Organic Chemistry: Hydrocarbons and Alkanes

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Questions and Answers

How is the name of alkynes derived from their corresponding alkanes?

The name of alkanes is modified by changing the ending -ane to -yne.

What is the general formula for alkynes?

The general formula for alkynes is C_nH_{2n-2}.

Describe the process to prepare ethyne using calcium dicarbide.

To prepare ethyne, calcium dicarbide is placed in a test tube, and water is added until it reacts completely, producing ethyne gas.

What observation would you make when ethyne is ignited?

<p>Upon ignition, ethyne produces a bright flame and burns with high temperature.</p> Signup and view all the answers

What happens when a few drops of bromine water are added to ethyne gas?

<p>The bromine water decolorizes, indicating the reaction of ethyne with bromine.</p> Signup and view all the answers

What type of forces primarily hold cyclohexane molecules together?

<p>Van-der-Waals forces</p> Signup and view all the answers

Identify the state of cyclohexane when there are 15 or more carbon atoms.

<p>Waxy solids</p> Signup and view all the answers

What is the IUPAC name for a compound with six carbons and a C=C double bond starting at carbon atom 2?

<p>Hex-2-ene</p> Signup and view all the answers

How would you describe the flame produced when ethene is ignited?

<p>Yellow luminous flame which is slightly smoky</p> Signup and view all the answers

What observable change occurs when ethene reacts with limewater?

<p>Limewater turns milky</p> Signup and view all the answers

What happens to the color of bromine water when mixed with ethene?

<p>Bromine water turns from red to colourless</p> Signup and view all the answers

What is formed when potassium manganate(VII) is added to ethene?

<p>The solution turns from purple to colourless</p> Signup and view all the answers

Characterize the solubility behavior of ethene.

<p>Insoluble in water but dissolves in some organic solvents</p> Signup and view all the answers

What characterizes a saturated compound?

<p>A saturated compound is characterized by having only single bonds between the atoms in the molecule.</p> Signup and view all the answers

List the first four members of the alkane series and their states at room temperature.

<p>The first four members are methane (gas), ethane (gas), propane (gas), and butane (gas).</p> Signup and view all the answers

Why are alkanes insoluble in water?

<p>Alkanes are insoluble in water because they are non-polar and only exhibit Van-der-Waals forces between their molecules.</p> Signup and view all the answers

Define a homologous series in organic chemistry.

<p>A homologous series is a series of compounds with similar chemical properties that vary in physical properties by a CHâ‚‚ unit.</p> Signup and view all the answers

What are structural isomers?

<p>Structural isomers are compounds that have the same molecular formula but different structural formulas.</p> Signup and view all the answers

What makes carbon unique compared to other elements?

<p>Carbon is unique because it can form stable rings and chains of atoms, allowing for diverse organic compounds.</p> Signup and view all the answers

Name a common source of hydrocarbons.

<p>Common sources of hydrocarbons include coal, natural gas, and petroleum.</p> Signup and view all the answers

What are the two primary components of Liquefied Petroleum Gas (LPG)?

<p>Propane and Butane.</p> Signup and view all the answers

Why are mercaptans added to LPG?

<p>To give an unpleasant smell for gas leak detection.</p> Signup and view all the answers

What is the primary use of kerosene?

<p>As a domestic heating fuel and fuel for aircraft.</p> Signup and view all the answers

How does the length of the alkane chain affect the octane number?

<p>Shorter alkane chains result in higher octane numbers.</p> Signup and view all the answers

What is autoignition in the context of petrol engines?

<p>Premature ignition of the petrol-air mixture before the spark occurs.</p> Signup and view all the answers

What was the purpose of adding lead to petrol in the past?

<p>To reduce engine knocking.</p> Signup and view all the answers

What kind of compounds generally have a higher octane number?

<p>Cyclic compounds have a higher octane number than straight-chain compounds.</p> Signup and view all the answers

Name one use of naphtha in the petrochemical industry.

<p>As a source for medicine, plastics, and synthetic fibres.</p> Signup and view all the answers

What is the primary characteristic of benzene that contributes to its stability?

<p>Benzene's stability is due to the delocalization of electrons across the benzene ring.</p> Signup and view all the answers

What observable change occurs when bromine water is mixed with an alkene?

<p>Bromine water changes from red to colourless.</p> Signup and view all the answers

How does the process of fractional distillation separate crude oil?

<p>Fractional distillation separates crude oil based on the different boiling points of its components.</p> Signup and view all the answers

What type of flame does ethyne produce when burned, and what is the consequence of this combustion?

<p>Ethyne burns with a luminous smoky flame, producing a significant amount of soot.</p> Signup and view all the answers

Define aromatic compounds and provide an example.

<p>Aromatic compounds are organic compounds that contain a benzene ring, such as benzene (C6H6).</p> Signup and view all the answers

What happens to KMnO4 when it is used in reactions with unsaturated hydrocarbons?

<p>KMnO4 changes from purple to colourless.</p> Signup and view all the answers

Why are refinery gases essential for a refinery?

<p>Refinery gases, such as methane, ethane, propane, and butane, serve as important fuel sources for the refinery.</p> Signup and view all the answers

What distinguishes aliphatic compounds from aromatic compounds?

<p>Aliphatic compounds consist of open chains of carbon atoms, while aromatic compounds contain a benzene ring.</p> Signup and view all the answers

What is the initial temperature of the hydrochloric acid solution measured before mixing?

<p>Temperature of HCl solution before mixing = X°C.</p> Signup and view all the answers

What should be done immediately after the sodium hydroxide is added to the hydrochloric acid solution?

<p>Stir the mixture well and quickly place a lid on the cup.</p> Signup and view all the answers

In the equation heat liberated = m c ∆T, what does each variable represent?

<p>m represents mass in kg, c is the specific heat capacity, and ∆T is the temperature rise.</p> Signup and view all the answers

Why should the answer for the heat of reaction be given a negative sign?

<p>Because the reaction is exothermic, indicating heat is released.</p> Signup and view all the answers

State Hess's Law in your own words.

<p>Hess's Law states that the total heat change in a reaction is the sum of the heat changes in each stage.</p> Signup and view all the answers

In calculating the heat of formation of methane, which reaction's enthalpy change is required?

<p>C(s) + 2H2(g) → CH4 ΔH = ?</p> Signup and view all the answers

What is the heat of formation of water according to the provided data?

<p>The heat of formation of water is -285.8 kJ mol^-1.</p> Signup and view all the answers

Why is it important to avoid splashing when mixing the acid and the base?

<p>To prevent loss of solution and ensure accurate temperature readings.</p> Signup and view all the answers

Flashcards

Hydrocarbon

A compound containing only carbon and hydrogen atoms.

Fossil Fuel

Fuel formed from ancient plant and animal remains.

Alkane

A hydrocarbon with only single bonds between atoms; a saturated hydrocarbon.

Saturated Compound

A compound where all available bonds are used and are single bonds.

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Homologous Series

A series of similar compounds, with each differing by a CHâ‚‚ unit.

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Isomer

Compounds with the same molecular formula, but different structural formulas.

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Structural Isomer

Isomers with the same molecular formula, but different arrangements of atoms.

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General Formula of Alkanes

CnHâ‚‚n+â‚‚

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Alkynes general formula

CnH2n-2

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Ethyne formula

C2H2

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Ethyne use

Oxyacetylene welding and cutting

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Ethyne solubility

Soluble in non-polar solvents, insoluble in water

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Ethyne preparation method

Reacting calcium dicarbide with water

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Cyclohexane solubility in water

Cyclohexane is insoluble in water due to non-polar nature of alkenes, which only have Van der Waals forces.

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Alkane boiling/melting points

Larger alkanes have stronger Van der Waals forces, resulting in higher boiling and melting points.

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5-methylhex-2-ene

Organic compound with a six-carbon chain, a double bond starting at carbon 2, and a methyl substituent at carbon 5.

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Preparation of ethene

A method to create ethene gas by heating ethanol over a catalyst (aluminum oxide).

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Ethene properties: flame

Ethene burns with a yellow, luminous, slightly smoky flame.

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Ethene properties: limewater

Ethene reacts with limewater, turning it milky.

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Ethene properties: bromine

Ethene decolorizes bromine water (red to colorless).

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Alkyne characteristic

Alkynes contain a carbon-carbon triple bond and are highly unsaturated compounds.

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Aromatic Hydrocarbons

Organic compounds containing a benzene ring structure.

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Benzene

A highly toxic, carcinogenic hydrocarbon with a special stable benzene ring structure.

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Delocalized Electrons

Electrons shared by more than two atoms.

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Fractional Distillation

Separation of crude oil into fractions based on boiling point differences.

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Refinery Gases

Gases like methane, ethane, propane, and butane obtained from fractional distillation.

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Crude Oil

A thick, black, liquid mixture of hydrocarbons from the earth.

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Fractions

Groups of compounds with similar boiling points in crude oil.

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Aliphatic Compound

An organic compound with open or closed chains of carbon atoms similar in properties.

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Octane Number

A measure of a fuel's resistance to knocking.

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Knocking

Premature ignition of a fuel-air mixture in an engine.

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2,2,4-trimethylpentane

A reference hydrocarbon with an octane number of 100.

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Heptane

A reference hydrocarbon with an octane number of 0.

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LPG

Liquefied petroleum gas, often propane or butane.

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Mercaptans

Sulfur compounds added to LPG to give it an odor.

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Higher Octane Number

Better resistance to knocking, smoother engine.

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Factors Affecting Octane Number

Chain length and branching structure of the hydrocarbon

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Heat of Formation

The heat change that occurs when one mole of a compound is formed from its elements in their standard states. It's a specific type of enthalpy change.

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Standard State

The most stable form of a substance under standard conditions (usually 25°C and 1 atm pressure).

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Exothermic Reaction

A reaction that releases heat energy into the surroundings. The temperature of the surroundings increases.

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Hess's Law

The total enthalpy change for a reaction is the same whether it occurs in one step or multiple steps.

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Heat of Reaction

The amount of heat absorbed or released during a chemical reaction. It's a measure of the energy change in a reaction.

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Calculate Enthalpy Change

Using Hess's Law, you can calculate the enthalpy change of an unknown reaction by combining known enthalpy changes of other reactions.

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What does ΔH represent?

The enthalpy change (heat change) of a reaction.

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What is the SI unit for ΔH?

The standard unit for enthalpy change is the joule (J) or kilojoule (kJ).

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Study Notes

Organic Chemistry

  • Organic chemistry is the study of carbon compounds.
  • Carbon forms stable rings and chains of its atoms.
  • Most organic compounds are covalent compounds.
  • Carbon has the ability to form covalent bonds with itself and other elements.

Hydrocarbons

  • A hydrocarbon is a compound containing only carbon and hydrogen.
  • Common sources are coal, natural gas, and petroleum.
  • Fossil fuels are formed from the remains of plants and animals millions of years ago.

Alkanes

  • Alkanes are hydrocarbons where all atoms are linked by single bonds.
  • They are saturated compounds.
  • Saturated compounds have only single bonds between atoms.
  • Example alkanes: methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, and decane.
  • Each alkane and its molecular formula are listed in table.

Homologous Series

  • A homologous series is a series of compounds with similar chemical properties that show gradations in physical properties.
  • Each member differs from the previous member by a (CH2) unit.

Isomers

  • Structural isomers are compounds with the same molecular formula but different structural formulas.
  • Example of structural isomerism for butane is shown in the text(structural formulas).

Naming of Alkanes

  • IUPAC rules are used for naming organic compounds.
  • Side chains are called alkyl groups (e.g., methyl, ethyl).
  • Prefixes like "di-" are used when two identical groups are present.
  • Example of naming a compound with formulas and steps are shown in the text.

Alkenes

  • Alkenes are hydrocarbons with one or more carbon-carbon double bonds.
  • They are unsaturated compounds.
  • General formula for alkenes is CnH2n.
  • Example of alkenes: ethene (ethylene), propene, and butene(isomerism begins).
  • Properties of alkenes (soluble in non-polar solvents, insoluble in water) are noted.
  • Naming of alkenes (longest chain containing the C=C double bond, numbering the chain) are noted.

Alkynes

  • Alkynes are hydrocarbons containing carbon-carbon triple bonds.
  • They are highly unsaturated.
  • General formula for alkynes is CnH2n-2.
  • Example is ethyne (acetylene).
  • Properties of alkynes (insoluble in water due to Van der Waals forces)are noted.

Aromatic Hydrocarbons

  • Aromatic compounds contain a benzene ring structure.
  • Benzene has a delocalized electron system.
  • Aromatic compounds are often highly stable and not reactive.
  • Example given is benzene.

Oil Refining

  • Crude oil is a mixture of hydrocarbons, separated by fractional distillation.
  • Fractional distillation separates crude oil into various fractions (different boiling points).
  • Fractions have different uses, e.g., petrol, kerosene, diesel, etc.
  • Octane number is a measure of a fuel's tendency to resist knocking.
  • Catalytic cracking breaks down long-chain hydrocarbons into shorter ones.

Heat of Reaction

  • Heat of reaction is the heat change when a specific number of moles of reactants react completely.
  • ΔH (delta H) is used to indicate the heat change.
  • Exothermic reactions release heat (+ΔH), and endothermic reactions absorb heat (- ΔH).

Heat of Combustion

  • Heat of combustion is the heat change when one mole of a substance is completely burned in excess oxygen.
  • Bomb calorimetry is used to measure heats of combustion.

Heat of Neutralization

  • Heat of neutralization is the heat change when one mole of H+ ions from an acid reacts with one mole of OH- ions from a base

Other

  • Hydrogen production methods (steam reforming and electrolysis) are mentioned.

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