially negative or negative that have a bond with the partially positive hydrogen atom

A higher energy species is

• More reactive (wants to change)

• Less stable

• Analogous to a strong acid and/or strong base

A lower energy species is

• Less reactive

• More stable

• Analogous to a weak acid and/or weak base

K_a – Acid Dissociation Constant

The equilibrium constant (K_eq) for an acid reacting with water

• A^- = Conjugate base

• H₃O⁺ = Conjugate acid

• HA = Brønsted acid

• Water (the Brønsted base) is considered a pure liquid and doesn’t appear in the equilibrium expression

• The stronger the acid, the larger the [A–][H3O+], the larger the K_a

• The weaker the acid, the smaller the [A–][H3O+], the smaller the K_a

pK_a

• Simpler way to depict K_a

• pK_a = -log(K_a)

• The stronger the acid, the smaller (more negative) the pK_a

• The weaker the acid, the bigger (more postitive) the pK_a

In order to qualitatively compare the relative strength of acids:

• Consider the stability of the conjugate base

• The more stable the conjugate base (A^-), the stronger the parent acid (HA)

• Product (Conjugate base) formation is favored when: Products (Conjugate base) are lower in energy

• Reactants (Parent acid) are more reactive when: Reactants (Parent acid) are higher in energy

How to qualitatively access relative acidity

May need to find the corresponding conjugate base first. If so, determine what the molecules would look like after they give away a partial positive proton.

1) Most important: An atom holding a negative charge in a conjugate base

• Same row: Electronegativity

• Same column: atom size

  • spread out—charge across a bigger surface area = lowering energy

2) 2nd most important: Consider resonance in the conjugate base

• Equally preferred: Energy is blended out

• Spread out charge = lower energy

• The more variations of resonance structures, the more stable the conjugate base

3) 3rd most important: Consider e-donating and e-withdrawing groups in conjugate base

• If the polar charge is facing away from the atom with the negative charge, then it’s more stable

• If the polar charge is facing towards the atom with the negative charge, then it’s not stable

• My assumption (not in notes): If both are facing away from the atom with the negative charge, then compare the electronegativity and size of the more electronegative atom.

The reaction is extensive if

the reacting acid is much stronger than the produced conjugate acid

The conjugate base relationship with the reactants preferred

• More stable conj. base / Weaker base = reactants preferred / reverse reaction is effective

• Less stable conj. base / Stronger base = reactants are even more preferred / reverse reaction is even more effective

K_eq relationship with pK_a

K_eq = 10^{(pK_a of conjugate acid - pK_a of reacting acid)}

pK_a = -log(K_a)