Chapter 11

List the substances CCl4 , CBr4 , and CH4 in order of increasing boiling point.

CH4 < CCl4 < CBr -- Because all three molecules are nonpolar, the strength of dispersion forces determines the relative boiling points. Polarizability increases in order of increasing molecular size and molecular weight, CH4 < CCl4 < CBr; hence, the dispersion forces and boiling points increase in the same order

In which mixture do you expect to find ion-dipole forces between solute and solvent to exist: CH3 OH in water or Ca(NO3 )2 in water?

Ca(NO3)2 in water, because calcium nitrate is a strong electrolyte that forms ions and water is a polar molecule with a dipole moment. Ion-dipole forces cannot be present in a CH3OH/H20 mixture because CH3OH does not form ions

What happens when you increase the pressure on a gas?

Can drive transformations from gas to liquid to solid because the increased pressure brings the molecules closer together, thus making intermolecular forces more effective.

Which characteristic of a liquid is more like a gas than a solid? A.its density B.its compressibility C.its molar volume at STP D.its ability to flow

D. Its ability to flow

state of a substance

Depends largely on the balance between the kinetic energies of the particles (atoms, molecules, or ions) and the interparticle energies of attraction. The kinetic energies, which depend on temperature, tend to keep the particles apart and moving. The interparticle attractions tend to draw the particles together.

When the molecules of two substances have comparable molecular weights and shapes, dispersion forces are approximately equal in the two substances

Differences in the magnitudes of the intermolecular forces are due to differences in the strengths of dipole-dipole attractions. The intermolecular forces get stronger as molecule polarity increases, with those molecules capable of hydrogen bonding having the strongest interactions.

three types of intermolecular attractions that exist between electrically neutral molecules

Dispersion Forces Dipole-Dipole attractions Hydrogen Bonding (and ion-dipole)

van der Waals forces

Dispersion forces and dipole-dipole attractions

Dipole-Dipole trend

For molecules of approximately equal mass and size, the strength of intermolecular attractions increases with increasing polarity -- boiling point increases as dipole moment increases

Increasing intermolecular attractions

From least to most: Gas, Liquid, Crystalline Solid Gas - kinetic energy Liquid - kinetic and particle-particle attraction are similar Solid - particle-particle attraction

Dispersion force (London dispersion forces or induces dipole-induced dipole interactions)

Intermolecular forces resulting from attractions between induced dipoles.

Molecular shape affects intermolecular attraction

Linear molecule - larger surface area enhances intermolecular contact and increases dispersion force Spherical molecule - smaller surface area diminishes intermolecular contact and decreases dispersion force

Ion-Dipole Forces

The force that exists between an ion and a neutral polar molecule that possesses a permanent dipole moment. The magnitude of the attraction increases as either the ionic charge or the magnitude of the dipole moment increases. Ion-dipole forces are especially important for solutions of ionic substances in polar liquids, such as a solution of NaCl in water.

Hydrogen bonds and boiling points

The greater the number of hydrogen bonds possible, the more tightly the molecules are held together and, therefore, the higher the boiling point

intermolecular forces

vary over a wide range but are generally much weaker than intramuscular forces (ionic, metallic, or covalent bonds). therefore, less energy is required to vaporize a liquid or melt a solid than to break covalent bonds.

when does a liquid boil?

when vapor pressure equals atmospheric pressure -- when bubbles of its vapor form within the liquid. The molecules of the liquid must overcome their attractive forces to separate and form a vapor. The stronger the attractive forces, the higher the temperature at which the liquid boils

How is polarizability related to molecular size and mass?

Polarizability increases as the number of electrons in an atom or molecule increases, therefore, the strength of dispersion forces increases with atomic or molecular size. dispersion forces tend to increase in strength with increasing molecular weight

Why does the O side of H2O point toward the Na+ ion?

Positive ends of polar molecules are oriented toward negatively charged anion Negative ends of polar molecules are oriented toward positively charged cation

Solid

Retains own shape and volume Does not expand to fill its container Is virtually incompressible Does not flow Diffusion within a solid occurs extremely slowly

Dipole-Dipole interactions

A force that become significant when polar molecules come in close contact with one another. The force is attractive when the positive end of one polar molecule approaches the negative end of another.

intermolecular forces and boiling points

A liquid boils when bubbles of its vapor form within the liquid. The molecules of the liquid must overcome their attractive forces to separate and form a vapor. The stronger the attractive forces, the higher the temperature at which the liquid boils. Similarly, the melting points of solids increase as the strengths of the intermolecular forces increase. melting/boiling points are higher for chemical bonds vs substances w/intermolecular forces

Hydrogen bonding

An attraction between a hydrogen atom attached to a highly electronegative atom (usually F, O, or N) and a nearby small electronegative atom in another molecule or chemical group. Occurs between an H atom that is bonded to N, O, or F in one molecule and an N, O, or F atom in another molecule

Gas

Assumes both volume and shape of its container Expands to fill its container Is compressible Flows readily Diffusion within a gas occurs rapidly

Liquid

Assumes shape of portion of container it occupies Does not expand to fill its container Is virtually incompressible Flows readily Diffusion within a liquid occurs slowly

Substance A is a liquid at room temperature and pressure, while substance B is a gas under the same conditions. Both are molecular substances. Based on this observation, we can say that the intermolecular attractions in substance A are __________ those in substance B. A.the same strength as B.stronger than C.weaker than

B. Strong than -- The liquid state becomes more stable than the gaseous state either when the intermolecular attractions become stronger or the temperature decreases, so that the kinetic energies of the molecules decrease. Since substance A is a liquid at the same temperature where substance B is a gas, we can conclude that the intermolecular forces must be stronger in substance A.

The polarizability of an atom increases as the ___________ increases. A.All of the above answers would correctly complete the sentence. B.principal quantum number of the valence electron shell C.first ionization energy D.effective nuclear charge

B. principal quantum number of the valence electron shell -- Polarizability describes the ease with which the electron distribution around an atom can be distorted. As the principal quantum number of the valence electron shell increases (for example,moving down a group in the periodic table), the valence electrons are less tightly bound to the nucleus, which leads to an increase in polarizability.

One remarkable consequence of hydrogen bonding is seen in the densities of ice and liquid water

In most substances the molecules in the solid are more densely packed than those in the liquid, making the solid phase denser than the liquid phase. By contrast, the density of ice at 0 °C (0.917 g/mL)0 °C (0.917 g/mL) is less than that of liquid water at 0 °C (1.00 g/mL),0 °C (1.00 g/mL), so ice floats on liquid water.

When the molecules of two substances differ widely in molecular weights, and there is no hydrogen bonding, dispersion forces tend to determine which substance has the stronger intermolecular attractions

Intermolecular attractive forces are generally higher in the substance with higher molecular weight.

What major type of attractive interaction must be overcome for water to evaporate?

Mainly hydrogen bonds, which hold the individual H2 OH2 O molecules together in the liquid.

What happens when the temperature of gas decreases?

The average kinetic energy of its particles decreases, allowing the attractions between the particles to draw the particles close together, forming a liquid, and then to virtually lock them in place, forming a solid.

Why are intermolecular forces so much weaker than ionic bonds?

The charges responsible for intermolecular forces are generally much smaller than the charges in ionic compounds & distances between molecules are often larger than the distances between atoms held together by chemical bonds

Polarizability

The ease with which the electron cloud of an atom or a molecule is distorted by an outside influence, thereby inducing a dipole moment.

How can you change the average kinetic energy of particles?

You can change a substance from one state to another by heart or cooling. Example: NaCl is a solid at room temp, melts at 1074 K and boils at 1686 K under 1 atm pressure

Dispersion and periodic table trends

as atomic/molecular weight increases moving down the periodic table, dispersion forces grow stronger, which in turn lead to higher boiling points

Which of the following substances is most likely to be a liquid at room temperature? a. formaldehyde, H2CO b. fluoromethane, CH3F c. hydrogen cyanide, HCN d. hydrogen peroxide, H2O2 e. hydrogen sulfide, H2S

d. hydrogen peroxide, H202 (the rest are gas)

intermolecular attractive forces - solid

strong enough to hold particles close together and lock them virtually in place. not compressible because particles have little free space between them.

intermolecular attractive forces - liquid

strong enough to hold particles close together, resulting in denser and less compressible than gas, forces aren't strong enough to keep particles from moving past one another (why it can be poured)

Comparing Intermolecular Forces

identify the intermolecular forces operative in a substance by considering its composition and structure -Dispersion forces are found in all substances Dispersion: 0.1-30 kJ/mol Dipole-Dipole: 2-15 kJ/mol Hydrogen bonds: 10-40 kJ/mol Ion-dipole: 50 kJ/mol --All interactions are considerably weaker than covalent and ionic bonds

Hydrogen bonding importance in biology

plays important roles in many chemical systems, especially those of biological significance. For example, hydrogen bonding helps stabilize the three-dimensional structure of proteins, which is critical to their function. Hydrogen bonding is also responsible for the double-helical structure of DNA, which is key to its genetic function