13 - Fuels and Heats of Reaction

To be accompanied with past exam questions. CALCULATIONS

What is a hydrocarbon?

Compound containing hydrogen and carbon only

State the systematic IUPAC names of compounds A and B

A: Octane

B: 2,2,4 - trimethylpentane

Explain why compounds A and B are referred to as saturated alkanes

Only contain single bonds

Draw the molecular structure of methylpropane, including all atoms and bonds

A molecule of octane can undergo catalytic cracking to produce a molecule of methylpropane and a molecule of compound C.

State the systematic IUPAC name of three of the possible isomers of compound C. In each case, draw the molecular structure of the isomer, including all atoms and bonds.

Give the systematic IUPAC name for the product in the equation

2,2,4 - trimethylpentane

Pentane and 2-methylbutane are structural isomers of C₅H₁₂. Give the systematic IUPAC name of the other structural isomer of C₅H₁₂ and draw its molecular structure

A molecule of C₁₅H₃₂ was converted into a molecule of 2-methylbutane, a molecule of but-1-ene and one other molecule during catalytic cracking. What was the molecular formula of the third molecule formed?

C₆H₁₂

What is the difference in the molecular formulae of two successive members of the alkane homologous series?

CH₂

Explain the term fractionation of crude oil

Separation into components according to size by distillation

With the aid of labelled diagram explain how crude oil is fractionally distilled

Separation on basis of boiling point /

smaller or lower b.p. at top /

larger or higher b.p. at base

Show on your diagram where the refinery gas and oil fractions separate

Refinery gas (LPG) above gas oil (diesel) shown

What property of kerosene allows it to be separated from the other constituents of crude oil?

Boiling point / molecular mass

What is the purpose of adding mercaptans to natural gas or to LPG?

Safety / to give gas a smell / to enable leaks to be detected

Give a major use for

i) kerosene

ii) residue / bitumen

i) Aviation fuel / home heating oil

ii) roofing / road making

Explain the term structural isomers

Compounds with the same molecular formula but different structural formula

Compare petrol (a mixture of light gasoline and naphtha) and diesel (gas oil) in terms of

i) average molecular mass of the hydrocarbons present,

ii) boiling point.

i) petrol has smaller average M_r / diesel has greater average M_r

ii) petrol has lower boiling point / diesel has higher boiling point

Explain the term octane number of a fuel

Measure of the tendency of a fuel to auto-ignite or cause knocking

Give the systematic IUPAC name and draw the structure of a molecule of the alkane fuel assigned an octane number of zero

Heptane

Give the systematic IUPAC name and draw the structure of a molecule of the alkane fuel assigned an octane number of 100

2,2,4 - trimethylpentane

What molecular characteristics are required to create a hydrocarbon fuel with a high octane number?

Short chain //

branched chain //

cyclic structure

[any two]

List three methods commonly used in oil refining to increase the octane number of a fuel

Isomerisation //

reforming / dehydrocyclisation //

catalytic cracking //

adding oxygenates

Give an example of an oxygenate used in petrol

MTBE, methanol, ethanol, propanol, butanol

What is the advantage of adding tetraethyllead to petrol?

To reduce auto-ignition /

To increase to octane rating

Why was the use of tetraethyllead in car engines discontinued?

Toxic / poisonous / harmful / carcinogenic / damages catalytic converters / pollutant

Name the process that can convert compound A to compound B

Isomerisation

Why is isomerisation carried out?

Increase octane number / reduce auto-ignition / increase branching

Identify the processes B and C

B: Catalytic cracking //

C: Dehydrocyclisation

Why is catalytic cracking carried out in oil refining?

Gives useful products in higher demand / gives products with higher octane rating

Explain the term knocking in the engine

Auto-ignition: tendency to premature ignition

How is hydrogen gas manufactured on an industrial scale?

Steam reforming of natural gas /

electrolysis of water

Give two advantages of using hydrogen as a fuel

High kilogram calorific value / non-polluting products / can be piped / can be produced from water / does not contribute to global warming etc

Suggest a disadvantage of hydrogen as fuel

Explosive / fire risk / difficulty storing fuel

Suggest a reason why elemental hydrogen (H₂) does not occur in nature on Earth as a fuel source

Too reactive (explosive, flammable, combustible) / too light

What are the two main hydrocarbon components of LPG?

Propane //

butane

Define heat of reaction

The heat change when the number of moles of reactants indicated in the balanced equation for the reaction, react completely

Define heat of combustion

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

Define heat of formation

Heat change in KJ when one mole of a substance is formed from its elements in their standard states

Heat of neutralisation

Heat change in KJ when one mole of H⁺ ions from an acid reacts with one mole of OH^- ions from a base

Explain why the same amount of energy is released when one mole of hydrogen is used up in a fuel-cell as when one mole of hydrogen is burned in excess oxygen

Same reaction / Hess’ law

What apparatus is used to accurately measure the heat of combustion of a fuel?

Bomb calorimeter

Give one use made of the kilogram calorific values of various fuels

Compare efficiencies of fuels

Suggest one advantage of using bioethanol as a motor fuel

Bioethanol is renewable / environmentally friendly / reduces knocking / cheaper

Why to A, B and C separate in the same fraction in the distillation of crude oil?

[note: they are structural isomers]

Same number of carbons / same molecular masses / same molecular formulae

Explain which of A, B and C is most likely to auto-ignite in a petrol engine

A because it has the longest chain / A because it is least branched /

A is straight chained / A has lowest octane number rating

Give the systematic IUPAC names for A, B and C

A: pentane

B: 2-methylbutane

C: 2,2-dimethylpropane

Identify the hydrocarbon gas produced by anaerobic decomposition of either animal waste or vegetation

Methane (CH₄)

Explain why high molecular mass alkanes have high boiling points

Stronger intermolecular bonds (forces) / more electrons present

Write a balanced equation for the dehydrocyclisation reaction in which heptane is converted into methylbenzene and hydrogen

State Hess’ Law

If a reaction takes place in a number of stages, the sum of the heat changes in the separate stages is equal to the heat change if the reaction was carried out in one stage

Draw the molecular structure of ethylbenzene

What are the two reference hydrocarbons used to assign octane numbers to fuels?

heptane //

2,2,4 - trimethylpentane

Write the balanced equation for the complete combustion of butane in an adequate supply of oxygen