Factors That Affect Rate of Dissolving and Solubility Section 7.2

Solubility Solubility of a solute is the amount of solute that dissolves in a given volume of solvent at a certain temperature. Solubility is determined by the intermolecular forces between solute and solvent particles

RATE OF DISSOLVING The rate of dissolving is how fast a solute will dissolve in a solvent. The rate of dissolving depends on temperature agitation particle size

Temperature Generally the solute will dissolve faster in a solvent at higher temperatures. At higher temperatures the molecules move faster and have more kinetic energy. The more Ek, the more movement, and the more collisions between the solute and solvent and dissolving occurs.   The solubility of gases decreases as the temperature increases.

Agitation Stirring a solute in a solvent increases the rate of dissolving. Increased movement, increases collisions and therefore more dissolving.

Particle Size The smaller the particle the more surface area (and more particles). A large chunk of sugar does not dissolve as fast as an equal amount of powdered sugar. The more surface area the more contact spots are available and more dissolving occurs.

SOLUBILITY AND PARTICLE ATTRACTIONS Why will some solutes dissolve in a particular solvent and not in others? For a solute to dissolve: The forces holding the molecules of solute together must be broken. The forces holding the molecules of solvent together must be broken. Then new attractions are formed between the solute and solvent.

Solute molecules that are bonded together must break apart Solute molecules that are bonded together must break apart. This requires energy. Solvent molecules that are bonded together must break apart. This requires energy.

 Attractions between solute and solvent form. This gives off energy.

The sum total of energy given off must be greater than the energy required to break molecules apart. Recall exothermic reactions. This is a key concept in chemistry and you will study it further in Chemistry 621.

Example Iodine is I2. This molecule is held together many weak dispersion forces. Water is H2O. This molecule is held together by H-bonds and dipole-dipole forces.   The attractions between the water molecules is so strong that only a strong attraction to other molecules will break it. Iodine does not attract the water at all.

Example Iodine is I2. This molecule is held together many weak dispersion forces. Kerosene also a nonpolar molecule.   The attraction of the different molecules for each other is greater than the attraction between the molecules. Therefore iodine will dissolve in kerosene.

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SOLUBILITY AND INTERMOLECULAR FORCES Polar molecules dissolve other polar molecules because of dipole - dipole attractions and H-bonds. A polar solute is attracted to the opposite pole of a polar solvent.   H-bonds of a solute are attracted to electronegative atoms of a solvent and vice versa.

Ionic compounds have a crystal lattice structure Ionic compounds have a crystal lattice structure. When placed in a polar solvent, such as H2O, the negative end of the water molecule is attracted to a positive surface ion and vice versa. These are ion-dipole attractions. If these ion-dipole attractions are strong enough and replace the ionic bonds then dissolving occurs. Generally an ionic compound will dissolve in a polar solvent.

When ionic compounds dissolve in water, they become hydrated. Hydrated means that each ion is surrounded by water molecules. Hydrated ions are called electrolytes because they are able to move and conduct electricity.

Ionic bonds are very strong and some ionic compounds do not dissolve in water. The ionic bond is so strong that ion-dipole bonds do not overcome the ionic bonds and the crystal structure stays intact.

SOLUBILITY OF COVALENT COMPOUNDS Soluble Covalent Compounds Most covalent compounds do not dissolve in water because they are nonpolar while water is polar. Some molecules that have polar bonds will dissolve in water such as methanol, ethanol, and sugars. These all contain an O-H bond. Sucrose (sugar) molecules are separated and then surrounded by water molecules (hydrated). However, sucrose molecules do not conduct electricity and covalent compounds dissolved in water are non-electrolytes. (Electrons of the molecules do not move).

Insoluble Covalent Compounds Nonpolar covalent compounds do not have any attraction points to the polar water molecule and therefore do not dissolve.   Nonpolar compounds will dissolve in other nonpolar compounds. The attractive forces within the solute and the solvent are replaced by attractive forces between the solute and solvent. Like Dissolves Like

Exceptions Some compounds may have a polar end and a nonpolar end and can therefore dissolve in both polar and nonpolar solvents. An example is acetic acid, CH3COOH.

ELECTRONEGATIVITY AND SOLUBILITY You can use the electronegativity difference of a binary compound to predict if the compound is ionic, polar, or nonpolar. Then based on that outcome and the idea of like dissolving like, you can predict if the compound will dissolve in water. Water will dissolve polar and ionic compounds, but not nonpolar.

Section Review on Page 254 #s 1 - 5   Chapter 7 Review Page 277 #s 2 - 10

FACTORS THAT AFFECT SOLUBILITY The three factors that affect solubility are molecular size, temperature, and pressure.   Although similar, do not confuse with the 3 factors that affect the rate of dissolving.

Molecule Size and Solubility When similar types of molecules are compared, those with a smaller size tend to dissolve easier.   Methanol, CH3OH, is very soluble while pentanol, CH3CH2CH2CH2CH2OH, is not very soluble at all. These two types of substances have the OH as a polar end and a nonpolar end, CH’s. The larger the molecule, the larger the nonpolar end and the more difficult to dissolve in a polar solvent.

Temperature and Solubility Temperature affects solubility which is why the temperature is always given with the solubility data.   We know that a solvent can hold (dissolve) more solid solute at higher temperatures. For example, the solubility of caffeine is 2.2g/100mL at 25oC. But at 100oC the solubility of caffeine is 40g/100mL. Temperature does not affect the solubility of liquids in liquids. The bonds within liquid molecules are not as strong as bonds in solids. There are fewer forces of attraction to overcome in liquids.

Gas particles have a lot of kinetic energy. When gas particles dissolve in a liquid some of that energy is lost. If, however, the liquid is at a high temperature, the gas particles gain more kinetic energy and will come out of the solution (evaporate). The solubility of gases decreases as the temperature increases.

Pressure and Solubility Pressure changes do not affect solid or liquid solutions. Pressure does affect the solubility of a gas. The solubility of a gas is directly proportional to the pressure of the gas above the liquid.   The less pressure above a lake the less soluble oxygen is. When a can of pop is opened and the carbon dioxide is released the gas pressure above the liquid decreases and the solubility of the gas in the soft drink decreases. Eventually all the gas will leave the soft drink (if left open) and the pop goes “flat”.

The greater the pressure, the more gas will dissolve in the liquid. Scuba divers experience a lot of pressure underwater. This increased pressure, increases the solubility of nitrogen gas in the blood. As the diver surfaces the pressure decreases and the nitrogen comes out of the blood solution. The diver must rise slowly, so this process can go slowly. If the diver rises too fast he can experience ‘the bends’, a painful and sometimes fatal condition caused by the nitrogen in the blood.

Heat Pollution: A Solubility Problem Many industries and power plants use water to help cool machinery. This cool water is piped around the areas to absorb the heat. The water is then returned to the nearby lakes or streams. Even though the water was not used or had chemicals added, this is still considered a form of pollution.   The returned warmer water can raise the temperature of the lake or stream. Water at higher temperatures holds less dissolved oxygen. This is harmful to fish and other aquatic life. Lake Erie, in some parts, is anoxic, without oxygen. It was a dead lake, nothing could live or grow in these areas. This was a result of various types of pollution and the shallow nature of the lake.

Finish Section Review on Page 254 #s 6-8 Do Review Sheet