Investigating The Alkanes

Physical Properties
Boiling Points

• What are the boiling points of methane, pentane and hexane? 

Methane has a boiling point of -161 degrees C, pentane 36 degrees C and hexane 68 degrees C. 

• Which is the first straight chain alkane to be a liquid at room temperature and pressure? 

Pentane. Methane and butane are both gases at room temperature and pressure. 

• In terms of intermolecular forces, explain why the boiling points of the alkanes increase with increasing molecular mass. 

Alkanes have Van der Waal's forces between them. With increasing molecular mass, there is also increasing surface area of straight chain alkanes, resulting in a stronger attraction between alkane molecules. This means more energy is required to break these bonds for a change of state. 

• What is the effect of branching on the boiling points of the alkanes?

Branching tends to lower the boiling point since it decreases the surface area. 

Solubility In Water

Measure out 2-3 cm3 of hexane into a test tube and add about twice this volume of water. Shake, then stand the test tube in a rack. 

• Does the hexane dissolve in water? In terms of intermolecular forces, explain why the two liquids behave in this way. 

The hexane doesn't dissolve in water but forms an immiscible layer instead. This is due to a difference of polarity between the two liquids. Polar solvents dissolve polar compounds best and non-polar solvents dissolve non-polar compounds. While hexane is non-polar, water is polar - so the two liquids do not mix. 

• Is hexane more or less dense than water? How do you know? 

Hexane is less dense than water as it forms the top layer of the two immiscible layers. 

Chemical Properties 

Reaction With Some Common Reagents

Add about 2cm3 of hexane to 2 cm3 of the reagent in a test tube, shake and look for any signs of a chemical reaction having occurred. Reagents: sodium hydroxide solution, bromine water, potassium manganate (VII) solution, and concentrated sulphuric acid. 

• Does hexane appear to react with any of these substances?

Hexane reacts with none of the substances - they all form immiscible layers. All these substances are aqueous solutions, meaning they are polar and hexane, a non-polar substance, won't react with them. 

• The alkanes were once more commonly called 'paraffins'. Why was this name used? 

Paraffin - means little reactivity. The alkanes don't react. 

• In which substance is bromine more soluble - hexane or water? Why? 

Bromine is more soluble in hexane - they are both non-polar and will dissolve into each other.
If it's bromine water, then water molecules are attached to the bromine molecules, and it'll more readily dissolve in water. 

• In which substance is potassium manganate (VII) more soluble - hexane or water? Why? 

Potassium manganate is more soluble in water - they are both polar and will dissolve into each other. 

Combustion Of Alkanes

Fill a test tube with methane from the gas tap. Stopper the tube and stand it in a test-tube rack. Light a split, unstopper the tube and apply the lighted splint to the mouth of the tube. 

• Write a balanced equation for the reaction that occurs.

CH4 + 2 O2 ----------> CO2 + 2 H2O

Using a pipette, place 3 drops of hexane on a watch glass. Light a long splint and use this to light the hexane. 

• Write a balanced equation for the reaction that occurs. 

2 C6H14 + 19 O2 ----------> 12 CO2 + 14 H2O

• Which burns with the sootier flame? Explain why hexane should burn with a sootier flame. 

Hexane - because it has a higher percentage of carbon than methane. 

Put a small piece of paraffin wax on a watch glass and attempt to ignite it with a lighted splint. 

• Can the wax be easily ignited? 

No. 

• Why is the wax harder to ignite than methane even though they both contain alkanes?

Paraffin is a much longer alkane chain and is therefore harder to ignite. 

• Why does a candle have a wick? 

The wick creates a mechanism called capillary action, in which the wick draws the molten wax to the flame, transported the liquid wax as fuel. When the fuel reaches the flame it then vaporizes and burns. 

Cracking Paraffin Oil

1. Put 3-4 cm depth of mineral wool into a boiling tube and add enough paraffin oil to thoroughly soak the mineral wool. 
2. Put several pieces of broken porcelain pot in the boiling tube. 
3. Set up the apparatus as shown in the diagram. 
4. Gently heat the porcelain pieces, allowing the first bubbles of gas to escape. 
5. Then heat more strongly, occasionally warming the oil. 
6. Collect four or five tubes of gas

• Why are the first bubbles of gas not collected?

These first bubbles are displaced air which was in the delivery tube before the experiment began. 

• Why is the porcelain heated strongly before the oil is warmed? 

The porcelain acts as a catalyst and needs to be heated strongly before the oil vaporizes and makes contact with it. The catalyst needs to be activated by the heat. If the catalyst pieces are colder than the boiling point of the paraffin, the oil will condense on them and not react. 

• Why is the porcelain broken into small pieces?

To create a greater surface area. 

• Add a few drops of bromine water to one of the test tubes containing gas, quickly stopper and shake. What happens? Write an equation for the reaction that takes place. 

The bromine water is decolorized. 

C2H4 + Br2 + H2O ----------> C2H4BrOH + HBr

• Try to light the gas in one of the the test tubes with a lighted splint. What happens? Write an equation for the reaction that occurs. 

Water vapour is readily produced. 

C2H4 + 3 O2 ----------> 2 CO2 + 2 H2O

• Name the type of reaction carried out in the main experiment. Why are reactions of this sort important in the petrochemical industry? 

Thermal cracking. Long chain alkanes can be broken down to shorter chain alkanes and alkenes which are much more useful to the petrochemical industry. 

• Given that the molecular formula of paraffin oil is C20H42, suggest an equation for the reaction that has occurred in this experiment. 

C20H42 ----------> C8H18 + C8H16 + C4H8