4.5 Physical Properties: Solubility

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4.5 Physical Properties: Solubility

When a solid, liquid or gaseous solute is placed in a solvent and it seems to disappear, mix or become part of the solvent, we say that it dissolved. The solute is said to be soluble in the solvent. The forces of attraction between the solute and solvent are the key to understanding their solubility. The general rule is "Like dissolves like". In other words, a polar or charged solute will dissolve in another polar or charged solvent and a non polar solute will be insoluble in a polar or charged solvent. This means that ionic substances generally dissolve in polar solvents (like water) and non-polar molecules are generally soluble in non-polar solvents.

Solvent Solute Soluble / insoluble according to

"Like dissolves like" rule

H

₂

O HCl

H

₂

O NaCl

H

₂

O CCl

₄

CCl

₄

I

₂

CCl

₄

NaCl

H

₂

O CH

₃

CH

₂

OH CCl

₄

CH

₃

CH

₃

Solubility of Covalent molecules

To understand why "like dissolves like” the balance between the forces holding the solute and solvent

particles together needs to be considered. Consider water (solvent) and ethanol (solute)

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The force of attraction between the water molecules and between ethanol molecules is weaker than the force of attraction between the combined water and ethanol molecules making ethanol soluble in water.

In order to dissolve the hydrogen bonds between the water molecules and the hydrogen bonds between the ethanol molecules are first broken. New hydrogen bonds are then made between the water and ethanol molecules.

Short chained organic molecules with a polar head tend to be soluble in polar solvents like water.

However, as the length of the non polar hydrocarbon increases, the non-polar chain will eventually outweigh in size the polar 'head' and the molecule will become insoluble in polar solvents. The molecule will now dissolve in non-polar solvents. Consider the table below which shows the solubility, melting point and boiling point of alcohols. All alcohols have a characteristic polar “head” the –O-H functional group attached to the hydrocarbon chain.

Name of

Alcohol Formula Solubility in

water Bpt Mpt

methanol CH

₃

OH soluble 64.5 -97.7

ethanol CH

₃

CH

₂

OH soluble 78.3 -114.1

propanol CH

₃

CH

₂

CH

₂

OH soluble 97.2 -126.2

butanol CH

₃

CH

₂

CH

₂

CH

₂

OH insoluble 117.1 -89.3

pentanol CH

₃

CH

₂

CH

₂

CH

₂

CH

₂

OH insoluble 138.0 -78.2

octanol CH

₃

CH

₂

CH

₂

CH

₂

CH

₂

CH

₂

CH

₂

CH

₂

OH insoluble 195.2 -14.9

Notice how the boiling points and melting points increase as the hydrocarbon chain increases in length.

This is because the number of electrons is increasing, increasing the strength of the intermolecular van

der Waals forces, therefore increasing the amount of heat energy that needs to be absorbed in order

to break them.

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Consider what happens when the larger alcohol pentanol is mixed with water. The -OH end of the alcohol molecules can form new hydrogen bonds with water molecules, but the non polar hydrocarbon "tail"

doesn't form hydrogen bonds. That means that quite a lot of the original hydrogen bonds being broken aren't replaced by new ones. In place of those original hydrogen bonds van der Waals forces form between the polar water and the non polar hydrocarbon "tails". These attractions are much weaker meaning that pentanol will not mix with water and instead forms an insoluble layer on top of the water.

Therefore, when a polar solute dissolves in a polar solvent the

intermolecular bonds between the solute and solvent are broken and then new intermolecular bonds are formed between the solute and solvent molecules. Bond breaking is an endothermic process. The solute molecules must absorb enough heat energy from the surrounding solvent to break the

intermolecular forces and separate the molecules. Bond making however is exothermic. The solute molecules release heat energy into the surrounding solvent when new bonds between the solute and solvent are made. Releasing heat energy decreases the solute molecules kinetic energy allowing them to get close enough to form these new intermolecular forces (IMF are short range forces).

If the heat energy needed to break IMF (endothermic) > heat energy need to make new IMF (exothermic) then the overall change will be endothermic and the temperature of the resulting solution will decrease. The temperature change is proportional to the strength of the intermolecular forces.

If the heat energy needed to break IMF (endothermic) < heat energy need to make new IMF

(exothermic) then the overall change will be exothermic and the temperature of the resulting solution will increase.

Solubility of Ionic Compounds

If an ionic substance dissolves in water it means the force of attraction the polar water molecules have for the ions is greater than the force of attraction the positive and negative ions in the lattice have for one another. The partial negative charge on the oxygen atom of the water is attracted to the positive metal ions of the giant ionic lattice and the partial positive charge on the hydrogen atoms of the water are attracted to the negative non-metal ions.

Not all ionic compounds are soluble in water and most can be classified as either soluble, insoluble or sparingly soluble. As a general rule, a soluble substance is one where ≥ 1g of solute dissolves in 100g of solvent. In an insoluble solute ≤ 0.1g of solute dissolves in 100g of solvent. In a sparingly soluble solvent, approximately 0.1-1g of solute dissolves in 100g of solvent.

The process of NaCl dissolving in water is represented in the following way in a chemical equation

NaCl

_(s)

+ H

₂

O(l) → Na

⁺_(aq)

+ Cl

^-_(aq)

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+ H

2

O(l) →

Distinguishing between dissociation and ionization

Dissociation refers to the breaking of chemical bonds, whether the product is ions or neutral molecules.

The amount of energy required to break a specific bond is called its bond dissociation energy.

Ionization refers to any process that forms ions. For example sodium chloride dissolving in water is ionization. Likewise an atom losing an electron to become a positive ion in a mass spectrometer is also ionization. Sodium chloride dissolving in water can also be called dissociation or ionization because bonds are broken and ions are produced.

Check out this NaCl dissolving animation

http://www.northland.cc.mn.us/biology/Biology1111/animations/dissolve.html

Solvent

Intermolecular forces between

solvent molecules

Solute

Forces of attraction in

solute

Soluble /

insoluble Reason

H

₂

O

Hydrogen bonds between polar

molecules

NaCl

Intramolecular Electrostatic

attraction between Na

⁺

and Cl

^-

ions

soluble

Soluble because the force of attraction between the positive Na

⁺

and negative Cl

^-

ions is weaker than the electrostatic force formed between the polar water molecules and the

individual ions.

H

2

O

Hydrogen bonds between polar

molecules

Al

2

O

3

Intramolecular Electrostatic

attraction between Al

³⁺

and O

^2-

ions

insoluble

Insoluble because the

electrostatic force between the small highly charged Al

³⁺

and O

^2-

ions is stronger than the

electrostatic force formed

between the polar water

molecules and the ions.

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H

₂

O

Hydrogen bonds between polar

molecules

C

₆

H

₁₄

Van der waals forces between the

non-polar molecules

insoluble

Insoluble because the water molecules attract each other and hexane molecules attract each other more strongly than hexane molecules attract water. The two will not mix.

Hexane C

₆

H

₁₄

Van der waals forces between non-

polar molecules

Xylene C

₈

H

₁₀

Van der waals forces between the

non-polar molecules

soluble

Soluble because the attraction between non polar hexane and non polar xylene molecules is stronger than the attraction between hexane molecules and xylene molecules .

NOTE: Because all polar and non-polar covalent molecules contain van der waals forces, polar solvents can also attract non-polar molecules. However, the strength of this attraction is not strong enough to cause the molecules to dissolve.

Activity

1. Draw labeled molecular representations on A3 paper to show the following solutes, solvents and solutions. The diagrams should:

i) Use correct structural formulae in the correct shape. Showing three molecules/ions is sufficient.

ii) Identify the intramolecular and strongest intermolecular forces.

iii) Be neat and visually appealing in a way that helps interpretation.

iv) Be chemically accurate and have sufficient details (shape, bond angles, sizes of ions) 2. For diagrams 5, 6 and 7 explain, giving a clear account the reasons for the solubility.

Diagram 1 - Pure carbon tetrachloride, CCl

4

(l) Diagram 2 - Pure potassium chloride, KCl(s) Diagram 3 - Pure methanol, CH

3

OH(l) Diagram 4 - Pure water, H

2

O(l)

Diagram 5 - Potassium chloride, KCl is soluble in water. Write a chemical equation to represent this.

Diagram 6 - Carbon tetrachloride, CCl

4

, is insoluble in water.

Diagram 7 -Methanol, CH

3

OH is soluble in water.

Questions

1. Explain why octanol (C

6

H

13

OH) is less soluble in water than ethanol (C

2

H

5

OH).

2. Explain why the lipid tristerin, (C

₁₇

H

₃₅

COO)

₃

C

₃

H

₅

is soluble in fat but not water.

3. Explain why CaCO

₃

is insoluble in water.

4. Explain why the temperature rises when water and ethanol are mixed.

5. Explain why the volume decreases when water and ethanol are mixed.

6. (N03/H) Identify which of the compounds butane (C

4

H

10

), and propan-1-ol are (CH

3

CH

2

CH

2

OH) insoluble and soluble in water and give your reasoning. [3]

7. (M99/H) Dodecanol, C

₂₀

H

₄₁

OH is only slightly soluble in water. Explain this property. [2]

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