chemistry chapter 12

viscosity

Among the three compounds, ethane is the only gas at 25°C, and it has the lowest viscosity because its molecules are so far apart that the intermolecular interactions that hinder flow are very weak. At the same temperature, nonane has the highest viscosity because it has the longest molecules, allowing them to make more contact with one another and possibly also get entangled.

A sealed vessel contains only water and water vapor. Describe how the number of molecules of water in the liquid and gas phases is related to equilibrium vapor pressure. Vapor pressure is the point when evaporation and condensation of a substance reach equilibrium. The water molecules continue to evaporate and condense in the same proportions.

As a liquid heats up, the molecules gain kinetic energy and increasingly collide with one another. The equilibrium point described by vapor pressure occurs upon reaching the maximum amount of water vapor molecules that can exist at any one time. The total number of gas and liquid molecules remains constant but individual molecules shift between the liquid and vapor phases.

Describe what happens to the relative strength of intermolecular forces and the kinetic energy of the molecules when a piece of ice melts

As the ice melts, the kinetic energy of the molecules increases until it can overcome the organized hydrogen bonding interactions that hold the molecules in the ice crystalline structure. Random hydrogen bonding interactions continue to exist to hold the water molecules close together in the resulting liquid.

Describe how the electron density distribution of a molecule changes when the molecule approaches a dipole or charged particle.

As the molecule approaches the polar molecule or ion the electron density distribution shifts. If the approaching charge is negative, the electron density distribution shifts so that the electron density is highest in the parts of the molecule that are farthest from the approaching charge. If the approaching charge is positive, the electron density distribution shifts so that the electron density is highest at the parts of the molecule that are closest to the approaching charge.

The viscosity of water is 1.14 ✕ 10−3 N·s/m2 at 15°C. Predict whether the viscosity will increase or decrease when the temperature is raised to 55°C. (Note that viscosity is given in the unit newton-seconds per meter squared.)

As the temperature increases, the water molecules move faster and can more readily overcome the intermolecular attractive forces that hinder flow. The viscosity of water can be expected to decrease when the temperature is raised from 15°C to 55°C.

Arrange the following compounds in decreasing order of boiling points: decane, ethane, octane, and hexane. Explain your answer

Because boiling points decrease as molar mass decreases, the compounds can be arranged in order of decreasing boiling points as follows. decane > octane > hexane > ethane

Describe dipole-dipole interactions.

Dipole-dipole interactions are electrostatic attractions between partial charges (of opposite signs) on neighboring dipolar molecules. The greater the dipole moments of the molecules involved, the greater the intermolecular attraction. Dipole-dipole interactions cause the molecules in a liquid or solid to arrange themselves in an orderly orientation.

Predict the solubility of propane and ethanol in water. Explain your reasoning.

Ethanol is very soluble in water because ethanol is polar and can have dipole-dipole and hydrogen bonding interactions with water. On the other hand, propane is nonpolar cannot have dipole-dipole or hydrogen bonding interactions, and is poorly soluble in water.

Rank the molecules in order of the expected strength of their London dispersion forces (from strongest to weakest). H3C−CH3, H3C−CH2-CH2−CH2-CH3, H3C−CH2-CH2−CH2-CH2−CH2-CH2−CH3

H3C−CH2-CH2−CH2-CH2−CH2-CH2−CH3 > H3C−CH2-CH2−CH2-CH3 > H3C−CH3

Classify hydrogen bond attraction as a type of intermolecular force.

Hydrogen bond attraction is a type of dipole-dipole force.

Identify the types of molecules or atoms that exhibit hydrogen bond attractions.

Hydrogen bond attraction occurs when an atom with an extremely high electronegativity is bound to a hydrogen atom.

Describe how hydrogen bond attractions affect the physical properties of a compound.

Hydrogen bond attractions between molecules of a substance lead to a high boiling point, surface tension, specific heat, cohesion, and capillary action.

Which type of intermolecular force is typically the strongest? Which is typically the weakest?

Hydrogen bonds and ion-dipole interactions tend to be the strongest intermolecular forces. London dispersion forces tend to be the weakest intermolecular forces.

Rank the molecules in order of the expected strength of their London dispersion forces (from strongest to weakest). Br2, F2, I2

I2 > Br2 > F2

Sodium chloride (NaCl) dissolves in water (H2O). Identify the intermolecular interaction that allows the sodium and chloride ions to dissociate

Ion-dipole interaction

"like dissolves like"

Polar molecules dissolve in other polar molecules and nonpolar molecules dissolve in other non polar molecules.

The surface tension of water is 7.28 ✕ 10−2 J/m2 at 20°C. Predict whether the surface tension of octane would be higher or lower than that of water at the same temperature. Explain your answer.

Since octane is a hydrocarbon, the main intermolecular forces within it are London dispersion forces. Water molecules, by contrast, form hydrogen bonding forces, which are much stronger than dipole-dipole interactions.As a result, more energy is needed to separate water molecules from one another than to separate octane molecules from one another. Therefore, the surface tension of octane can be expected to be lower than that of water when the two are considered at the same temperature.

ion- dipole interaction

The attraction or repulsion of a charged species (ion) and a polar molecule. Ex: NaCl + H2O

Dipole- dipole interactions

The attraction or repulsion of a polar molecule and another polar molecule. Ex: H2O + methanol

Is the boiling point of propane higher or lower than that of ethanol? Explain your answer.

The boiling point of propane is lower than that of ethanol because ethanol molecules have dipole-dipole and hydrogen bonding interactions with one another, whereas propane molecules do not.

Sodium chloride (NaCl) dissolves in water (H2O). Describe the arrangement of the dissolved ions and the water molecules.

The cation is attracted to the partially negative oxygen atom, and the anion is attracted to the partially positive hydrogen atoms.

Describe hydrogen bond attractions between water molecules in the liquid phase.

The partial charge on a water molecule attracts the opposite partial charge on adjacent water molecules.

Describe how the strength of dipole-dipole forces is related to the magnitude of the dipole moment of a substance.

The size or strength of a dipole moment, i.e., its magnitude, is determined by its geometry and differences in electronegativity between its component atoms. The larger the differences in electronegativity between two atoms, the stronger the dipole moment. The dipole-dipole interactions between two molecules with large dipole moments cause a strong interaction between the two.

Describe the property of the water molecule that leads to hydrogen bond attractions.

The water molecules have permanent dipoles with greater partial charge distribution due to the higher electronegativity of the oxygen atom and the location of the nonbonding electrons.

Phase change diagram

Triple point: where all three phases are represented

How will the vapor pressure of a liquid in a closed container be affected when the following changes are made to the system? The liquid is replaced with another liquid that contains stronger intermolecular forces between its molecules.

Vapor pressure will decrease.

How will the vapor pressure of a liquid in a closed container be affected when the following changes are made to the system? Temperature is increased.

Vapor pressure will increase.

How will the vapor pressure of a liquid in a closed container be affected when the following changes are made to the system? The liquid is placed in a wider, shallower container that allows the liquid to spread thinner, increasing its surface area

Vapor pressure will not change.

Sodium chloride (NaCl) dissolves in water (H2O). Describe the property of the water molecule that allows the sodium and chloride ions to dissociate.

Water is a polar molecule, so it retains a partial positive charge on the hydrogen atoms, and a partial negative charge on the oxygen atom.

dipole- induced dipole forces

an attraction between a polar molecule and a non polar molecule that is forced by environment to exhibit polarization. polarization: charge separation/ distribution.

surface tension

attraction of a liquid to the surface of another substance. (meniscus)

Water (H2O) has a boiling point of 100°C, and hydrogen iodide (HI) has a boiling point of −35.2°C.Why is the boiling point of water higher than that of hydrogen iodide?

because the intermolecular forces in water are stronger than in hydrogen iodide

atomization

creation of atoms from gas (enthalpy of formation)

ionization

creation of ions from atoms Ex: Cl(g) + e- -> cl- (g)

The dipole moment of diazomethane is 1.50 D and the dipole moment of cyanamide is 4.28 D. Both substances have approximately the same molecular mass (about 42 amu). Which of these two substances has stronger dipole-dipole attractive forces?

cyanamide

What intermolecular forces can exist in a gaseous sample of HCl?

dipole-dipole and London dispersion

Consider the straight-chain hydrocarbons ethane (C2H6), pentane (C5H12), and nonane (C9H20). At 25°C, ethane is a gas, and pentane and nonane are liquids. Among the three named compounds, predict which one has the lowest viscosity at 25°C.

ethane

the stronger the intermolecular forces, the __ the boiling point

higher

as boiling point increases, molar mass..

increases

Consider the straight-chain hydrocarbons ethane (C2H6), pentane (C5H12), and nonane (C9H20). At 25°C, ethane is a gas, and pentane and nonane are liquids. Among the three named compounds, predict which one has the highest viscosity at 25°C.

nonane

Hydrogen bonding

strongest of the intermolecular forces. The attraction of an -OH, -NH, or -FH to a molecule with a dipole. If a hydrogen is bonded DIRECTLY to an oxygen, nitrogen, or fluorine, that molecule can form hydrogen bonds to another electronegative atom.

capillary action

the pull of a liquid into a small diametered tube based on attraction between liquid and the mouth of the capillary

viscosity definition

the resistance of a liquid to flow based on "thickness".

heat of hydration

the smaller the size of an atom, the more energy required to hydrate it, so tiny atoms with higher charge have highest heat of hydration.

Induced- dipole induced- dipole (london dispersion forces)

weakest of all the intermolecular forces. Attraction between 2 non polar molecules