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 (B)

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 (B)</p> Signup and view all the answers

Incomplete combustion occurs when there is an excess of oxygen.

<p>False (B)</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 (B)</p> Signup and view all the answers

The initiation step produces alkanes as final products.

<p>False (B)</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. (A)</p> Signup and view all the answers

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

<p>False (B)</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. (A)</p> Signup and view all the answers

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

<p>False (B)</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. (A)</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 (B)</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 (D)</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 (B)</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 (B)</p> Signup and view all the answers

Ethane is an example of a polar molecule.

<p>False (B)</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 (C)</p> Signup and view all the answers

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

<p>False (B)</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

Flashcards

Hydrogenation of Alkenes

A chemical reaction involving the addition of hydrogen to an alkene, resulting in the formation of an alkane. Usually takes place with a platinum or nickel catalyst.

Cracking of Alkanes

Breaking down large alkane molecules into smaller alkanes and alkenes by applying heat and a catalyst.

Addition Reaction

A chemical reaction involving the addition of hydrogen to an alkene, resulting in the formation of an alkane.

Alkanes

Organic molecules containing carbon and hydrogen atoms, with only single bonds between carbon atoms.

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Alkenes

Organic molecules containing carbon and hydrogen atoms, with at least one carbon-carbon double bond.

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Complete Combustion

A chemical reaction where a substance is burned in the presence of oxygen, producing carbon dioxide (CO2) and water (H2O) as the main products.

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Incomplete Combustion

A chemical reaction where a substance is burned in a limited supply of oxygen, producing carbon monoxide (CO), carbon dioxide (CO2) and water (H2O) as the main products.

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Combustion of Alkanes

The process where alkanes are burned to release energy, often used as a fuel source.

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Free Radical Substitution

A type of reaction where a hydrogen atom in an alkane is replaced by a halogen atom.

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Carbon monoxide poisoning

Carbon monoxide (CO) is a toxic gas that binds to haemoglobin in the blood, preventing it from carrying oxygen. This leads to suffocation, dizziness, loss of consciousness, and ultimately death.

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Free-radical substitution in alkanes

Free-radical substitution is a reaction where a hydrogen atom in an alkane is replaced by a halogen atom, like chlorine or bromine.

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Initiation step of free-radical substitution

The initiation step in free-radical substitution involves breaking the bond of a halogen (Cl-Cl or Br-Br) with the energy from UV light, producing two free radicals.

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Propagation step of free-radical substitution

In the propagation step, the free radicals react with the alkane molecule, creating new free radicals, and continuing the chain reaction.

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Termination step of free-radical substitution

The termination step in free-radical substitution involves two free radicals colliding and combining to form a stable molecule, stopping the chain reaction.

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The role of UV light in free-radical substitution

Ultraviolet light (sunlight) is necessary for the free-radical substitution reaction to occur because it provides the energy needed to break the halogen bond, initiating the process.

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The relationship of sunlight and bromine in free-radical substitution

The disappearance of bromine's color when mixed with an alkane and exposed to sunlight suggests that the ultraviolet light is essential for the free-radical substitution reaction to take place.

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Termination Step (Free Radical Substitution)

The step in a free radical substitution reaction where two free radicals combine to form a single unreactive molecule, stopping the chain reaction.

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Homolytic Fission

A type of chemical bond where an electron is shared equally between two atoms.

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Fish Hook Arrow

An arrow with a single barbhead used to show the movement of a single electron in a chemical reaction. Commonly used for showing the formation or breaking of covalent bonds involving a single electron.

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Double-Headed Arrow

An arrow with two barbheads shows the movement of a pair of electrons in a chemical reaction. It is commonly used to represent the movement of electrons in the formation or breaking of covalent bonds.

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Free-radical Initiation

The first step of free-radical substitution reactions, where a molecule is broken down into two highly reactive free radicals by homolytic fission, usually using UV light.

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Free-radical propagation

The second step in free-radical substitution, where free radicals react with an alkane, replacing a hydrogen atom with a halogen atom, and creating a new free radical.

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Free-radical Further Substitution

This involves the formation of multiple halogenated alkane products, leading to a mixture of compounds.

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Free Radicals

Highly reactive species with an unpaired electron. These are extremely reactive and are key to free-radical reactions.

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Chain Reaction

This is a special kind of reaction where multiple steps are linked together in a chain-like manner.

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Substitution Reaction

A type of reaction where a single atom or group of atoms is replaced within a molecule. In free-radical substitution, a hydrogen atom in an alkane is replaced by a halogen atom.

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Initiator

The reactant molecule that initiates the free-radical chain reaction. This is broken down into free radicals in the initiation step.

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Why are alkanes unreactive?

The strong covalent bonds between carbon and hydrogen atoms in alkanes make them difficult to break, limiting their reactivity.

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How do alkanes lack polarity?

The difference in electronegativity between carbon and hydrogen atoms is very small, leading to an almost equal sharing of electrons and a nonpolar molecule.

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Why don't alkanes react with polar reagents?

Polar reagents are attracted to partial charges, which alkanes lack due to their nonpolar nature.

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How do nucleophiles and electrophiles relate to alkane reactivity?

Nucleophiles are negatively charged and attracted to electron-deficient areas, while electrophiles are positively charged and attracted to electron-rich areas. Alkanes have neither, so they don't attract these reagents.

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What are the main reactions of alkanes?

Alkanes are primarily unreactive, only participating in combustion reactions and halogen substitution.

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How does ethane's structure explain its lack of polarity?

Ethane is an example of an alkane with almost equal electronegativity between carbon and hydrogen atoms, resulting in a nonpolar molecule.

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What happens during combustion of alkanes?

Alkanes react in combustion by burning with oxygen to release energy, producing carbon dioxide and water as the main products.

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What happens during halogen substitution of alkanes?

In halogen substitution, a hydrogen atom in an alkane is replaced by a halogen atom, primarily driven by light or high temperatures.

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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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