CHAPTER 4

SOLUTIONS Chemistry for AUSMAT

Prepared by Mr Venu (MCKL)

Formation of solution

• When homogeneous mixture formed (two substances mixed together), one substance has dissolved in another.

• No distinct boundaries bet. the two substances mixed.

• Dissolved substances has broken up into individual substances.

• Eg. Glucose (solid lattice) break away to mix with water molecules.

• This glucose solution = homogeneous mixture of glucose and water molecules.

• When Heterogeneous mixture formed (adding insoluble substance eg. Oil or sand), two phases (layers) will form.

• Solvent: large amount of solution.

• Solute: least amount of solution.

• Eg 1g of glucose + 500ml of water. (solvent=water, solute=glucose).

• See table 4.1, pg 94.

Solution process

• Stronger bond need more energy to break (the bond).

• Energy released when new bond formed.

• Stronger bonds, need more energy to make the bonds.

• Other factors of dissolving process: entrophy (the tendency of a system to move towards a greater state of disorder.

Mixing polar solutes and polar solvents

• Like-dissolves-like: polar solutes dissolve in polar solvent.

• Eg. HCl + Water.

Methanol + Water (pg 94).

Propanone + Water (pg 95)

• The previous reasoning can also be used to explain the solubility of polar solutes in other polar solvents apart from water.

• For example, methanol and ethanol are both soluble in propanone because during the dissolving process, the two alcohols form hydrogen bonds with propanone, and hydrogen bonds and dipole– dipole forces are broken.

Falkirk Wheel : Scotland The Falkirk Wheel transports boats between the Union Canal and the Forth and Clyde canal. It opened in 2002 and is the only rotating boat lift in the world. It completes a full spin in 8 minutes).

Mixing a non polar solute with polar solvent

Mixing a non-polar solute with a non-polar solvent

Do all polar substances dissolve in water, a highly polar solvent? • Some molecules contain a polar group, but the rest of the

molecule is essentially non-polar.

• For example, the alcohol butan-1-ol, CH3CH2CH2CH2OH, has an OH group that is polar.

• However, the chain of carbon and hydrogen atoms is nonpolar and this can have an influence on the solubility of the alcohol in water.

• *Solubility (in water) of alcohols decrease as the length of the carbon chain increases.

• even though the OH end of the alcohol molecule is able to form a hydrogen bond with water molecules, the hydrocarbon chain, being reasonably large compared to the water molecules, will have to push some water molecules apart to fit in between them.

• alcohol molecule becomes less polar as the number of carbon and hydrogen atoms in the chain increases.

• Because the molecules with the longer hydrocarbon chain also contain more electrons, the dispersion forces between them formed.

Prediction of solubility of polar and non-polar substances

Predictions concerning degree of solubility

• polar (and ionic) solutes will dissolve in polar solvents

• non-polar solutes will dissolve in non-polar solvents

• the solubility of a polar solute, and an ionic compound, decreases as the polarity of the solvent decreases

• the solubility of a non-polar solute increases as the polarity of the solvent decreases.

Predict the solubility of the following substances in water and in petrol. • ammonia, NH3?

• Water

•

• Petrol

•

• methane, CH4

• Water

•

• Petrol

•

• calcium chloride?

• Water

•

• Petrol

•

• Ethanol?

• Water

•

• Petrol

•

• naphthalene (C10H8)?

• Water

•

• Petrol

•

Dissolving ionic compounds in water

• When ionic compound dissolves in water:

• Bonds will be broken and new bonds will form.

• >>>>hydrogen bonds between the water molecules must be disturbed

• >>>>ionic bonds holding the ions in the lattice must be broken.

Attractive forces do exist between ions and polar molecules • This type of attractive force is called an ion–dipole force.

• It can be stronger than hydrogen bonds (but not as strong as the various forms of strong bonding).

• Positive ions attract the partial negatively charged ‘end’ of the water molecule, the lone pairs of the oxygen atoms.

• Vice versa.

• Water molecules cluster around each ion in layers.

• For smaller monatomic ions like Li, four water molecules usually form.

• But for larger ions, typically six water molecules.

*For some ions, these water molecules remain bonded to the ion even when the solvent is evaporated.

• ions with the polar molecules around them are called complex ions.

• Other than water, other polar molecules can form complexes…

• The polar molecules that are attracted to the positive and negative ions, during the formation of a complex ion, are often called ligands.

• A complex ion is therefore composed of a positive or negative ion and ligands.

Review exercise 4.2

• 1 Which of the following substances would you predict to be soluble in water?

• sodium phosphate

• propan-1-ol, C3H7OH

• turpentine, C10H16 (approximate)

• zinc carbonate ZnCO3

• iron(III) sulfide Fe2S3

• hydrogen bromide, HBr

A sulphide is a simple chemical combination of an element and the Sulphur atom e.g. Hydrogen Sulphide, H2S. A sulphite is a chemical combination of an element and a compound of Sulphur and Oxygen, namely 3 Oxygen atoms to every Sulphur atom. E.g. Calcium Sulphite CaSO3

• 2 Draw the particles present in an aqueous solution of potassium cyanide, labeling the bonds between and within the particles. Identify which bonds are weak and which are strong.

• 3 Explain what bonds are broken and formed during the dissolving of potassium bromide in water.

• Include diagrams to support your answer.

The mole concept

Review exercise 4.3

•Work in pairs.

•Q1-Q4.

Solution concentrations

• quantity of solute dissolved in a particular quantity of solution.

• commonly used to express:

• mole per litre, mol L-1

• gram per litre, g L-1

• parts per million, ppm

• percentage composition by mass.

Concentration in mole per litre, mol L-1 (molarity)

Concentration in gram per litre (g L-1)

Concentration in parts per million (ppm)

• In simple terms, the concentration in parts per million can be thought of as the

• mass in grams of solute dissolved in

• 1 000 000 g of solution.

Concentration expressed as percentage composition • percentage of solute in a particular solution.

Review exercise 4.4

• Question 3 and 6.

Diluting and mixing solutions

• When more solvent is added to a solution, the concentration of the solution decreases.

• In this dilution process, the volume of the solution is increased, but the amount of solute remains constant.

• mole of solute in concentrated solution = mole of solute in dilute solution

• See Example 4.14