Polar Covalent Bonds: Acid and Bases

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Chapter 2

• Polar Covalent Bonds Acid and Bases

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Chapter 2 - Definitions

• Polar Covalent Bonds electrons are even
  distributed between two atoms in a molecule.
• Electronegativity- the attractiveness of an atom
  to an electron in a bond.
• Dipole Moment is the total net molecular
  polarity.
• Formal Charge assigns specific charges to
  individuals atoms inside molecule, particularly
  to atoms that have an apparently abnormal
  number of bonds.
• Resonance Forms describes the movement of
  electrons that accounts for the electron
  densities of molecules.
• Bronsted-Lowery acid is an proton donator.

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Chapter 2 - Definitions

• Bronsted-Lowery base is a proton acceptor.
• Acidity constant, Ka determines the strength of
  an acid.
• Lewis acid - is a substance that accepts an
  electron pair.
• Lewis base are a substance that donates an
  electron pair.

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What is Electronegativity?

• Electronegativity is the desire of an atom to
  gain an additional electron to fill its octet or
  the strength of the atom to pull electrons.
• The higher number the stronger the
  electronegativity.
• Largest electronegativity F 4.0, Cl 3.5,
  O 3.5, N 3.0, Br 2.8, C 2.5

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Why is Electronegativity So Important?

• The electronegativity of two atoms in a bond
  determines what type of bond forms.
• There are basically 2 types of bonds with a 3rd
  also considered a type of bond.
• Ionic
• Covalent
• Polar Covalent (between a Ionic and covalent
  bond.)

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Similar Electronegativities
Same color denotes similar electronegativities
and a covalent bond.
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Some Examples

• Na 0.9, Cl 3.0 (Ionic)
• C 2.5, C 2.5 (Covalent)
• C 2.5, O 3.5 (Polar Covalent)

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What is the Difference Between Bonding?

• Imagine that atoms play tough of war with the
  electrons that they share in a bond.
• There would be three possibilities.
• 1) One atom wins and takes the electron (ionic
  bond)
• 2) Both atoms are even matched sharing the
  electrons evenly. (covalent)
• 3) One atom is stronger than the other atoms and
  has the electron over its side more than the
  other atom. (polar covalent)

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Ionic Bonds

• Ionic Bonds are where an electron is donated to
  another atom. This creates two charged species.
  These charged atoms or molecules are normally
  free in solution but are held together when solid
  by electrostatic attractions.

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Covalent Bond

• Describes the sharing of electrons between two
  atoms. There are two different types of covalent
  bonds.
• Nonpolar covalent bonds is defined as the even
  distribution of electrons between 2 atoms.
• Polar covalent bonds is defined as the uneven
  distribution of electrons between two atoms.

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Nonpolar Covalent Bonds

• Are bonds that are formed between atoms with
  similar electronegativities.
• Example chains of carbon (C) bonded to hydrogens
  (H) (Hydrocarbons)

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Polar Covalent Bonds

• Describes the unequal sharing of electrons in a
  covalent bond.

• This makes the oxygen considered partially
  negative because the electrons around it more.
  The carbon is considered partially positive
  because the shared electrons are mainly around
  the oxygen.

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Polar Covalent Bonds and Polar Molecules

• Polar covalent bonds can be found in both
  individual bonds and in entire molecules.
• To calculate individual bond polarity you need to
  use the electronegativities of the two atoms.
• To calculate the dipole moment of a molecule you
  need to determine the center of positive and
  negative charges. If they are not the same then
  there is an overall polarity of the molecule
  (called a polar molecule).

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Dipole Moment

• The dipole moment of a molecule describes the
  region of the molecules where the electron
  density is highest and lowest.
• The dipole moment maintains a vector from low
  electron density to high electron density.
• If the electron density is equal across the
  molecules then the molecules is nonpolar molecule
  (evenly distributed)

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Non-Polar Molecules

• Are molecules which either maintain atoms with
  similar electronegativities or molecules whose
  dipoles are even in all directions.
• CH4 (nonpolar)
• CCl4 (Nonpolar)
• CH2Cl2 (Polar)

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Non-Polar Molecules

• These molecules are molecules whose dipoles are
  even spread in all directions or maintain similar
  electronegativities.

Carbon Tetrachloride
Methane
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Questions
Carbon Tetrachloride
Methane
Dichloromethane
Pick the polar molecule(s). And its(their)
vectors.
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Polar Covalent Molecules

• The electronegativity of the oxygen and nitrogen
  atoms are different there by causing a overall
  dipole in the molecule making the molecules polar.

Water
Ammonia
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Properties of Covalent Molecules

• Remember (LIKE DISSOLVES LIKE)
• Polar Covalent Molecules Water, Methanol,
  Ethanol.
• Nonpolar Covalent Molecules (oils,
  hydrocarbons) Propane, decane, etc..
• Oils and water do not mix because one is polar
  covalent and the other is nonpolar covalent.

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Formal Charges

• Formal charges - assigns specific charges to
  individuals atoms inside molecule, particularly
  to atoms that have an apparently abnormal
  number of bonds.
• This is used when you see charge separation in a
  molecules to indicate if the atom is positive or
  negative.

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Formal Charge

• Formal charge the charge on each atom in a
  molecule.
• Formal of valence e- of
  valence e-
• charge (free atom)
  (bound atom)
• Formal of valence e- Half of
  of
• Charge bonding
  e- nonbonding e-

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Resonance

• Resonance is the movement of either free
  electrons or p electrons to form other possible
  structures.
• Because movement of electron occurs frequently
  resonance structures try to show how the
  electrons might move.
• Only the movement of double (p bonds) or free
  electrons are found. Movement of the sigma bonds
  would breakup the molecule instead of create
  resonance structures.

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Examples of Resonance
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Rules of Resonance

• 1) Individual resonance forms are imaginary, not
  real.
• 2) Resonance forms differ only in the placement
  of their p or nonbonding electrons.
• 3) Different resonance forms of a substance dont
  have to be equivalent.
• 4) Resonance forms obey normal rules of valency.
• 5) The resonance hybrid is more stable than any
  individual resonance form.

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Resonance Forms
1) Number 1 and 2 are resonance forms 2) Although
resonance forms many not be equivalent, 1 and 2
are equal.
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Question
Why does this oxygen have a negative charge?
This oxygen has a negative charge because it has
taken an electron from a hydrogen giving it a
negative electron and a charge of -1. This
completes its octet with 8 electrons
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Bronsted-Lowry Acid Base

• Bronsted-Lowry Acid - is a substance that donates
  a proton (H). It keeps the electron and becomes
  negative.
• Bronsted-Lowry Base is a substance that accepts
  a proton (H). It gives its proton so it becomes
  positive.

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Examples of Bronsted Lowry Acids and Bases
1
2
3
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Conjugate Acid and Bases

• Conjugate Acids and bases are if you look at a
  reaction as a reversibility, these product would
  be the acid base found on the opposite side.
• If you look at the reaction above the acid an
  base are clearly defined, however if you switch
  the reaction then these would be the acid and
  base, as shown below.

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Acid Base Strength

• Acids differ in their ability to donate an (H)
  proton.
• Some acids break apart and donate their proton
  well (100), while others acids only gives an
  proton about 50 percent or less of the time.
• The ability to give protons donates the strength
  of the acid.

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How do you Calculate the Strength of an Acid?

• The way that you normally determine the strength
  of an acid is to use the equilibrium constant.
  In dilute solutions this is rewritten as the
  bottom equation.

In Dilute Solutions
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What is the Ka Range of Acids?

• The strongest acids has a range from 1015 and
  weaker acids are about 10-60.
• This is a wide difference so the use of pKa is
  used.
• P -Log (Number)
• So the pKa of an acid whose Ka 1015 equals
• -Log(1015)
• -(15)
• -15 is the pKa of a strong acid

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Some Examples of pKa
Weakest

• CH3CH2OH 16.00
• H2O 15.74
• HCN 9.34
• H2PO4- 7.21
• CH3CO2H 4.76
• HNO3 -1.3
• HCl -7.0

Strongest
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What is the pKb?

• Just the same way that the acid strength can be
  determined the basic strength can also be
  determined. This strength is opposite of what
  the acidic strength would be.

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Strength of pKb
Strongest

• CH3CH2O-
• HO-
• CN-
• HPO42-
• CH3CO2-
• NO3-
• Cl-

Weakest
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Predicting Acid Base Reactions

• The predict if an acid base reaction will occur
  you have to determine if the stronger acid and/or
  stronger base are on the left side. If this is
  not true then the reaction will not proceed.

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Organic Acids

• Rule of thumb Organic Acids often are found
  where oxygen's are. The more oxygen's the
  stronger the acid.
• Some examples are the alcohols and carboxylic
  acids

Alcohol
Carboxylic Acid
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Which is the stronger acid?
Alcohol
Carboxylic Acid
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Organic Bases

• Rule of Thumb Organic Bases can maintain both an
  oxygen or a nitrogen. Nitrogen almost always
  functions as a base however oxygen can function
  as both an acid or base.
• When something has a plus charge it is a base.

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Which Oxygen Functions as a Base?
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Lewis Acids and Bases

• Lewis Acids is a substance that accepts an
  electron pair.
• Lewis Bases are a substance that donates an
  electron pair.
• These are much broader and can often be used in
  both organic and inorganic chemistry.

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Lewis Acid

• Maintains an empty or vacant orbital. If you
  think of H this is a Lewis acid because it has
  given up its single electron to another atom and
  has only a proton.
• Another example is Mg2.

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Lewis Base

• Maintains a filled orbital. If you think of
  Nitrogen containing molecules they maintain 1
  filled lone pair of electrons to share making
  them a Lewis base. Any atom with a filled lone
  pair of electrons to share. Oxygen, Nitrogen,
  Sulfur.
• Examples are amines and sulfides a sulfur
  containing molecule.

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Which of these are considered Lewis Bases?
Ammonia
Methane
Water
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Non-Covalent Interactions

• Just like ionic atoms and molecules pair up due
  to charge, dipole interaction makes the
  intermolecular forces occur.
• Intermolecular forces use the partial positive
  and negative charges cause by dipoles to pair.
  These pair partial positive to partial negative.

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Types of Non-Covalent Interactions

• Hydrogen Bonding the strongest of these forces
  shows the attractive forces of a Hydrogen bond
  atom to an electronegative atom of O and N.
• Vander Walls Forces weaker interactions of non
  hydrogen bound atoms. For example Cl, Br, etc
• Dispersion Forces forces other molecules away
  because the electron distribution is constantly
  changing in a non-uniform fashion.

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Things to Know

• Electronegativity
• Bonding Ionic, Covalent (nonpolar, polar)
• Polar and Non-Polar Molecules
• Resonance
• Acids and Bases (Bronsted-Lowry)
• Acid Strength (Ka, pKa)
• Organic Acids and Bases
• Know examples of Lewis Acids and Bases
• Non-Covalent Interactions (, how do they work)