Fuels & Heats of Reaction - Chpt. 21

Also cover: IUPAC naming, structural formulas, Bond Energy, Neutralisation & Formation Questions

Saturated compound

Compound that contain only single bonds between atoms in a molecule (alkanes)

Hydrocarbon

Compound containing carbon and hydrogen only

Fossil Fuels

Fuels formed from the remains of plants and animals that lived millions of years ago

Gradation in physical properties

• Boiling points increase with molecule size due to increased van der Vaals forces

• Greater number of electrons in electron cloud allow temporary dipoles to form more easily

Homologous series criteria

1. series of compounds of similar chemical properties
2. show gradations of physical properties
3. have general formula
4. have similar method of preperation
5. each number differs from the previous by a CH2 unit

Structural isomers

Compounds with same molecular formula but different structural formulas

Example of a Structural Isomer & draw structure

Butane and Methylpropane

Sidechains/Substituents

Groups attached to main chain eg methyl, ethyl, etc

Unsaturated compound

Compound that contains one or more double/triple bonds between atoms in a molecule (alkenes & alkynes)

General Formula for Alkanes

Cn H2n+2

General Formula for Alkenes

Cn H2n

How to prepare ethene

Used for clothes: polyethene

1. Ethanol soaked glass wool end of test tube; aluminium oxide in centre heated gently by bunsen burner (flame not on ethanol or only get ethanol vapour passing over aluminium oxide and not converting to ethene)

2. Gas collected in test tube; first tube discarded as only contains air displaced from apparatus

3. Delivery tube taken out of water before turning of bunsen burner to prevent suck-back (cold water could crack boiling tube)

Tests on Ethene

1. Gas: colourless with sweetish small; insoluble in water but dissolves in organic solvents, eg. cyclohexane (same as methane)

2. Combustion: Yellow luminous slightly smokey flame (produces water & CO₂), limewater turns milky as Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

3. Bromine: Bromine water red to colourless as double bond is broken (1,2-dibromoethane or 2-bromoethanol when bromine water): test for unsaturation

4. Acidified KMnO₄: Purple to colourless as double bond breaks: test for unsaturation

General Formula for Alkynes

Cn H2n-2

Prepration of Ethyne

• No naked flames (ethyne is explosive)

• Used to artificially ripen fruits

1. Set up apparatus, calcium carbide in buchner flask, with water in tap, acidified copper sulfate solution next, which removes impurites (hydrogen sulfide, phosphine, and ammonia), and then a water bath to trap ethyne.

2. Calcium Carbide fizzes in buchner flask, turning from sandy to white solid, buchner flask becomes warm

CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂

Tests for Ethyne

1. Gas: Colourless sweetish smelling gas (with impurities smells bad), insoluble in water, soluble in cyclohexane

2. Combustion: Luminous smoky flame (produces water and CO₂)

3. Bromine: Red to clear: unsaturated

4. Acidified KMnO₄: Purple to clear: unsaturated

Aliphatic compound

Organic compound that consists of open chains of carbon atoms & closed chains of compounds (rings) that resemble them in chemical properties

Aromatic compounds

Compounds that contain benzene ring structure in molecules eg. Methylbenzene (C₇H₈ - preferred as non-carcinogenic) & Ethylbenzene (C₈H₁₀)

Steps in fractional distillation

1. After analysis, crude oil is heated to 600C

2. Bubble caps slow rate vapour rises through fractionating collumn, allowing for better seperation

3. Vapour molecules condenses & are collected in trays

Refinery/Petrolium gas

• Methane, ethane, propane, butane

• Used in domestic gas

• Mercaptans (sulfur compounds) are added to add smell and propane and butane are pressurised into LPGs (Liquified Petroleum Gas)

Petrol (light gasoline

• C5 - C10

• Used as motor fuel

Naphtha

• C7 - C10

• Some is used in petrol due to the overlap, some used for petrochemicals such as medicines, plastics and solvents

Kerosene/Paraffin oil

• C10 - C14

• Domestic heating oil, aircraft fuel

Deisel/Gas

• C14 - C19

• Fuel for cars, buses, trucks, trains & domestic oil

Lubricating oil

• C14 - C35

• High boiling range so cannot be used for fuel. Used as lubricant to reduce wear and tear of engines

Fuel oil

• C30 - C40

• Used to power ships, power stations & heating plants

Bitumen

• C35

• Black tar on roads, waterproofing & Roofing

Auto-ignition/Knocking

Premature ignition (explosion) of petrol-air mixture before normal ignition of mixture by spark takes place

Octane number

Measure of tendency of fuel to resist knocking (the higher the better) 2,2,4-trimethylpentane being 100, heptane being 0. (reference hydrocarbons) Octane number of a substance can be measure by comparing its performance to the other two in a test engine

Factors Affecting Octane No.

1. Length of carbon chain

2. Degree of branching

3. Cyclical Structure

Why is tetraethyl lead not used anymore

Poisoned the metal catalysts in the catalytic converter

How to Increase Octane No.

1. Isomerisation: Changing straight-chain alkanes into isomers; heated in presence of catalyst, chains break, rejoin in branched chains (pentane and hexane)

2. Catalytic Cracking: Braking down of long-chain hydrocarbon molecules by action of heat and catalysts into short-chain molecules for which there is greater demand

3. Dehydrocyclisation: Use of catalysts to form ring compounds

4. Adding Oxygenates: Adding methanol, ethanol and MTBE

Ways to form hydrogen

1. Steam reforming of natural gas by reacting methane w steam (CH4 + H20 = 3H2 + CO)
2. Electrolysis of water. Produced in oil when dehydrocylisation happens (H2O = H2 + ½O2)

Heat of Reaction

Heat change when numbers of moles of reactants indicated in balanced equation for reaction react completely

Heat of combustion

Heat change when one mole of substance is completely burned in excess oxygen

Use of bomb calorimeter

Measure heat of combustion of a compound

Kilogram calorific value

Heat energy produced when 1kg of fuel is completely burned in oxygen

Bond Energy

Average energy required to break one mole of particular covalent bond & seperate neutral atoms completely from each other

Heat of Neutralisation

Heat change when one mole of H+ ions from an acid reacts w one mole of OH- ions from a base

To Determine Heat of Reaction of Hydrochloric Acid w Sodium Hydroxide

Steps

1. Known volume hydrochloric acid solution placed in polystyrene cup with thermometer in each, temp. recorded

2. Sodium hydroxide solution added quickly to hydrochloric acid solution; lid placed on, stirred continuously, max temp. recorded

Explinations

• For better result, solutions of HCl and NaOH used are stronger, otherwise temperature rise would be too small to accurately measure

Steps to improve accuracy

• Accurate thermometer that reads to 0.1 degrees/attached to datalogger

• Burette could be used instead of a graduated cylinder to measure, however not to mix as it would be too slow

• Lid minimises heat loss, ensuring all heat given out is recorded

• Quick addition of base avoids heat loss

• Avoid splashing, as any loss of liquid would lead to inaccurate readings

Calculations

Heat given out = mass (kg) x specific heat capacity (J kg⁻¹ K⁻¹) x rise in temperature (t₂ - t₁)

Heat of Formation

Heat change that takes place when one mole of a compound in its standard state is formed from its elements in their standard states

Hess's Law

If a chemical reaction takes place in a number of stages, sum of heat changes in seperate stages is equal to heat change if reaction is carried out in one stage 🔺️H1 = 🔺️H2 + 🔺️H3

Heat Released equation

Mass × Specific heat capacity × temperature rise