Chap. 7: How Molecules Mix

Question 1

Review:

**Dipole **

Answer 1

Electrons in a compound are not distributed evenly. Instead, they tend to congregate preferentially to one side of the compound.

Question 2

What do you call a molecule that has a dipole?

Answer 2

A polar molecule.

Question 3

Are ion-dipole attractions weaker or stronger than ionic bonds?

Answer 3

Ionic-dipole attractions are weaker than ionic bonds.

Question 4

What happens to polar molecules when they are near an ionic compound?

Answer 4

The opposite charges electrically attract one another.

For example:

When sodium chloride molecules are near water molecules, a positive sodium ion attracts the negative side of a water molecule, and a negative chloride ion attracts the positive side of a water molecule.

(Ion-Dipole Attraction)

Question 5

Dipole-Dipole Attractions

Answer 5

An attraction between two polar molecules.

Question 6

Hydrogen Bond

Answer 6

An unusually strong dipole-dipole attraction. Usually occurs between molecules that have a hydogen atom covalently bonded to a small, highly electronegative atom (usually nitrogen, oxygen, or fluorine).

Question 7

Induced Dipole

Answer 7

A temporarily uneven distribution of electrons.

Question 8

What is an example of a dipole-induced dipole attraction?

Answer 8

Plastic wrap sticking to glass.

These wraps are made of very long nonpolar molecules that are induced to have dipoles when placed in contact with glass, which is highly polar.

Question 9

**Polarizable **

Answer 9

A term for larger atoms that can form an induced dipole more easily.

Question 10

Solvent

Answer 10

The component in a solution that is present in the largest amount.

For example:

When a teaspoon of sugar is mixed with 1 liter of water, the sugar is the solute and the water is the solvent.

Question 11

Solute

Answer 11

Any component in a solution that is not the solvent.

For example:

When a teaspoon of sugar is mixed with 1 liter of water, the sugar is the solute and the water is the solvent.

Question 12

Dissolving

Answer 12

The process of a solute’s mixing with a solvent.

Question 13

To make a solution, a solute must dissolve in a solvent; that is, the solute and solvent must form a (homogenous/heterogenous) mixture.

Answer 13

Homogenous mixture.

Question 14

Saturated Solution

Answer 14

A solution in which no more solute can be dissolved.

For example:

When you add sugar to a glass of water, the rapidly dissolves. As you continue to add sugar, however, there comes a point when it no longer dissolves. Instead, it collects at the bottom of the glass, even after stirring. At this point, the water is saturated with sugar (the water cannot accept anymore sugar).

Question 15

Unsaturated Solution

Answer 15

A solution that has not reached the limit of solute that will dissolve.

Question 16

Concentration of Solution

Answer 16

The quantity of solute dissolved in a solution.

concentration = amount of solute / amount of solution

Question 17

Amount of Solute

Answer 17

amount of solute = concentration of solution x volume of solution

Question 18

Volume of Solution

Answer 18

volume of solution = amount of solute / concentration of solution

Question 19

How many grams of sucrose are in 3 liters of an aqueous solution that has a concentration of 2 grams of sucrose per liter of solution?

Answer 19

Question 20

A solution you are using in an experiment has a concentration of 10 grams of solute per liter of solution. If you pour enough of this solution into an empty laboratory flask so that the flask contains 5 grams of the solute, how many liters of the solution have you poured into the flask?

Answer 20

Question 21

Mole

Answer 21

6.02 x 10²³

One mole of gold = 6.02 x 10²³ gold atoms

One mole of sucrose molecules = 6.02 x 10²³ sucrose molecules

Question 22

How many moles of sucrose are in 0.5 liter of a 2-molar solution? How many molecules of sucrose is this?

Answer 22

2-molar means 2 moles of sucrose per liter of solution. To get the amount of solute:

(2 moles/L) (0.5 L) = 1 mole, which is the same as 6.02 x 10²³ molecules.

Question 23

Does 1 liter of 1-molar solution of sucrose in water contain 1 liter of water, less than 1 liter of water, or more than 1 liter of water?

Answer 23

Molarity: The number of liters of *solution *(not solvent).

When sucrose is added to a given volume of water, the volume of the solution increases.

So if 1 mole of sucrose is added to a given volume of water, the result is more than 1 liter of solution.

Therefore, 1 liter of a 1-molar solution requires less than 1 liter of water.

Question 24

Solubility

Answer 24

The ability of a solute to dissolve in a solvent.

Solubility mainly depends on the attractions between the fundamental particles of the solute and solvent.

Question 25

Soluble

Answer 25

If a solute has any solubility in a solvent, then that solute is soluble.

Question 26

What is an example of an infinitely soluable solute?

Answer 26

Ethanol and water.

Infinitely soluble: A solute that has no practical point of saturation in a given solvent.

The hydrogen bonds among water molecules are about as strong as those between ethanol molecules. These two liquids therefore mix together quite well in just about any proportion.

We can even add ethanol to water until the ethanol, rather than the water, becomes the solvent.

Question 27

What is an example of a solute that has very little solubility in a given solvent?

Answer 27

Oxygen in water.

Only 0.004 gram of oxygen can dissolve in 100 millileters of water.

The only attractions that occur between oxygen molecules and water molecules are relatively weak dipole-induced dipole attractions.

Question 28

Insoluble

Answer 28

A material that does not dissolve in a solvent to any extent.

For example:

Sand and glass (although they are both soluable in hydrofluoric acid).

Styrofoam (although it is partially soluable in nail polish remover).

Question 29

Why isn’t sucrose infinitely soluble in water?

Answer 29

The attraction between two sucrose molecules is much stronger than the attraction between a sucrose molecule and a water molecule.

Because of this, sucrose dissolves in water only as long as the number of water molecules is way more than the number of sucrose molecules.

When there are two few water molecules to dissolve any additional sucrose, the solution is saturated.

Question 30

Does solubility change with temperature? How?

Answer 30

Yes.

Take, for example, sucrose in water.

By heating water almost to the boiling point, the hot water molecules will have greater kinetic energy and therefore will collide with the solid solute (sucrose) more vigorously.

The collisions will break up the particle-to-particle attractions in the solid.

Question 31

Although the solubilities of many solid solutes (like sucrose) are greatly increased by rises in temperatures, the solubilities of other solutes (like sodium chloride) are only mildly affected. Why?

Answer 31

There’s a number of factors:

• The strength of the chemical bonds in the solute molecules.
• The way the molecules are packed together.

Question 32

Precipate

Answer 32

A solute that has come out of a solution.

For example:

When a sugar solution saturated at a high temperature is allowed to cool.

Question 33

The solubility of gases in liquids (decreases/increases) with increasing temperature.

Answer 33

The solubility of gases in liquids decreases with increasing temperature.

With an increase in temperature, the solvent molecules have more kinetic energy. This make it more difficult for a gaseous solute to remain in solution, because the solute molecules are literally ejected by the high-energy solvent molecules.

Question 34

Why do warm carbonated beverages go flat faster?

Answer 34

The higher temperature causes the molecules of the carbon dioxide gas to leave the liquid solvent at a higher rate.

Question 35

The solubility of a gas in a liquid also depends on

_______________________.

Answer 35

The pressure of the gas immediately above the liquid.

Question 36

A (lower/higher) gas pressure above the liquid means more of the gas dissolves.

Answer 36

A higher gas pressure above the liquid means more of the gas dissolves.

Question 37

Hard Water

Answer 37

Water containing large amounts of calcium and magnesium ions.

When hard water is heated, its calcium and magnesium ions tend to bind with negatively charged ions also found in the water to form solid compounds.

These can clog water heaters and boilers.

You’ll also find hard water coating the inside of a used tea kettle.

Hard water also inhibits the cleansing actions of soaps and detergents.

Question 38

How does hard water inhibit the cleansing actions of soaps and detergents?

Answer 38

The sodium ions of soap and detergent molecules carry a +1 charge.

Calcium and magnesium ions (in hard water) carry a +2 charge.

The negatively charged portion of the polar head of soap or detergent molecule is more attracted to the double positive charge of calcium and magnesium ions than to the single positive charge of sodium ions.

Soap or detergent molecules will therefore give up their sodium ions to bind selectively with calcium or magnesium ions.

Soap or detergent molecules bound to calcium or magnesium ions tend to be insoluble in water.

Question 39

How can you determine if a bond between two atoms is polar, nonpolar or ionic?

Answer 39

By each atom’s electronegativity and the difference of electronegativity between the two atoms.

If the difference in electronegativity is:

0 – 0.4 = COVALENT (strongest bond)

0.5 – 1.7 = POLAR COVALENT (strongest bond, electrons shared equally)

> 1.7 = IONIC (electrons have been transferred)

For example:

Find the bond in NF₃.