Reactions of Alkenes and Alcohols

1. What are "alkenes" ?

Answer: Hydrocarbons with carbon–carbon double bonds (C=C).


2. Write down the general formula of alkenes.

Answer: CnH2n


3. List the first four members of the alkene homologous series.

Answer: ethene, propene, butene and pentene


4. What do alkenes react with in combustion reactions?

Answer: oxygen


5. How do alkenes react with hydrogen, water and halogens?

Answer: By the addition of atoms across the carbon–carbon double bond (so that the double bond becomes a single carbon–carbon bond).


6. Write down the functional group for the alcohol homologous series.

Answer: –OH


7. List the first four members of the alcohol homologous series.

Answer: methanol, ethanol, propanol and butanol


8. Name the process used to make an aqueous solution of ethanol.

Answer: fermentation


9. List the first four members of the carboxylic acid homologous series.

Answer: methanoic acid, ethanoic acid, propanoic acid and butanoic acid


10. Why are carboxylic acids weak acids?

Answer: They only partially ionise in solution.


11. Propene is an alkene. The displayed formula of propene is shown in the figure below.


a) Write down the functional group of the alkene homologous series.

Answer: C=C


b) Give the molecular formula of propene.

Answer: C3H6


c) Write the word equation for the reaction between steam and propene.

Answer: propene + steam  →  propane


d) State the type of chemical reaction that happens when propene reacts with chlorine.

Answer: addition


12. Ethene is the first member of the homologous series of alkenes.

a) Draw the displayed formula of ethene.

Answer: 


b) Explain why ethene can be described as a hydrocarbon.

Answer: As ethene contains only hydrogen and carbon atoms.


c) Justify how ethene can be described as unsaturated.

Answer: Contains C=C bond and two fewer hydrogen atoms than the alkane with the same number of carbon atom.


13. Butene (C4H8 ) is an alkene.

a) Write down the number of atoms in a molecule of butene.

Answer: The number of atoms in a molecule of butene is 12


b) Compare the combustion reaction of butene with the combustion reaction of butane. Your answer should include the names of any products formed.

Answer: Butane will be more likely to undergo complete combustion whilst butene will be more likely to undergo incomplete combustion. The butene flame is smokier than the butane flame. Combustion of butene is more likely to make pollutants such as carbon and carbon monoxide.


c) Write a word equation for the combustion of butene to make carbon dioxide, carbon and one other product.

Answer: butene + oxygen  →  carbon dioxide + carbon + water


14. Pentene (C5H10 ) is a reactive hydrocarbon.

a) Identify the homologous series that pentene belongs to.

Answer: alkene


b) Pentene reacts with chlorine to make one product. Write the symbol equation for this reaction.

Answer: C5H10 + Cl2  →  C5H10Cl2


c) Pentene will react with hydrogen to make pentane. Identify this type of reaction.

Answer: hydrogenation reaction or addition reaction


d) Pentene reacts with steam. Draw the displayed formula of the product formed. Name the product.

Answer: 


15. Alcohols are a homologous series and can be used as fuels.

a) Explain why alcohols are not classified as hydrocarbons.

Answer: They contain oxygen.


b) Write the word equation for the complete combustion of ethanol.

Answer: ethanol + oxygen  →  carbon dioxide + water


c) Suggest what will happen if a sample of methanol is left open to the air.

Answer: Oxidise to form a carboxylic acid or methanoic acid.


16. Ethanol is the second member of the alcohol homologous series. Ethanol can be made from a fermentation reaction using the apparatus in the figure below.


a) Write the symbol equation for the fermentation of glucose (C6H12O6 ) using yeast.

Answer: C6H12O6  →  2C2H5OH + 2CO2


b) Suggest what observations you would make during this reaction.

Answer: bubbles going through the limewater ; limewater would go from colourless to cloudy ; glucose solution and yeast could get foamy


c) Suggest the purpose of the oil on the glucose solution.

Answer: To prevent any oxygen getting into the solution (anaerobic respiration) so that ethanol is made.


d) Ethanol made from fermentation is a mixture. Describe a method for collecting a pure sample of ethanol.

Answer: This could be done through distillation. Heat the reaction mixture until the ethanol evaporates (at 78°C). Collect the vapours and allow them to condense.


17. Vinegar has a sharp taste and a pH less than 7. It is a carboxylic acid known as ethanoic acid.

a) Write down the functional group of ethanoic acid.

Answer: –COOH


b) Suggest the product when ethanoic acid is reacted with ethanol.

Answer: ester or ethyl ethanoate


c) Ethanoic acid (C2H4O2 ) will react with calcium carbonate (CaCO3 ) to make calcium ethanoate [Ca(C2H3O2)2 ], water and a gas. Write a balanced symbol equation for this reaction.

Answer: 2C2H4O2 + CaCO3  →  Ca(C2H3O2)2 + H2O + CO2


d) Describe a method to identify the gas produced.

Answer: Gas to be bubbled through limewater, which will turn cloudy if the gas is carbon dioxide.


18. Ethanoic acid can be used to pickle vegetables. The displayed formula of ethanoic acid is shown in the figure below.


a) Write down a balanced symbol equation that shows ethanoic acid behaving as a weak acid.

Answer: CH3COOH  ⇋  CH3COO + H+


b) Suggest the pH of a solution of ethanoic acid.

Answer: Any number between 3 and 6


19. The figure below shows some examples of structures of organic molecules. Which of these compounds ... is a hydrocarbon? .... can be used to make addition polymers? ... is the product of the reaction between ethene and a halogen? ... is the product of the reaction when ethanol is left open to the air? ... is the product of the reaction between A and an alcohol?


Answer: Compound C is a hydrocarbon and can be used to make addition polymers. Compound E is the product of the reaction between ethene and a halogen. Compound A is the product of the reaction when ethanol is left open to the air. Compound D is the product of the reaction between A and an alcohol.


20. Ethene is the first member of the alkene homologous series.

a) Write the balanced symbol equation for the reaction of steam with ethene.

Answer: C2H4(g) + H2O(g)  →  C2H6O(g)


b) Write the balanced symbol equation for the reaction of ethene with hydrogen.

Answer: C2H4 + H2  →  C2H6


c) Name the type of reaction when iodine reacts with ethene.

Answer: addition



21. Ethanol is an important fuel. Pure ethanol can be produced in industry by fermentation or by hydration of ethene. The table below summarises the two methods. Use this information and your own knowledge to evaluate the two methods of production of ethanol. Your answer should include the balanced symbol equations for each reaction.


Answer: Fermentation is C6H12O6  →  2CO2 + 2C2H6O  and  hydration of ethene is C2H4 + H2O  →  C2H6O
Fermentation is a more sustainable way to make ethanol as plants are a renewable resource and less energy is required as the reaction mixture is only heated to a maximum of 40°C. However fermentation has a lower atom economy and further processing is needed to get a pure product. In contrast, hydration of ethene is quicker and produces 100% pure product with less waste as it has a 100% atom economy.


22. Carboxylic acids are a homologous series of organic compounds. They can be made from alcohols in an oxidation reaction.

a) Give the molecular formula of methanoic acid.

Answer: CH2O2  or  HCOOH


b) Draw the displayed formula of propanoic acid.

Answer: 


c) Ethanoic acid can react with ethanol to make water and another organic chemical. Give the name of the organic product and the homologous series that it belongs to.

Answer: The organic product is ethyl ethanoate and it belongs to ester series.


d) Describe what happens when butanoic acid is added to an equal volume of water and stirred.

Answer: Butanoic acid will dissolve. The acid group (–COOH) partially ionises and releases H+(aq), making the solution acidic.


23. Ethene and ethane are both hydrocarbons. The figure below shows the structure of these two molecules. Describe the difference in reactions between ethene and ethane. Explain your answer in terms of bonding.


Answer: Ethene is unsaturated (has C=C) and this makes it more reactive than ethane. Both molecules will undergo combustion, but ethene is more likely to undergo incomplete combustion and have a smokier flame. Ethene will undergo addition reactions, whereas ethane will not. Ethene can be a monomer (or make polymers), whereas ethane cannot.


24. Pentene is an organic molecule.

a) Write down the molecular formula of pentene.

Answer: C5H10


b) Explain why pentene is unsaturated whereas pentane is saturated.

Answer: Pentene contains two fewer hydrogen atoms than the pentane with the same number of carbon atoms. Pentene has a double carbon-carbon bond (C=C), whereas pentane does not.


c) Write a balanced symbol equation for the reaction of pentene with bromine.

Answer: C5H10 + Br2  →  C5H10Br2


25. Sodium is a Group 1 metal and can react with alcohols to make a salt and hydrogen.

a) Write a balanced symbol equation for the reaction of methanol with sodium.

Answer: 2Na + 2HCOOH  →  2NaCOOH + H2


b) Describe a method to collect the gas produced in this reaction. Include the test and the observations that will identify the gas as hydrogen. Give a risk assessment with your answer.

Answer: Fill a test tube with the alcohol and trap the sodium beneath it (collect the gas via displacement). Use a lighted splint, and after a while you will hear a (squeaky) pop if the gas is hydrogen. Keep the naked flame from the ethanol and sodium as they are both flammable. Wear gloves because sodium and ethanol can damage the skin and wear eye protection to prevent any splashes of chemicals into the eyes.


26. Esters can be made by reacting an alcohol with a carboxylic acid as shown in the equation in the figure below.


a) Which alcohol is the functional group?

Answer: O–H


b) Name the two products of this reaction.

Answer: ethyl ethanoate and water


c) Explain the role of the catalyst.

Answer: To speed up the chemical reaction by providing an alternative reaction pathway with a lower activation energy.


27. A solution of ethanol was left open to the air and a chemical reaction took place. The figure below shows the displayed formula of the organic product.


a) Describe the structure and bonding in this molecule.

Answer: Covalent bonding between the atoms. Each bond is made from a shared pair of electrons. The product has a simple molecular structure or two shared pairs.


b) Give the name and homologous series of this chemical.

Answer: The name is ethanoic acid and the homologous series is carboxylic acid.


c) Write a balanced symbol equation for the complete combustion of this chemical.

Answer: C2H4O2 + 3O2  →  2CO2 + 2H2O


d) Ethanol can be made by fermentation. Describe a method to make a sample of pure ethanol using yeast and a sugar solution. Include the reaction conditions in your answer.

Answer: Mix yeast and sugar solution together. Warm to about 30ÂșC and remove air to leave for a few days. Distil the mixture by heating and evaporating the ethanol and then condense and collect the distillate.