4.5 Physical Properties: Solubility

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1 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 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 are generally soluble in non-polar solvents. Solvent Solute HCl Soluble / insoluble according to "Like dissolves like" rule NaCl CCl 4 CCl 4 I 2 CCl 4 NaCl CH 3 CH CCl 4 CH 3 CH 3 Solubility of Covalent To understand why "like dissolves like the balance between the holding the solute and solvent particles together needs to be considered. Consider water (solvent) and ethanol (solute) 1 P a g e

2 The force of attraction between the water and between ethanol is weaker than the force of attraction between the combined water and ethanol making ethanol soluble in water. In order to dissolve the hydrogen bonds between the water and the hydrogen bonds between the ethanol are first broken. New hydrogen bonds are then made between the water and ethanol. Short chained organic 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 ethanol CH 3 CH soluble propanol CH 3 CH 2 CH soluble butanol CH 3 CH 2 CH 2 CH insoluble pentanol CH 3 CH 2 CH 2 CH 2 CH insoluble octanol CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH insoluble 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, therefore increasing the amount of heat energy that needs to be absorbed in order to break them. 2 P a g e

3 Consider what happens when the larger alcohol pentanol is mixed with water. The -OH end of the alcohol can form new hydrogen bonds with water, 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 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. Bond breaking is an endothermic process. The solute must absorb enough heat energy from the surrounding solvent to break the intermolecular and separate the. Bond making however is exothermic. The solute release heat energy into the surrounding solvent when new bonds between the solute and solvent are made. Releasing heat energy decreases the solute kinetic energy allowing them to get close enough to form these new intermolecular (IMF are short range ). 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. 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 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) + (l) Na + (aq) + Cl - (aq) 3 P a g e

4 + (l) Distinguishing between dissociation and ionization Dissociation refers to the breaking of chemical bonds, whether the product is ions or neutral. 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 Solvent Intermolecular between solvent Hydrogen bonds between polar Solute NaCl Forces of attraction in solute Intramolecular Electrostatic attraction between Na + and Cl - ions Soluble / insoluble soluble Reason 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 and the individual ions. Hydrogen bonds between polar 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 and the ions. 4 P a g e

5 Hexane C 6 H 14 Hydrogen bonds between polar Van der waals between nonpolar C 6 H 14 Xylene C 8 H 10 Van der waals between the non-polar Van der waals between the non-polar insoluble soluble Insoluble because the water attract each other and hexane attract each other more strongly than hexane attract water. The two will not mix. Soluble because the attraction between non polar hexane and non polar xylene is stronger than the attraction between hexane and xylene. NOTE: Because all polar and non-polar covalent contain van der waals, polar solvents can also attract non-polar. However, the strength of this attraction is not strong enough to cause the 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 /ions is sufficient. ii) Identify the intramolecular and strongest intermolecular. 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, (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) 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] 5 P a g e

6 Bibliography Clark, Jim. Chem Guide < Clugston, Michael and Rosalind Flemming. Advanced Chemistry. Oxford: Oxford University Press, Derry, Lanna, Maria Connor and Carol Jordan. Chemistry for use for the IB Diploma Standard level. Melbourne: Pearson Education, Green, John and Sadru Damji. Chemistry for use with the International Baccalaureate Programme. Melbourne: IBID Press, Neuss, Geoffrey. IB Diploma Programme Chemistry Course Companion. Oxford: Oxford University Press, IB Study Guides, Chemistry for the IB Diploma. Oxford: Oxford University Press, Organisation, International Baccalaureate. Online Curriculum Centre. < "Chemistry Data Booklet." International Baccalaureate Organisation, March "Chemistry Guide." International Baccalaureate Organisation, March "IB Chemistry Examination Papers." Cardiff: International Baccalaureate Organisation, P a g e