Alkanes (Cambridge (CIE) O Level Chemistry): Theory Questions

Ozone is a form of oxygen. Ozone is present in the upper atmosphere and it prevents dangerous solar radiation from reaching the Earth's surface.

Some of the chemicals that diffuse into the upper atmosphere decompose ozone. Chemicals that have this effect are methane (CH₄), chloromethane (CH₃Cl) and an oxide of nitrogen (NO₂).

i) Which of these three chemicals diffuses the most slowly? Give a reason for your choice.

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ii) Chloromethane is formed when seaweed decomposes.

Name the compounds in the environment from which seaweed might have obtained carbon, hydrogen and chlorine.

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iii) How can chloromethane be made from methane?

Give the reagent and the condition.

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iv) The oxides of nitrogen are atmospheric pollutants.

Describe how they are formed.

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The reaction between methane and chlorine takes place only in the presence of ultraviolet light.

Explain the role of the ultraviolet light in this reaction.

A mixture of methane and chlorine is sealed in a flask and kept in complete darkness.

State what happens in the flask.

A student suggests that the flask could be warmed in a water bath instead of being exposed to ultraviolet light.

Explain why the student's suggestion would not work.

Hexane is an alkane.

Aqueous bromine is added to hexane and the mixture is kept in the dark.

Describe what is observed.

Explain your answer.

Hexane does react with chlorine when the mixture is placed in ultraviolet light.

Explain why hexane reacts with chlorine in ultraviolet light but does not react with aqueous bromine in the dark.

Propane also reacts with chlorine in the presence of ultraviolet light.

Give the structural formula of a product in which one hydrogen atom has been replaced by a chlorine atom.

Butane, C₄H₁₀, reacts with chlorine in the presence of ultraviolet light.

Several products are formed. One of them has the molecular formula C₄H₈Cl₂.

i) Deduce how many hydrogen atoms of the butane molecule have been replaced in this product.

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ii) Explain how the molecular formula shows this.

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Give the molecular formula of the product in which only one hydrogen atom of the butane molecule has been replaced.

A student writes the following equation for the formation of C₄H₈Cl₂.

C₄H₁₀ + Cl₂ → C₄H₈Cl₂ + H₂

The equation balances, but it does not represent the reaction correctly.

Explain why.

Alkanes are generally unreactive.

Explain why, in terms of the bonds in an alkane molecule.

Compound X is an unbranched hydrocarbon.

One molecule of X contains four carbon atoms.

Aqueous bromine is added to X in the dark. The mixture stays orange.

i) Deduce the molecular formula of X.

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ii) Explain how the result with aqueous bromine helped you.

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Name compound X.

Many organic compounds which contain a halogen have chloro, bromo or iodo in their name.

The following diagram shows the structure of 1-chloropropane.

Draw the structure of an isomer of this compound.

Describe how 1-chloropropane could be made from propane.

Suggest an explanation why the method you have described in part (b) does not produce a pure sample of 1-chloropropane.

The alkanes are generally unreactive. Their reactions include combustion, substitution and cracking.

The complete combustion of an alkane gives carbon dioxide and water.

i) 10 cm³ of butane is mixed with 100 cm³ of oxygen, which is an excess. The mixture is ignited.

Give the volume of unreacted oxygen left, in cm³, and the volume of carbon dioxide formed, in cm³.

C₄H₁₀ (g) + 6.5O₂ (g) → 4CO₂ (g) + 5H₂O (l)

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ii) Why is the incomplete combustion of any alkane dangerous, particularly in an enclosed space?

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The equation for a substitution reaction of butane is given below.

CH₃−CH₂−CH₂−CH₃ + Cl₂ → CH₃−CH₂−CH₂−CH₂−Cl + HCl

 i) Name the organic product. 

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ii) This reaction does not need increased temperature or pressure. 

What is the essential reaction condition? 

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iii) Write a different equation for a substitution reaction between butane and chlorine. 

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Alkenes are more reactive and industrially more useful than alkanes.

They are made by cracking alkanes.

i) Draw the structural formula of the polymer poly(propene).

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ii) Give the name and the structural formula of the alcohol formed when but-1-ene reacts with steam.

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A student is asked to write an equation for a reaction of propane with chlorine.

The student writes three equations. Each one balances.

1  C₃H₈ + Cl₂ → C₃H₇Cl + HCl

2  C₃H₈ + Cl₂ → C₃H₆Cl₂ + H₂

3  C₃H₈ + Cl₂ → C₃H₈Cl₂

State which equation represents the reaction correctly.

Name the type of reaction it represents.

Explain what is wrong with equation 2.

Explain what is wrong with equation 3.

Bromine chloride, BrCl, is a compound containing two different halogens.

Bromine chloride reacts in a similar way to chlorine.

Bromine chloride reacts with ethane in the presence of ultraviolet light.

Name the type of reaction that takes place.

Two different pairs of products can be formed in this reaction.

Suggest an equation for each.

Explain why two different pairs of products are possible.

Ethane and ethene both react with chlorine, but they react in different ways.

Ethane reacts with chlorine in the presence of ultraviolet light.

i) Name the type of reaction that takes place.

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ii) Give the formula of the inorganic product of this reaction.

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Ethene reacts with chlorine at room temperature, with no ultraviolet light needed.

Name the type of reaction that takes place.

Explain, in terms of the bonding in their molecules, why ethane and ethene react with chlorine in different ways.