Hydrocarbons Overview

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Questions and Answers

Which class of hydrocarbons consists of only single carbon-carbon bonds?

  • Alkenes
  • Alkanes (correct)
  • Aromatic compounds
  • Alkynes

Alkenes are compounds that contain only single carbon-carbon bonds.

False (B)

What is the general application of ethene in the context of hydrocarbons?

It is used to ripen bananas.

A series of compounds with similar chemical properties that differ by a _______ unit is called a homologous series.

<p>CH2</p> Signup and view all the answers

Match the following hydrocarbons with their characteristics:

<p>Alkanes = Saturated with single bonds Alkenes = Unsaturated with double bonds Alkynes = Highly unsaturated with triple bonds Aromatic = Contains a benzene ring</p> Signup and view all the answers

What is the primary method for separating crude oil into useful fractions?

<p>Fractional distillation (D)</p> Signup and view all the answers

Structural isomers have different molecular formulas but the same structural formula.

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

What is a characteristic of aromatic compounds?

<p>They have a fruity smell.</p> Signup and view all the answers

Which compound is NOT typically considered a refinery gas?

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

The octane number of a fuel is irrelevant to its performance and efficiency.

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

What is added to fuels to provide a smell for safety?

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

The process of breaking long carbon chains into shorter ones using a catalyst is called __________.

<p>catalytic cracking</p> Signup and view all the answers

Match the following factors with their effects on octane number:

<p>Short carbon chains = Increase octane number Branching = Increase octane number Tetraethyl lead = Increase octane number (toxic) Hydrogen gas = Clean burning fuel</p> Signup and view all the answers

Which of the following methods is NOT used to produce hydrogen?

<p>Methane combustion (C)</p> Signup and view all the answers

Hydrogen gas is easier to store and transport than hydrocarbons.

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

In the absence of external influence, boiling points of hydrocarbons tend to __________ with increasing molecular mass.

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

What determines whether a substance is soluble in water or organic solvents?

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

An exothermic reaction has a positive change in heat (∆H).

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

What device is used to measure the heats of combustion of fuels and foods?

<p>Bomb calorimeter</p> Signup and view all the answers

The law of conservation of energy states that energy cannot be created or destroyed, but only changed from one form to another, which is a key principle in __________.

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

Match the following types of bonds with their description:

<p>Sigma bond = Head-on overlap of orbitals Pi bond = Sideways overlap of p-orbitals Single bond = 1 pair of electrons shared (1 sigma bond) Double bond = 2 pairs of electrons shared (1 sigma and 1 pi bond)</p> Signup and view all the answers

Which statement about benzene is true?

<p>Benzene has delocalized electrons. (C)</p> Signup and view all the answers

Hess's law states that the heat change of a reaction only depends on the initial state of the reactants.

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

How many pi bonds are present in a triple bond?

<p>2</p> Signup and view all the answers

Flashcards

Hydrocarbons

Compounds that consist only of carbon and hydrogen atoms.

Alkanes

A class of hydrocarbons with only single carbon-carbon bonds. They are saturated, meaning they can't hold any more hydrogen atoms.

Alkenes

Compounds that contain one or more double carbon-carbon bonds. They are unsaturated, meaning they can hold more hydrogen atoms.

Alkynes

Compounds that contain at least one triple carbon-carbon bond. Also unsaturated, they can hold even more hydrogen atoms.

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Structural Isomers

Compounds with the same molecular formula but different arrangements of atoms.

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Fractional Distillation

A process where crude oil is separated into different components (fractions) based on their boiling points. Smaller hydrocarbons boil at lower temperatures and are collected at the top.

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Aromatic Compounds

Compounds that contain a benzene ring in their structure. These compounds often have a strong, sweet smell.

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Aliphatic Compounds

A class of hydrocarbons where carbon atoms are linked in straight or branched chains. They are not aromatic.

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Octane number

A measure of a fuel's resistance to premature ignition, causing 'knocking' in the engine.

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Auto ignition

The premature ignition of the fuel-air mixture in an engine, causing a knocking sound and power loss.

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Isomerisation

A process that converts long, straight-chain alkanes into their branched isomers, increasing octane number.

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Catalytic cracking

A process that breaks down long-chain hydrocarbons into shorter molecules using a catalyst, producing alkenes and increasing octane number.

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Dehydrocyclisation

A process that forms ring compounds from hydrocarbons, producing hydrogen gas and increasing octane number.

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Adding oxygenates

Adding compounds like methanol, MTBE, and ethanol to gasoline to increase octane number and reduce pollution.

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Steam reforming of natural gas

The chemical process of converting methane and steam into hydrogen gas and carbon monoxide.

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Electrolysis of water

The use of electricity to decompose water into hydrogen and oxygen gas.

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Solubility of Non-Polar Compounds

Non-polar molecules, like those with many C-H bonds, dissolve best in other non-polar solvents (like oils) but not in polar solvents (like water). This follows the rule "like dissolves like."

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Heat of Reaction (∆H)

The amount of heat released or absorbed during a chemical reaction. Exothermic reactions release heat (∆H is negative), while endothermic reactions absorb heat (∆H is positive).

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Sigma (σ) Bond

A specific type of covalent bond formed by the overlapping of atomic orbitals along the internuclear axis, resulting in electron sharing and a strong bond.

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Pi (Ï€) Bond

A type of covalent bond formed by the sideways overlap of p-orbitals above and below the internuclear axis. It is weaker than a sigma bond and allows for rotation around the bond axis.

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Covalent Bonding: Sigma and Pi Bonds

The sharing of electrons between 2 atoms, forming a single, double, or triple bond. Single bonds have one sigma bond, double bonds have one sigma and one pi bond, and triple bonds have one sigma and two pi bonds.

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Benzene: Aromatic Structure

A special type of cyclic molecule with a delocalized electron system, creating a very stable structure. The delocalized electrons are represented by a circle within the hexagon shape of the benzene ring.

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Delocalization of Electrons

The sharing of electrons among multiple atoms in a molecule, leading to increased stability. It is responsible for the unique properties of molecules like benzene.

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Heat of Combustion

The heat change when one mole of a substance is completely burned in the presence of excess oxygen.

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Study Notes

Hydrocarbons

  • Hydrocarbons are defined as compounds containing only carbon and hydrogen.
  • Three types of hydrocarbons: Alkanes, Alkenes, Alkynes.

Alkanes

  • Consist of only single carbon-carbon bonds.
  • Saturated compounds.
  • All carbon atoms are tetrahedral.
  • Boiling points are low (influenced by Van Der Waals forces) and increase with increasing molecular size.
  • Properties are similar, forming a homologous series.
  • Application: Fuel (e.g., methane, ethane, propane, butane, pentane, hexane, heptane, octane)

Homologous Series

  • A series of compounds with similar chemical properties.
  • Gradations in physical properties.
  • A general formula for members.
  • Each successive member differs by a CHâ‚‚ unit.

Alkenes

  • Consist of one double carbon-carbon bond.
  • Unsaturated.
  • Boiling points are low (influenced by Van Der Waals forces) and increase with increasing molecular size.
  • A homologous series.
  • The C=C bond is planar.
  • Application: Ethene is used to ripen bananas.

Alkynes

  • Consist of a triple bond.
  • Highly unsaturated.
  • Ethyne is the only alkyne to study.
  • Application: Welding and cutting.

Isomers

  • Structural isomers are compounds with the same molecular formula but different structural formulas.
  • Examples of isomerisations in alkanes (methyl group positioning) and alkenes (double bond positioning).

Aromatic Compounds

  • Aliphatic compounds: Carbon atoms joined in straight chains.
  • Aromatic compounds: Contain a benzene ring.
  • Aromatic compounds have a fruity smell.

Oil Refining

  • Fractional distillation separates crude oil into fractions based on boiling points.
  • Smaller hydrocarbons (low boiling points) separate at the top.
  • Larger hydrocarbons (high boiling points) separate at the bottom.
  • Refinery gases include methane, ethane, propane, butane.
  • Mercaptans (sulfur compounds) are added for safety to provide a smell.
  • Liquefied Petroleum Gas (LPG): Propane and butane.

Octane Number

  • Measurement of a fuel's resistance to knocking (premature ignition).
  • Higher octane number = better fuel.
  • 2,2,4-trimethylpentane = 100
  • Heptane = 0
  • Factors increasing octane number: Short carbon chains, branching, cyclic compounds.
  • Methods of increasing octane number: Isomerization, catalytic cracking, dehydrocyclization, adding oxygenates.

Hydrogen

  • Advantage: High energy output, cleaner burning than hydrocarbons.
  • Methods of producing hydrogen: Steam reforming of natural gas and electrolysis.
  • Problems with hydrogen: Difficult to store and transport (explosive), hard to liquefy.

Heat of Reaction

  • Heat change during a reaction (calculated based on a balanced equation).
  • Exothermic: Products have less energy than reactants (ΔH negative).
  • Endothermic: Products have more energy than reactants (ΔH positive).
  • Bomb calorimeter: Measures heats of combustion of fuels and foods.
  • Bond energy: Average energy to break bonds in the gaseous state.
  • Heat of combustion: Heat change when 1 mole of a substance is completely burned.
  • Heat of formation: Heat energy when 1 mole of a compound is formed from its elements.
  • Hess's Law: Heat change depends only on initial and final states.
  • Law of conservation of energy: Energy cannot be created or destroyed.

Sigma and Pi Bonding

  • Covalent bonding represented in terms of sigma (σ) and pi (Ï€) bonds.
  • Sigma: Head-on overlap of orbitals.
  • Pi: Sideways overlap of p-orbitals.
  • Single bond: 1 sigma bond.
  • Double bond: 1 sigma & 1 pi bond.
  • Triple bond: 1 sigma & 2 pi bonds.
  • Benzene: Aromatic ring, highly stable due to delocalization of pi electrons.

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