Characteristics of molecular matter

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Characteristics of molecular matter

• Gases, liquids, solids on the molecular level
• Transitions between states
• Chemical forces a second look
• Intermolecular forces
• Determining forces in play by evaluating formulas

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Dealing with a very common misconception
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Which choice best shows what happens when water
evaporates?
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Microscopic View of Molecular Solids, Liquids and
Gases

• No ionic substances, metals, network covalent
  substances or atomic substances
• Gaseous state - molecules in constant motion,
  widely separated, not attracted to each other,
  elastic collisions
• Liquid state - molecules moving slowly, close to
  each other, attractions to each other that are
  constantly forming and breaking
• Solid state - molecules close together locked
  into position, strong attractions for each other
  only have vibrational motion

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Transitions between states

• Addition of energy to the molecules, usually as
  heat (increase in temperature), causes transition
  to more dispersed state (state with more kinetic
  energy motion)
• Melting ice, boiling water
• Removal of energy from the molecules through
  cooling causes transition to more organized state
  (state with more potential energy attractions)
• Freezing water, condensing steam

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Gases, Liquids and Solids

• The state of matter depends on the competition
  between cohesive and disruptive forces
• Cohesive forces bonds and partial or full
  electrostatic attractions
• as T decreases cohesive forces can take hold
• all matter will be in the solid phase at absolute
  zero
• Disruptive force heat (increasing temperature)
• as T increases disruptive forces increase
• all matter will be in the gas phase at a high
  enough temperature
• And completely ionized at very high temperatures
• At ambient temperature (25oC)
• Cohesive forces in solids (fixed positional
  attractions) gt cohesive forces in liquids
  (transitory attractions) gt cohesive forces in
  gases (cannot take hold because molecules have
  too much kinetic energy)

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Changes of state

• At a phase change the temperature does NOT change
  until all of one phase is gone
• when melting, the temperature of a water/ice
  mixture stays at 0oC until ALL the ice is gone
  then the temperature starts to rise to the
  temperature of the surroundings

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Heats of Fusion and Vaporization

• Heat of Fusion
• Amount of heat needed to melt 1.00 g of a solid
• OR in reverse to freeze 1.00 g of liquid
• Q (fusion)
• for water Q (fusion) 80 cal/g (0.334 kJ/g)
• Heat of Vaporization
• Amount of heat needed to vaporize 1.00 g of a
  liquid
• OR in reverse to condense 1.00 g of vapor
• Q (vaporization)
• for water Q (vaporization) 540 cal/g (2.26
  kJ/g)
• Specific heat of water
• Applies at temperatures between 0 and 100 oC
  1.00 cal/g-oC (4.184 J/g-oC)
• Conclusion more cohesive forces have to be
  broken going from liquid to gas than in going
  from solid to liquid

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Heating curves

• Region A
• Below the melting point of ice ice is getting
  warmer but no melting
• Region B
• Melting point both ice and liquid water present
  NO CHANGE IN TEMPERATURE Heat of fusion
• Region C
• Last ice has melted and water temperature
  increases as heat added
• Region D
• Boiling point both liquid water and steam
  present NO CHANGE IN TEMPERATURE Heat of
  vaporization
• Region E
• Last liquid water has evaporated and steam
  temperature increases as heat is added

Heat added ?
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Evaporation of Liquids

• Liquids evaporate
• liquid molecules on surface obtain enough kinetic
  energy through collisions to escape
• The rate of evaporation depends on the strength
  of the cohesive forces in the liquid
• As temperature increases, evaporation increases
• Non equilibrium condition

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Evaporation
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Vapor Pressure of Liquids

• Liquids exert a vapor pressure
• after top put on vapor pressure increases until a
  constant value is reached
• the rate of liquid molecules leaving the surface
  the rate of vapor molecules being captured
• equilibrium condition
• As temperature increases, evaporation increases
  temporarily
• new equilibrium condition reached
• higher vapor pressure

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Boiling Points

• Boiling Point - The temperature at which the
  vapor pressure of a liquid equals the external
  pressure
• boiling points are lower at higher elevations
• food cooks slower
• in a pressure cooker boiling point of water is
  higher
• food cooks faster
• Normal Boiling Point - Temperature at which the
  vapor pressure of a liquid equals 1 atmosphere
  (760 mm Hg 760 torr) (sea level)
• The higher the boiling point the greater the
  OVERALL attractions in the liquid

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Vapor Pressure vs Temperature

• Vapor pressure curves for
• (a) carbon disulfide (CS2)
• (b) methanol, CH3OH
• (c) ethanol, CH3CH2OH
• (d) water, H2O
• (e) aniline, C6H5NH2
• ?The greater the vapor pressure, the less the
  attractions
• ?The lower the boiling point, the less the
  attractions
• THEREFORE ? attractions depend on size AND
  polarity

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Chemical Forces Round 2

• Major conceptual problem in chemistry
• Inability to distinguish between strong and weak
  forces
• Applying the wrong idea to explain a
  physical/chemical/biological phenomenon
• Electrostatic attractions
• Attractions between opposite charges OR partial
  charges
• Inter Particle Forces
• Inter between
• Particle a discrete chemical unit (ion or
  molecule)
• Strength of attraction is proportional to
  magnitude and proximity of charges
• Inverse square law
• Bonds
• Bond an overlap of electrons in orbitals
  between nuclei causing an attraction of one atom
  to another atom
• Metallic bonding pooling of electrons with many
  nuclei (no directionality)
• Covalent bonding sharing of electrons between
  nuclei (directionality)

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STRONG
STRONG
WEAK
WEAK
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Strong vs Weak Attractions

• Strong Forces
• Full ion ion interparticle forces in ionic
  substances
• Metallic bonds
• Covalent bonds in all covalent materials
• Weak Forces
• Attractions between covalent molecules
• Attractions between covalent molecules and ions
  (or ionic portions of covalent molecules)
• Attractions between ions (or ionic components of
  covalent molecules) in mixed states (primarily
  solutions)
• Think proteins, nucleic acids, membranes!!!
• All Forces
• Strength of attraction is proportional to the
  magnitude of the interaction
• 3 -3 attraction gt 1 -1 attraction triple bond
  is stronger than single bond
• Strength of attraction is proportional to
  proximity of interacting units
• The closer, the stronger

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Explaining the Properties of Matter

• The macroscopic properties of matter are a
  consequence of microscopic relationships between
  atoms/ions and groups of atoms/ions caused by the
  redistribution of electrons
• Redistribution of electrons is caused by
  electronegativity differences
• Large differences ? formation of ions
• Smaller difference ? polar covalent compounds
• Very small differences ? nonpolar covalent
  compounds
• Explanations can be in terms of strong OR weak
  forces OR a combination of strong and weak forces
• Vapor pressure, boiling point and melting point
• Solubility in various solvents
• Acidity and basicity
• Energy content of different kinds of matter
• Chemical reactivity
• Replication of DNA, binding of oxygen to
  hemoglobin, action of drugs, vision

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Explaining Boiling Points

• As size increases in a homologous group, boiling
  point increases
• Therefore attractions must be greater
• HF and H2O should have lower boiling points if
  size is the ONLY factor ? not the case
• Therefore another factor is responsible

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Intermolecular Forces

• Intermolecular Forces - Forces of attraction
  between molecules in the liquid, solid or mixed
  state (solutions)
• A special category of interparticle forces
• Types of attractions in liquids and solids
• London Forces - Instantaneous dipole-dipole
  attractions that exist between all molecules,
  nonpolar as well as polar.
• Only intermolecular attraction in nonpolar
  substances .
• Dipole-dipole Forces - Electrostatic attractions
  between the positive end of one polar molecule
  and the negative end of another polar molecule
• Present in polar substances
• Hydrogen Bonding - Dipole-dipole attractions
  between a hydrogen atom covalently bonded to N, O
  or F atom on one molecule and a N, O or F atom on
  another molecule
• an especially strong dipole- dipole attraction
• Types of attractions in solutions
• Between different chemical species
• Later when discussing solutions

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The London Force

• If the nature of the species does NOT change,
  as size increases, the of electrons increases,
  therefore the of induced dipoles that can form
  increases, therefore the of attractions
  increases, therefore the BP/MP increases
• BP/MP halogens
• I2 gt Br2 gt Cl2 gt F2
• BP/MP hydrocarbons
• C16H34 gt C8H18 gt CH4

• Induced Dipoles
• Dipole an item with two poles or ends
• Chemical dipoles have one end positive and the
  other end negative
• Induced to change a characteristic by an action
• The distribution of electrons
• Also known as van der Waals attractions or
  dispersion forces

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Models of the Dipole-dipole Force

• Attractions due to permanent dipoles
• Bond in which there is an electronegativity
  difference of about 0.3 or more
• Not O-H or N-H or F-H
• Not as strong as a hydrogen bond
• Present in all polar molecules

• If size stays the same ( of electrons) and
  polarity increases, the strength of individual
  attractions increase, the overall strength of
  attractions increases and the BP/MP increases

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The hydrogen bond (H-bond)

• The H-bond is NOT a bond
• Strongest weak electrostatic attraction
• negative end of the H-bond
• Highly electronegative atom
• en ? F 4.0 O 3.5 N 3.0
• Very small atom recall strong effective nuclear
  charge
• H-bond acceptor free electron pair required
• positive end of the H-bond
• Smallest atom that is involved in bonding
• H atom MUST be connected to F, O or N for it to
  become polarized positively (and have significant
  d)
• H-bond donor polarized H atom
• H-bonds can be mixed between different species
• Maximum strength depends on weaker contributor
• Solutions biological materials (DNA, proteins)

H- Bond
d-
d-
d
d
d
d
H-bond in water
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Number of H-bonds

• The number of H-bonds a species can form depends
  on the match of H-bond donors and H-bond
  acceptors per species
• HF 3 electron pairs (3 acceptors) 1 H-F bond
  (1 donor on H)
• 2 H-bonds possible in liquid (or solid) HF
• H2O 2 electron pairs (2 acceptors) 2 H-O bonds
  (2 donors)
• 4 H-bonds possible in liquid (or solid) H2O
• NH3 1 electron pairs (1 acceptor) 3 H-N bond
  (3 donors)
• 2 H-bonds possible in liquid (or solid) NH3

For illustration only showing 4 H-bonds in water
More typical H-bonding network in water
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Strength of H-bonds

• Total H-bond strength depends on
• Magnitude of polarization ? greatest for H-F
  (most electronegative) least for H-N
• Determines magnitude of partial charges d and d-
  
• Number of H-bonds that can form ? depends on
  match of donor and acceptor capability ? highest
  for H2O
• Determine proportional strength of H-bond in
  comparison to H-F
• H-F 1.9/1.9 H-O 1.4/1.9 H-N 0.9/1.9
• Excellent correlation to boiling points

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H-bond donors and acceptors
Solvents with H-bonding capacity

• Dimethyl ether
• Isomer of ethyl alcohol
• BP -23
• Cannot H-bond to itself!!
• No H-bond donor
• Dipole- dipole attraction

Water MP 0 BP 100
Hydrazine MP 1 BP 114

• Serine
• MP 228
• An amino acid
• Lots of donors
• acceptors
• Lots of H-bonding capacity high MP

Ethyl alcohol BP 78

• Small substances with significant H-bond donors
  and acceptors
• As nonpolar component increases, BP decreases
  (lower attractions)

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H-Bonding micro macroscopic

• Liquid H-bonds mobile
• At 25oC about 3 out of 4 possible H-bonds
• Solid H-bonds fixed and directed
• Below 0oC 4 per water

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Important Intermolecular Forces

• London Forces
• individually London force is weak
• cumulative London force is strong
• IM Rule 1 as the sizes increases (molecular
  weight), the number of potential temporary
  dipoles increases, therefore the number of
  attractions increase, therefore the substance is
  more likely to be a liquid or solid
• Hydrogen Bonding
• individually strong
• extremely strong cumulative effect
• Integrity of DNA and structural proteins
• IM Rule 2 as the of H-bonds increases, the
  number of attractions increases, therefore the
  substance is more likely to be a liquid or solid
  OR held together strongly in solution
• A few strong attractions can hold together matter
  as well as many weak attractions
• NOTE important dipole- dipole attractions are
  rare

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Melting points

• If the melting point is high ? attractive forces
  are strong
• If the attractive forces are strong ? the melting
  point will be high
• Very high melting points (gt500oC) are ALWAYS a
  consequence of ion ion attractions
• Lower melting points are due to cumulative WEAK
  ATTRACTIONS
• When ionic compounds melt, strong ion-ion
  attractions are broken when covalent compounds
  melt, weak intermolecular attractions are broken

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What is melting 1?

• Melting going from solid to liquid state
• Crystalline amphorous solids
• Liquids of components particles decomposed
  stuff
• Disruption of FIXED interparticle attractions
• Ionic substances
• Breaking ion-ion attractions
• Covalent substances
• Breaking intermolecular attractions
• London forces
• Dipole-dipole attractions
• H-bonds

liquid copper sulfate
Crystal of copper sulfate
Crystal of sodium chloride
Melting point 801
liquid sodium chloride
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What is melting 2?
Close-up particle
Attractions broken
Solid
Liquid
MP 114

• Iodine I2
• MW 253.8
• London
• Weak, many small
• Glucose C6H12O6
• MW 180
• H-bonds
• Few, strong
• Serine C3H7NO3
• Amino acid
• MW 105
• H-bonds polarity
• Few, strong

MP 150
MP 228
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Determining the Major Inter-species Attractive
Force

• Determine whether the species is covalent or
  ionic
• IF IONIC ? ion-ion force is major attractive
  force in play
• Determine whether the covalent substance is polar
  or nonpolar
• Make a Lewis structure if necessary for a
  decision
• IF NONPOLAR ? dispersion force is major
  attractive force
• Determine whether the polar covalent substance
  has H-bonding capacity
• Inspect Lewis structure
• IF H-O, H-N or H-F BONDS PRESENT ? H-bonding is
  major attractive force
• IF THESE TYPES OF BONDS NOT PRESENT ? dipole
  dipole force is major attractive force

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Matrix for Determining Dominant Interspecies
Attraction
Formula
IONIC
COVALENT
Look at Lewis structure
Ion-Ion attraction
NO
YES
Dispersion force
YES
NO
H- bond force
Dipole-dipole force
From previous flow chart