Alkanes Overview and Reactions
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Questions and Answers

What process is used to produce alkanes from alkenes?

  • Hydrogenation (correct)
  • Hydrolysis
  • Oxidation
  • Dehydrogenation

    • Alkenes contain C-C single bonds.

    False

    What catalyst is commonly used in the hydrogenation reaction to produce alkanes?

    Finely divided Pt or Ni

    Alkanes can be produced by cracking of longer ______.

    <p>alkane chains</p> Signup and view all the answers

    Match the following terms related to alkanes with their descriptions:

    <p>Hydrogenation = Addition of hydrogen to alkenes Cracking = Breaking down long alkane chains Alkene = Unsaturated hydrocarbon with double bonds Alkane = Saturated hydrocarbon with only single bonds</p> Signup and view all the answers

    What is produced during the complete combustion of alkanes?

    <p>Carbon dioxide and water</p> Signup and view all the answers

    Incomplete combustion occurs when there is an excess of oxygen.

    <p>False</p> Signup and view all the answers

    What type of reaction forms halogenoalkanes from alkanes?

    <p>Free-radical substitution</p> Signup and view all the answers

    Complete combustion of alkanes requires a ______ supply of oxygen.

    <p>plentiful</p> Signup and view all the answers

    Match the types of combustion with their characteristics:

    <p>Complete combustion = Produces carbon dioxide and water Incomplete combustion = Produces carbon monoxide</p> Signup and view all the answers

    What initiates the free-radical substitution reaction?

    <p>UV light</p> Signup and view all the answers

    The initiation step produces alkanes as final products.

    <p>False</p> Signup and view all the answers

    What type of reaction explains how free radicals attack unreactive alkanes?

    <p>Homolytic fission</p> Signup and view all the answers

    In the propagation step, the free radicals can attack another __________ molecule.

    <p>chlorine or bromine</p> Signup and view all the answers

    Match the step of the free-radical substitution reaction with its description:

    <p>Initiation = Formation of free radicals from UV light Propagation = Chain reaction producing halogenoalkanes Further substitution = Multiple substitution products are formed</p> Signup and view all the answers

    Which statement about the free-radical substitution reaction is true?

    <p>It involves a chain reaction.</p> Signup and view all the answers

    The further substitution process results in the selective formation of a single halogenoalkane.

    <p>False</p> Signup and view all the answers

    What is produced after the alkyl free radical attacks a chlorine or bromine molecule?

    <p>Halogenoalkane and a regenerated free radical</p> Signup and view all the answers

    What is the primary danger of carbon monoxide?

    <p>It binds to haemoglobin, preventing oxygen transport.</p> Signup and view all the answers

    Carbon monoxide has a distinct smell that helps in its detection.

    <p>False</p> Signup and view all the answers

    What type of light is necessary for free-radical substitution of alkanes?

    <p>Ultraviolet light</p> Signup and view all the answers

    The process of incomplete combustion often occurs in _____ due to limited oxygen.

    <p>car engines</p> Signup and view all the answers

    Match the steps of the free-radical substitution mechanism with their descriptions:

    <p>Initiation = Halogen bond is broken to form radicals Propagation = Radicals create further radicals Termination = Two radicals collide to end the reaction</p> Signup and view all the answers

    During free-radical substitution, what happens in the initiation step?

    <p>UV energy breaks the halogen bond.</p> Signup and view all the answers

    The disappearance of bromine color during a reaction is an indication that no reaction has taken place.

    <p>False</p> Signup and view all the answers

    What is the reason for the low reactivity of alkanes?

    <p>Strong C-C and C-H bonds</p> Signup and view all the answers

    What are the two halogens mentioned that can substitute hydrogen in alkanes?

    <p>Chlorine and bromine</p> Signup and view all the answers

    Alkanes can react with polar reagents due to their polarity.

    <p>False</p> Signup and view all the answers

    What is the electronegativity difference between carbon and hydrogen in alkanes?

    <p>0.4</p> Signup and view all the answers

    Alkanes mainly undergo _____ reactions in chemical processes.

    <p>combustion</p> Signup and view all the answers

    Match the following terms with their correct definitions:

    <p>Nucleophiles = Negatively charged species attracted to electron-deficient areas Electrophiles = Positively charged species attracted to electron-rich areas Alkanes = Hydrocarbons with single C-C and C-H bonds Polarity = Difference in electron distribution in a molecule</p> Signup and view all the answers

    Which of the following is a characteristic of alkanes?

    <p>Formed by C-H single bonds</p> Signup and view all the answers

    Ethane is an example of a polar molecule.

    <p>False</p> Signup and view all the answers

    What type of reactions do alkanes undergo with halogens?

    <p>substitution reactions</p> Signup and view all the answers

    What is the termination step in a free-radical substitution reaction?

    <p>The point when two free radicals react to form a single unreactive molecule</p> Signup and view all the answers

    The termination step of free-radical substitution always results in a pure halogenoalkane.

    <p>False</p> Signup and view all the answers

    What type of arrow is used to indicate the movement of one electron in free-radical mechanisms?

    <p>half-headed arrow or fish hook arrow</p> Signup and view all the answers

    The bond breaking in the initiation step is known as ___________ fission.

    <p>homolytic</p> Signup and view all the answers

    Match the mechanism steps in free-radical substitution with their descriptions:

    <p>Initiation = Formation of free radicals from stable molecules Termination = Formation of a single unreactive molecule from free radicals Propagation = Continuous reaction cycles producing free radicals Fission = Breaking of bonds into free radicals</p> Signup and view all the answers

    Study Notes

    Alkanes

    • Alkanes are hydrocarbons containing only carbon-carbon single bonds and carbon-hydrogen single bonds
    • They are saturated, meaning they have the maximum number of hydrogen atoms possible
    • They are relatively unreactive due to the strong carbon-carbon and carbon-hydrogen bonds
    • Alkanes are used as fuels because of their ability to combust releasing significant energy

    Producing Alkanes

    • Hydrogenation: Adding hydrogen to an alkene in the presence of a catalyst (like Pt/Ni) forms an alkane. This reaction is exothermic
    • Cracking: Breaking down larger alkane molecules, found in crude oil, into smaller, more useful hydrocarbon molecules (alkanes and alkenes). This process requires high temperatures and catalysts. Cracking is endothermic

    Combustion of Alkanes

    • Complete Combustion: Occurs in the presence of excess oxygen, producing carbon dioxide and water.
    • Incomplete Combustion: Occurs when there is a limited supply of oxygen, producing carbon monoxide and water, as well as some unburnt carbon. Carbon monoxide is a toxic gas, binding to haemoglobin which prevents it from binding oxygen.

    Free Radical Substitution

    • Alkanes can react with halogens (like chlorine or bromine) through a free radical substitution reaction
    • This reaction only occurs in the presence of UV light, breaking the halogen molecule into free radical species which interact with the alkane forming new radicals which can continue the reaction with more halogen molecules. This is commonly used to change alkanes to halogenoalkanes to make useful compounds.
    • The halogen-containing molecule and hydrogen gas will form. The initiation step is triggered by UV light which breaks the halogen molecule into single radicals which cause the chain reaction. The propagation steps show how the radicals repeat. Termination steps show how the reaction stops, with two radicals forming stable molecules.

    Chemical Reactivity of Alkanes

    • The carbon-hydrogen and carbon-carbon bonds in alkanes are very strong and difficult to break.
    • The almost equal electronegativities of carbon and hydrogen in an alkane mean the molecules are non-polar.
    • Due to their unreactivity, alkanes mainly undergo combustion reactions or free radical substitution reactions when exposed to high energy.

    Obtaining Useful Compounds by Cracking

    • Crude oil is a complex mixture of hydrocarbons.
    • Fractional distillation separates crude oil into fractions based on their boiling points. Each fraction contains a range of alkanes with similar boiling points.
    • Cracking breaks down large hydrocarbon molecules into smaller, more useful ones, like alkenes and alkanes, with simpler structures and lower boiling points, suitable for various purposes (e.g., gasoline).
    • Cracking typically is done by heating hydrocarbon molecules in the presence of a catalyst to cause them to break into smaller fragments.

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    CIE AS Chemistry Alkanes PDF

    Description

    Explore the fascinating world of alkanes, the saturated hydrocarbons that are essential fuels. This quiz will cover the properties, production methods such as hydrogenation and cracking, and the combustion processes of alkanes. Test your knowledge on their structure, reactivity, and applications.

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