Chem Unit 3 Chapter 7.2-7.5

What are the six changes of state?

Melting, freezing, vaporization, condensation, sublimation, and deposition.

What is melting?

Solid → liquid; heat is absorbed.

What is freezing?

Liquid → solid; heat is released.

What is vaporization?

Liquid → gas; heat is absorbed.

What is condensation?

Gas → liquid; heat is released.

What is sublimation?

Solid → gas without becoming liquid; heat is absorbed.

What is deposition?

Gas → solid without becoming liquid; heat is released.

Which changes of state are endothermic?

Melting, vaporization, and sublimation; energy is absorbed to overcome attractions between particles.

Which changes of state are exothermic?

Freezing, condensation, and deposition; energy is released as particles form stronger attractions.

What happens to temperature during a phase change in a pure substance at constant pressure?

Temperature remains constant while energy changes the physical state.

What is vapor pressure?

The pressure exerted by vapor above a liquid when evaporation and condensation are at equilibrium.

What happens to vapor pressure as temperature increases?

Vapor pressure increases because more molecules have enough energy to escape the liquid.

When does a liquid boil?

When its vapor pressure equals the external atmospheric pressure.

What is the relationship between vapor pressure and boiling point?

At the same temperature and external pressure, a liquid with higher vapor pressure generally has a lower boiling point.

How do stronger attractive forces affect boiling point?

Stronger attractions → higher boiling point because more energy is required to separate molecules.

How do stronger attractive forces affect vapor pressure?

Stronger attractions → lower vapor pressure because fewer molecules escape into the gas phase.

Why does water have a relatively high boiling point?

Water molecules form strong hydrogen bonds that require substantial energy to overcome.

How does decreasing atmospheric pressure affect boiling point?

Boiling point decreases because vapor pressure reaches the lower external pressure at a lower temperature.

What are the five major types of attractive forces in this chapter?

Ion-ion attractions, ion-dipole forces, hydrogen bonding, dipole-dipole forces, and London dispersion forces.

What is the general strength trend for attractive forces?

Ion-ion > ion-dipole > hydrogen bonding > dipole-dipole > London dispersion, as a general guide; actual strengths can vary.

What are London dispersion forces?

Temporary attractions caused by fluctuations in electron distribution that create instantaneous and induced dipoles.

Which substances have London dispersion forces?

ALL atoms and molecules, whether polar or nonpolar.

What is the strongest intermolecular force in a nonpolar molecule?

London dispersion forces.

How does molecular size affect London dispersion forces?

Larger, more polarizable electron clouds generally produce stronger dispersion forces.

What are dipole-dipole forces?

Attractions between the partially positive and partially negative ends of polar molecules.

Which molecules experience dipole-dipole forces?

Polar molecules.

What is hydrogen bonding?

A relatively strong attraction involving hydrogen bonded to N, O, or F and a lone pair on N, O, or F of another molecule.

What must a pure molecular substance contain to form hydrogen bonds between its molecules?

Hydrogen directly bonded to nitrogen, oxygen, or fluorine and an available lone pair on a suitable atom.

What are ion-dipole forces?

Attractions between an ion and a polar molecule, such as Na+ interacting with water.

When are ion-dipole forces especially important?

When ionic compounds dissolve in polar solvents such as water.

What are ion-ion attractions?

Electrostatic attractions between oppositely charged ions, such as Na+ and Cl- in an ionic solid.

What is the strongest intermolecular force in H2O?

Hydrogen bonding; water also has dipole-dipole and London dispersion forces.

What is the strongest intermolecular force in NH3?

Hydrogen bonding because N-H bonds and nitrogen lone pairs allow hydrogen bonds.

What is the strongest intermolecular force in HF?

Hydrogen bonding because hydrogen is directly bonded to fluorine.

What is the strongest intermolecular force in CH4?

London dispersion because CH4 is nonpolar.

What is the strongest intermolecular force in CO2?

London dispersion because CO2 is linear and nonpolar.

What is the strongest intermolecular force in HCl?

Dipole-dipole because HCl is polar but does not form typical hydrogen bonds between its own molecules.

What is the strongest intermolecular force in CH3OH (methanol)?

Hydrogen bonding because methanol contains an O-H bond.

What is the strongest intermolecular force in CH3OCH3 (dimethyl ether)?

Dipole-dipole because it is polar but lacks O-H, N-H, and F-H bonds.

What is the strongest attraction between dissolved Na+ ions and water?

Ion-dipole forces.

How do you determine the strongest attractive force present?

Check for ions first (ion-ion or ion-dipole), then hydrogen bonding, then molecular polarity (dipole-dipole); nonpolar molecules rely on dispersion forces.

What does like dissolves like mean?

Substances with similar polarity and compatible intermolecular interactions tend to dissolve in each other.

What dissolves best in polar solvents?

Polar and many ionic substances, when favorable solute-solvent attractions can form.

What dissolves best in nonpolar solvents?

Nonpolar substances because they interact through compatible dispersion forces.

Why does oil not dissolve in water?

Oil is nonpolar and cannot form sufficiently favorable interactions with polar water to mix well.

Why can NaCl dissolve in water?

Polar water molecules surround Na+ and Cl- ions through ion-dipole attractions.

What determines whether a molecule is polar?

Bond polarity and molecular geometry; the bond dipoles must produce a nonzero overall dipole.

How do you predict molecular polarity?

Identify polar bonds, determine the molecular shape, and check whether the bond dipoles cancel.

What makes a bond polar?

A difference in electronegativity causes unequal sharing of bonding electrons.

Can a molecule have polar bonds but be nonpolar overall?

Yes. Symmetrical arrangements can cancel bond dipoles, as in CO2 and CCl4.

Why is CO2 nonpolar?

Its linear shape makes the two C=O bond dipoles cancel.

Why is H2O polar?

Its bent shape prevents the O-H bond dipoles from canceling.

Why is NH3 polar?

Its trigonal pyramidal shape gives it a net dipole.

Why is CH4 nonpolar?

Its symmetrical tetrahedral shape gives no net molecular dipole.

Why is CCl4 nonpolar?

Its four polar C-Cl bonds are arranged symmetrically and cancel.

Why is CH2Cl2 polar?

Its tetrahedral bonds are not arranged symmetrically enough to cancel their dipoles.

What does hydrophilic mean?

Water-loving; usually polar or charged and interacts favorably with water.

What does hydrophobic mean?

Water-fearing; usually nonpolar and does not mix well with water.

What is hydration?

The process in which water molecules surround and interact with dissolved ions or molecules.

Which end of water points toward a positive ion?

The partially negative oxygen end.

Which end of water points toward a negative ion?

The partially positive hydrogen ends.

What does amphipathic mean?

Having both a hydrophilic polar or charged region and a hydrophobic nonpolar region.

Why are fatty acids amphipathic?

They have a polar carboxyl head group and a nonpolar hydrocarbon tail.

What is the structure of a fatty acid?

A long nonpolar hydrocarbon chain with a carboxyl (-COOH) group at one end.

What is a micelle?

A spherical arrangement of amphipathic molecules with hydrophobic tails inward and hydrophilic heads outward in water.

Why do micelles form in water?

Nonpolar tails cluster away from water while polar or charged heads remain exposed to water.

How does soap remove grease?

Soap molecules embed their nonpolar tails in grease while their hydrophilic heads interact with water, helping disperse the grease into droplets that rinse away.

What is the hydrophilic part of soap?

The charged carboxylate head group (-COO-).

What is the hydrophobic part of soap?

The long nonpolar hydrocarbon tail.

What is a saturated fatty acid?

A fatty acid containing no carbon-carbon double bonds in its hydrocarbon chain.

What is an unsaturated fatty acid?

A fatty acid containing one or more carbon-carbon double bonds in its hydrocarbon chain.

Why are many saturated fats solid at room temperature?

Straight saturated fatty acid chains pack closely together, producing stronger overall dispersion attractions.

Why are many unsaturated oils liquid at room temperature?

Cis double bonds bend fatty acid chains, preventing close packing and lowering melting points.

What is the difference between fats and oils?

Fats are generally solid at room temperature and often contain more saturated chains; oils are generally liquid and often contain more unsaturated chains.

What does a cis double bond do to a fatty acid chain?

Introduces a bend or kink that reduces close packing.

What is a triglyceride?

A lipid consisting of glycerol bonded to three fatty acids.

What is a phospholipid?

An amphipathic lipid generally consisting of a glycerol backbone, two fatty acid tails, and a phosphate-containing polar head.

Which part of a phospholipid is hydrophilic?

The phosphate-containing polar head.

Which part of a phospholipid is hydrophobic?

The two nonpolar fatty acid tails.

What is a phospholipid bilayer?

A double layer of phospholipids with hydrophilic heads facing water and hydrophobic tails facing inward.

Why do phospholipids form bilayers in water?

Their polar heads interact with water while their nonpolar tails cluster away from water.

What is the fluid mosaic model?

A model describing the cell membrane as a flexible phospholipid bilayer containing proteins and other molecules that can move laterally.

Why is the cell membrane considered fluid?

Phospholipids and many membrane proteins can move laterally within the bilayer.

Why is the cell membrane considered a mosaic?

It contains a mixture of phospholipids, proteins, cholesterol, and other molecules.

How does unsaturation affect membrane fluidity?

Cis double bonds reduce tight packing of fatty acid tails, generally increasing membrane fluidity.

What is a steroid?

A lipid characterized by four fused carbon rings.

What is the basic structure common to all steroids?

Four fused carbon rings.

What is cholesterol?

A steroid found in animal cell membranes that helps regulate membrane fluidity and is a precursor for steroid hormones.

How does cholesterol affect membrane fluidity?

It buffers membrane fluidity by restraining phospholipid movement at higher temperatures and preventing tight packing at lower temperatures.

What are examples of steroid hormones?

Estrogen, testosterone, and cortisol.

How are steroids structurally different from triglycerides?

Steroids have four fused rings, while triglycerides contain glycerol bonded to three fatty acids.

Why are most steroids largely hydrophobic?

Their structures consist mostly of nonpolar carbon-hydrogen bonds and fused hydrocarbon rings.