Organic Chemistry (review)

1. Crude oil is an important raw material. It is made mainly of hydrocarbons.

a) What does "hydrocarbon" mean?

Answer: Hydrocarbons contain only carbon and hydrogen.


b) Describe how crude oil is made.

Answer: Crude oil is made by ancient biomass (plankton) being trapped in mud.


c) Describe how crude oil is separated.

Answer: Crude oil is heated and evaporated. The vapour enters a fractionating column. The vapour rises up the column and each fraction will condense at a different height (temperature).


d) Methane is collected in the refinery gas fraction and decane (C10H22 ) can be found in kerosene. Compare the properties of these two alkanes.

Answer: Methane has a lower boiling point than decane. So, methane is a gas at room temperature, but decane is a liquid. Methane is colourless whereas decane is brown. Methane is more flammable than decane.


e) Long chain hydrocarbons like decane are often cracked. Write the equation for the cracking of decane to make ethene and one other alkane product.

Answer: C10H22  →  C2H4 + C8H18


2. Ethane (C2H6 ) is a hydrocarbon that can be extracted from crude oil.

a) Name the process used to obtain ethane from crude oil.

Answer: fractional distillation


b) Draw the displayed formula of an ethane molecule.

Answer: 


c) What is the general formula of an alkane?

Answer: CnH2n+2


d) Calculate the relative formula mass for one molecule of ethane. Relative atomic mass (Ar): C = 12 ; H = 1

Answer: (2 × 12) + (6 × 1) = 30


e) Ethane can be used as a fuel in camping stoves. Write a balanced symbol equation for the complete combustion of ethane.

Answer: 2C2H6 + 7O2  →  4CO2 + 6H2O


3. Propene (C3H6 ) is an alkene and can be made by cracking undecane.

a) Write a balanced symbol equation for the cracking of undecane (C11H24 ) to make three molecules of propene and one other product.

Answer: C11H24  →  3C3H6 + C2H6


b) Explain why propene is unsaturated.

Answer: Propene contains a carbon–carbon double bond (C=C) and has two fewer hydrogen atoms compared to the similar alkane.


c) Compare the combustion of propene and propane in air.

Answer: Both propene and propane are hydrocarbons and will react with air in an exothermic reaction. Propene is more likely to undergo incomplete combustion than propane. This means that the propene flame is smokier as carbon (soot) is made. The propene flame releases is a cooler flame (less energy) as incomplete combustion is less efficient than complete combustion.


d) A student was given two unlabelled colourless liquids. Describe a chemical test to determine which liquid was an alkane and which was an alkene. Include the observations that the student would make.

Answer: Add a few drops of bromine water to a sample of the liquid to be tested and shake well. If the solution turns from red/orange/brown to colourless then it was unsaturated and is the alkene. If there is no visible change (colour remains), then the liquid was the alkane.


4. A student is making ethanol by fermentation using the equipment shown in the figure below.


a) Describe the process of fermentation to produce ethanol in terms of the reactions that occur.

Answer: A mixture of glucose solution and yeast is added to the conical flask. This is warmed to about 40°C and fermentation occurs. The enzymes in the yeast break down the sugar to make ethanol and carbon dioxide.


b) Describe any observations that the student makes.

Answer: There would be bubbles in the conical flask as the sugar reacted and made a gas. There would be bubbles seen going through the limewater. The limewater would go from colourless to cloudy as it reacted with the gas showing that carbon dioxide was made. As the reaction progresses, there would be a yeasty (bread) smell.


c) Suggest how the ethanol can be purified and collected.

Answer: fractional distillation


d) Ethanol can oxidise in air to make ethanoic acid. Draw the displayed formula of this product. Name the homologous series it belongs to.

Answer: 


e) Ethanol can react with the product in the question above. Name the sweet-smelling substance that is made.

Answer: ethyl ethanoate


5. Polymers are an important material in everyday life.

a) Describe how addition polymers are made.

Answer: Many small molecules (monomers) join together to form very large molecules (polymers).


b) Describe how condensation polymers are made.

Answer: Condensation polymerisation involves monomers with two different functional groups. When these types of monomers chemically react, they join together, they also lose small molecules such as water.


c) Polypeptides are an important class of natural polymers. Identify whether a polypeptide is an addition or a condensation polymer.

Answer: They are examples of condensation polymers, as they are made from monomers with two different functional groups that chemically react to join them and they release a small molecule of water.