Fuels and Heats of reaction Flashcards
Hydrocarbon
A compound that contains only carbon and hydrogen
Saturated compound
a compound in which there are only single bonds between the atoms in the molecule
Unsaturated compound
One which contains at least one double or triple bonds netweem the atoms in the molecule
Structural isomers
Compounds with the same molecular formula but different structural formulae
Aliphatic (compound)
An organic compound that consists of straight (open) chains of carbon atoms and closed chain compounds with similar properties
Aromatic (compound)
An organic compound that contains a benzene ring structure in their molecules
Octane Number
Octane number of a fuel is a measure of the tendency of the fuel to resist knocking
Catalytic cracking
The breaking down of long chain hydro-carbon molecules into short chain molecules by heat and catalysts
Auto-ignition
Premature ignition (explosion) of the petrol-air mixture before normal ignition of the mixture by a spark takes place
Alkanes
These compounds are a family of hydrocarbons in which all atoms are linked by single bonds (saturated compounds)
Alkanes - formula
Cₙ H₂ₙ₊₂
Order
Meth Eth Prop But Pent Hex Hept Oct Non Dec
Alkane structual and condensed formulas
on hardback
Alkenes - formula
Cₙ H₂ₙ
Alkenes
- A homologous series of aliphatic hydrocarbons
- Lowest member of the series is ethene (2 carbon atoms)
- Each alkene molecule contains a carbon-carbon double bond
- Unsaturated compounds
Alkenes - methene
No methene due to double bonds
Alkynes endings
ends in -yne
Alkynes
Contain a carbon-carbon triple bond
Highly unsatured
Alkynes formula
Cₙ H₂ₙ₋₂
Ethyne
Name - Ethyne
Formula - C₂ H₂
Structural formula: H - C = C - H
Ethyne is the first of the series as you must have a carbon-carbon triple bond. (You need at least 2 carbons)
Endothermic reaction
Energy + reactants –> products
Activation energy
Activation energy is the maximum point on the reaction graph
Example of exothermic reaction and endothermic reaction
Exothermic reaction: Hydrochloric acid + magnesium
Endothermic reaction: water + ammonium nitrate
Bomb calorimeter
- use
- method
Used to find accurate values for heats of combustion
- Sample is ignited using electrically heated wire
- Excess oxygen ensures complete combustion of sample
- Heat produced by rapid + complete combustion of the sample heats, water + temperature is recorded
- Apparatus is well insulated to prevent heat loss
- Amount of heat used to start burning can be calculated and removed
Bond energy
- Average C-H bond energy in methane: 412 kJ mol⁻¹ ie. E(C-H) = 412 kJ mol⁻¹
- Energy of a particular bond can vary
- It is unusual to quote the average bond energies
Natural gas
- A colourless, odourless mixture of gases made up mostly of methane (CH₄). Other gases that can form part of natural gas include small amounts of ethane (C₂ H₆), propane (C₃ H₈), and butane (C₄ H₁₀)
- Methane burns relatively cleanly to produce heat energy + by-products of water and carbon dioxide
Why is natural gas useful?
- high kg calorific value
- Plentiful
- Easily distributed
Why mercaptans are added to natural gas
Due to natural gas being highly inflammable + explosive, mercaptans are added so it can be detected
Fractional distillation
A large-scale, continuous process which separates crude oil into a number of useful mixtures called fractions
eg. refinery gas is a fraction that can be used as a domestic gas
Octane number
Measure of the tendency of the fuel to resist knocking
eg. 2, 2, 4 - trimethlypentane is assigned an octane number of 100
heptane is assigned an octane number of 0
Factors affecting octane numbers
The shorter the alkane chain, the more branched the chain, and cyclic compounds all lead to higher octane numbers
Methods used to increase the octane number of petrol
- Isomerisation
- Catalytic cracking
- Dehydrocyclisation
- Adding oxygenates
Ways of manufacturing hydrogen
- steam reforming of natural gas
- electrolysis of water
Isomerisation
involves changing straight-chained hydrocarbons into branched-chained isomers. The branched-chained isomers burn more smoothly, therefore have higher octane number.
Catalytic cracking
involves breaking down long chained hydrocarbons (low demand) into short chained molecules (high demand). Short-chained hydrocarbonsa higher octane number + tend to be highly branched.
Dehydrocyclisation
involves conversion of straight chained hydrocarbons to form ring compounds. Then converted into aromatic compounds. Done in presence of a catalyst. As hydrogen is a byproduct of this reaction it is called ‘dehydro’. Aromatic compounds have high octane number but also carcinogenic.
Adding oxygenates
the addition of oxygen compounds to the petrol.
Tend to be of two types (a) addition of alcohols i.e. methanol (octane no. of 114) and (b) addition of ethers i.e. methyl tert-butyl ether (MTBE) (octane no. of 118).
These compounds (a) increase octane number of the petrol (b) cause less pollution as they reduce level of carbon monoxide in exhaust fumes.
Catalytic cracking allows what?
Allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules
Catalytic cracking method
Fractions containing large hydrocarbon molecules are vaporised + passed over a hot catalyst. This breaks chemical bonds in the molecules + forms smaller hydrocarbon molecules
Why do catalytic cracking
Fuels made from oil mixtures w/ large hydrocarbon molecules -> not efficient, do not flow easily, difficult to ignite
Crude oil –> often too many large hydrocarbon molecules + not enoughh small hydrocarbon molecules to meet demand
Heat of neutralisation
- Nearly always exothermic (-△H)
- Measured in kJ/mol or kJ mol⁻¹
- Always less than -57 kJ mol⁻¹
WRITE A NEGATIVE IN FRONT OF THE ANSWER IF ASKED TO CALCULATE HEAT OF NEUTRALISATION
How to find out if substance is acid or base
Acid: contains H⁺ ions eg. HCl
Base: Contains OH eg. NaOH
Calculating the heat liberated in a reaction
Heat liberated/given out = mass x specific heat capacity x rise in temperature
(m x c x (t₂ - t₁)
mass - kg
shc - J kg⁻¹ K⁻¹
Mercaptans
Chemicals added to odourless gases to give them an unpleasant smell so that gas leaks can be detected
Who discovered benzene?
Michael Faraday in 1825
Properties of benzene
Formula: C₆ H₆
Highly unsaturated
Highly toxic
Carcinogenic
Points about benzene that puzzled chemists
- Reactivity
2. Bond lengths
- Reactivity of Benzene
Predicted benzene could be highly reactive. Found to be very unreactive.
- Bond lengths of Benzene
Problems about alternating double + single bonds between the carbon atoms. When bond lengths measured, was found that all carbon-carbon bond lengths were same. Single bond is longer than double bond. Bond lengths in benzene are intermediate between that of a single bond + double bond.
Why benzene is unreactive
The electrons were delocalised.
Delocalisation of electrons gives the benzene molecule extra stability + accounts for its unreactivity
Angles on benzene molecule
The molecule is a flat molecule with all bond angles equal to 120°
Examples of aromatic hydrocarbons
Methylbenzene
Ethylbenzene
Methylbenzene properties
- Liquid at room temp
- Insoluble at water
- Will dissolve in organic solvents such as cyclohexane and propanone
- A good organic solvent
Fractional distillation - residue
bitumen for roads + roofing
Fractional distillation - fuel oils
fuels for ships, factories, central heating
Fractional distillation - Lubricating oil
lubricating oils, waxes, polishes
Fractional distillation - diesel oils
diesel fuels
Fractional distillation - kerosene (paraffin oil)
jet fuel, paraffin for lighting + heating
Fractional distillation - petrol (gasoline)
petrol for vehicles
Fractional distillation - naphta
chemicals
Fractional distillation - gases
liquefied petroleum gas
environmental effect of the release of methane
greenhouse effect
source of methane
flatulence of cows
why lead was removed from petrol
lead is toxic to humans
additives to increase octane number
- addition of alcohol: methane
- addition of ethers (methyl tert-butyl ether
additives used to increase octane number of a fuel
- Methanol / ethanol
- Methyl-tert-butyl-ether
How fractional distillation is carried out
- know diagram
- hot crude oil introduced at base of fractionating column
- vapur moves up through a series of trays
- fractions come off through outlets depending on their boiling points
- temp gradient shown, highest temp at bottom
something undergoing dehydrocyclisation diagram
literally just becomes a circle thing with the c forming the circle
eg. hexane -> cyclohexane + hydrogen
heat produced by a reaction
heat = mcΔΘ
heat of reaction
the heat change when the numbers of moles of reactants indicated in the balanced equation for the reaction react completely
heat of combustion
of a substance is the heat change when one mole of the substance is completely burned in excess oxygen
kilogram calorific value
of a fuel is the heat energy produced when 1kg of the fuel is completely burned in oxygen
bond energy
the average energy required to break one mole of a particular covalent bond and to separate the neutral atoms completely from each other
heat of neutralisation
heat change when one mole of H+ ions from an acid reacts with one mole of OH- ions from a base
heat given out
c(t₂ - t₁)
heat of formation
of a compound is the 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, the sum of the heat changes in the separate stages is equal to the heat change if the reaction is carried out in one stage
Law of conservation of energy
energy cannot be created or destroyed, but can only be converted from one form of energy into another
calculating heat of combustion
ΔH° (combustion) = ΣΔH 𝒻(Products) - ΣΔH 𝒻(Reactants)
measured in kJ mol⁻¹ !!!!
(div by the no. of moles to get it to 1 mole)
fractionation of crude oil
separation into components according to molecular size by distillation
components of liquid petroleum gas (LPG)
propane
butane
calculating heat of reaction (/when one mole of each solution is used)
use mcΔΘ
then div by no. of moles if u need to get it in terms of 1 mole
effect of length of carbon chain on boiling point
the longer the carbon chain (more carbon atoms), the higher the boiling point
hydrocarbon gas produced by anaerobic bacterial decomposition of either animal waste or vegetation
methane (CH₄)
uses of methane
fuel
steam reforming
production of fertilizer
why is the concentration of gas in the atmosphere a cause of concern?
global warming
enhanced greenhouse gas
Why do high molecular mass alkanes have high boiling points?
stronger intermolecular bonds
the physical property that is the basis for hydrocarbons being isolated in the process of fractional distillation
boiling point
relative molecular mass
the two reference hydrocarbons used to assign octane numbers to fuels
heptane and 2,2,4-trimethylpentane
2 advantages of adding oxygenates to fuel
- increase octane number, reduces knocking
- reduces pollution
two properties of hydrogen that made it the fuel choice for the space shuttle
- low density
- high kilogram calorific value
- minimally pollution (only product of combustion is water)
disadvantage of hydrogen as a fuel
- leaks easily out of containers
- explosive
- hazardous
- expensive
two ways hydrogen gas is produced industrially
- steam reforming of natural gas
- dehydrocyclisation
- electrolysis of water
balanced equations for combustion of something in oxygen
reactants: itself + O2
products: co2 and h2o
exothermic reaction profile graph
know what the graph looks like and how to label
fractions which contain molecules with the carbon chain lengths needed for petrol
- light gasoline/petroleum
- naphtha
two changes to the hydrocarbon molecule during dehydrocyclisation
- removal of hydrogen
- ring formation
what is the principal use made of oxygenates such as methyl tert-butyl ether, MTBE, in the petrochemicals industry?
- raise octane number
- prevent auto-ignition
- cleaner emissions
in which fraction of crude oil do pentane and its isomers occur?
light gasoline/petrol
how do molecules of naphtha differ from molecules of the gas oil fraction?
- have shorter chains
- gas oils have longer chains
fractional distillation separation
know the order, where each of the fractions end up
naphtha condenses higher up while gas oil comes off lower down
uses for catalytic cracking
- give useful products
- gives higher octane numbers
why is the use of benzene in petrol strictly controlled?
benzene is carcinogenic/toxic
two reasons why the addition of lead to petrol has been discontinued
- it poisons/destroys the catalyst in catalytic converter
- lead emission presents a health hazard (toxic to living things)
why methane is useful/excellent as a fuel
- high kilogram calorific value
- clean (burns to produce carbon dioxide and water)
- plentiful
- non-toxic
what does LPG stand for?
Liquefied petroleum gas
source from which methane is released to the atmosphere
- fossil fuels
- flatulence of cows
- slurry
- anaerobic decay
- coal(coal gas, coal mines)
why it is possible to measure experimentally the heat of formation of co2 and water but not that of ethanoic acid
ethanoic acid cannot be formed directly from elements
/
formation of ethanoic acid is not a combustion reaction
components of air feedstock that are removed prior to fractional distillation/before it is liquefied
- water vapour
- carbon dioxide
- dust
- oil particles
how the air intake is liquefied prior to fractional distillation
- cooled
- compressed (placed under pressure)
boiling points of the fractions
Top - lower boiling points
Bottom - higher boiling points
which of the fractional distillation products is oxygen?
the bottom
uses for the oxygen gas produced by the fractionation of liquid air
- steel-making
- breathing aid
- rocket fuel
- beauty treatment
- using bomb calorimeter for measuring heats of combustion
- medical treatment (eg. muscle injury)
fractional distillation of pure liquid air process
-diagram
- compressed air warms up in column
- oxygen collects at base of column + removed
- nitrogen at top of column
industrial use for hydrogen produced in dehydrocyclisation
hydrogenation (of vegetable oils) / fuel / preparation of hydrochloric acid/ammonia
suggest a reason why the heat of neutralisation of sulfuric acid is different to ethanoic acid
sulfuric acid: strong (dissociated/energy not needed to dissociate)
ethanoic acid: weak (undissociated/energy needed to dissociate)
when calculating heats of combustion/formation
- keep in mind moles in equation
- if no equation given, write the balanced equation so you know how many moles of each react!!
importance of oil refining
give useful products / higher octane numbers
balanced equation for combustion of ethanoic acid
CH3COOH + 2O2 –> 2CO2 + 2H2O
combustion of a hydrocarbon formula
CₓHᵧ + ⁽⁴ˣ⁺ʸ⁾/⁴O₂ —> xCO₂ + ʸ/²H₂O
using heat formula
If given in g, turn into kg, and turn kJ into J.
However, don’t convert the heat change if given in Kelvin to celsius, as the heat change would be the same in both units!!
calculating heat of neutralisation
use mcθ to find heat produced in reaction, then divide by no. of moles (taking balanced equation into account) to find heat for 1 mole
