Topic 2- Developing Fuels Flashcards
What are standard conditions?
- standard temperature= 298K
- standard pressure= 1 atm
- concentration= 1mol/dm^3
What is absolute 0?
0K when there is no kinetic energy/ are no vibrations in any particle (-273 degrees c)
What is the definition of enthalpy change of reaction?
The enthalpy change when molar quantities of reactants as stated in the equation react together under standard conditions.
What is the definition of enthalpy change of combustion?
The enthalpy change that occurs when 1 mole of substance is burned completely in oxygen under standard conditions.
What is the definition of enthalpy change of formation?
The enthalpy change when 1 mole of compound is formed from its elements when everything is in standard states.
What is enthalpy?
Enthalpy describes the energy transfer in terms of chemical reactions
What is an exothermic reaction?
When the enthalpy change has a negative value, the enthalpy of the the products is less than the enthalpy of the reactants.
What is an endothermic reaction?
When the enthalpy change is positive, the enthalpy of the products is greater than the enthalpy of the reactants
How can you find the enthalpy change of a reaction by transferring it to water?
A bomb calorimeter- using the calculation E= m x c x change in T
c=specific heat capacity of water (4.18)
What is an enthalpy cycle?
In reactions that require large amounts of energy to react the values for an enthalpy transfer are hard to find in standard conditions, the value must be found indirectly using an enthalpy cycle which involves an indirect route with enthalpy changes that are possible to measure
What are the problems with petrol?
- It is finite and probably won’t last more than 100 years
- It is needed as a feedstock to make fibres, detergents, paints etc. and so it may become to precious to just burn
- It pollutes, when burnt it releases large amounts of carbon dioxide
What is Hess’s law?
The enthalpy change for any chemical reaction is independent of the intermediate stages, so long as the initial and final conditions are the same for each route.
Which way will the arrows point provided with enthalpy change of combustion values for an enthalpy cycle?
The arrows will always point downwards.
Which way will the arrows point provided with enthalpy change of formation values for an enthalpy cycle?
The arrows will always point upwards
What is energy density?
The energy released per kilo of fuel.
enthalpy change of combustion
———————————— x1000
Relative molecular mass
What is required to make and break bonds?
To break a bond is endothermic (+), it requires energy to separate the nuclei and electrons
To make a bond is exothermic (-) so energy is given out to the environment.
What is bond enthalpy?
The energy required to make or break a bond
Why are oxygenated fuels less energy-rich than hydrocarbons?
The energy released during combustion comes from bonds forming with oxygen, if a fuel already has a bond with oxygen then it is partially oxidised and so will release less energy.
How is crude oil separated?
Crude oil is separated by fractional distillation. This produces fractions which will have a specific boiling range and contain molecules made of certain numbers of carbon atoms.
What are the problems with fractional distillation?
- Straight run gasoline is a very poor petrol
2. Supply and demand don’t match up
What is a winter blend of petrol?
Winter petrol needs to be more volatile, so short branched hydrocarbons are used. (Lower boiling point so less energy is needed for the fuel to evaporate) the chains are less tangled and so this makes them more volatile.
What is a summer blend of petrol?
In summer the fuel needs to be less volatile to reduce evaporation from the petrol tank which is wasteful and polluting, so longer unbranched hydrocarbons are used
What is knocking?
Auto-ignition. When the fuel-air mixture is compressed, it can ignite before the spark from the spark plug occurs causing 2 explosions in the cylinder.
What is a molecules octane number?
The molecules tendency to auto ignite, a lower octane number the more likely the molecule to auto ignite.
To get a high octane number we need:
-short chains
-branched chains
Which types of engines need auto ignition?
Diesel engines require auto ignition to work however auto ignition damages petrol engines.
How can we make high octane petrol?
- lead compounds- but they create toxic exhaust fumes and poison catalytic convertors.
- short and branched molecules- we can’t have too many or the petrol will be too volatile
- modifying the structures of hydrocarbons- isomerisation, reforming, cracking.
- adding other things- oxygenates
What are alkanes?
A hydrocarbon with only single bonds between carbon atoms. They are saturated. Each member differs by CH2
What are the prefixes for naming?
1C- meth~ 8C- oct~ 2C- eth~ 9C- non~ 3C- prop~ 10C- dec~ 4C- but~ 11C- undec~ 5C- pent~ 12C- dodec~ 6C- hex~ 7C- hept~
What is the test for unsaturation/ alkenes
Adding bromine water, the bromine changes colour from red/brown to colourless if an alkene is present
What are cycloalkanes?
A homologous series- alkenes join up at their ends to form rings (2H’s removed)
What are isomers?
Branches in chains, molecules have the same molecular formula but different structural formula.
What is structural isomerism?
The same molecular formula but atoms are bonded together in a different order.
Types of structural isomers include chain isomerism, position isomerism and functional group isomerism.
What is chain isomerism?
When a chain of carbon atoms branches, this can create different properties e.g boiling points
What is position isomerism?
When the functional group is situated in different positions in the molecules e.g alcohols
What is functional group isomerism?
Molecules have the same molecular formula but have different functional groups and are therefore in different homologous series e.g alcohols and ethers
What is the process of isomerism.
Turns straight chain alkenes into branched isomers. The process is done using an Al2O3 catalyst with platinum finely dispersed on it. The products then mass over a zeolite, which acts a molecular sieve which separates branched and straight chain alkanes.
What is the process of reforming?
Turns straight chain alkanes into cycloalkanes then into aromatic compounds.
Reforming is done at 500C using an Al2O3 catalyst with finely dispersed platinum- known as platforming.
What is the process of catalytic cracking?
Alkanes that are too big to be used in petrol are broken into a shorter alkane and an alkene/cycloalkane.
This used high temperatures and a zeolite catalyst.
What does adding oxygenates to petrol do?
Oxygenates e.g alcohols and ethers have high octane numbers and reduce the amount of carbon monoxide released.
What is a functional group?
An atom or group of atoms that modifies the behaviour of a carbon skeleton.
What is a polyhydric alcohol?
If there are more than 1 -OH groups on the carbon skeleton then the alcohol is polyhydric.
2 OH= diol
3 OH= triol
What is an ether?
An oxygen atom found between 2 carbon chains- called an ether linkage.
How do you name an ether?
The shorter chain becomes an alkoxy e.g 3 C’s propoxy
The longer chain is left as the parent alkane e.g pentane
Propoxypentane
What is important in making diesel fuel?
- cetane number- higher cetane number makes a more suitable fuel
- flash point- lowest temperature at which vapours can catch fire must be low for diesel
- cloud point- components of diesel can solidify in cold weather making the fuel look cloudy. Winter diesel has a lower cloud point.
What is entropy?
Entropy is the number of ways particles have of arranging themselves, ( the amount of disorder in a system)
What causes entropy to increase?
- the number of molecules increases
- if the different types of particles increases
- S gas > S liquid > S solid
- if the size of the atom/ molecule increases
- if there are more moles of the right hand side than left hand side
- if there is a more mixed system
What are primary pollutants?
The products of chemical reactions that occur, the emissions
Where do emissions from car engines come from?
CO2/ H2O- products of complete combustion of hydrocarbons
CO- incomplete combustion
NOx- N2 and O2 react together at high temperatures within the engine
SOx- fuels contain some sulphur compounds which react with O2
CxHy- some is unburnt as it passes through the system
Particulates- incomplete combustion
What are the concerns with pollutants?
CO2- greenhouse gas
CO- toxic to humans
NOx- cause acid rain and the formation of photochemical smog
SOx- cause acid rain
CxHy- cause formation of photochemical smog and are carcinogenic
Particulates- irritate the lungs
What are secondary pollutants?
Pollutants produced by reactions between primary pollutants
What is photochemical smog?
Photochemical smog is produced by the action of the sun’s rays on primary pollutants produced by burning hydrocarbons. It includes ground level ozone which is very reactive.
What is avagadro’s law for gases?
1 mole of any gas occupies the same volume at the same temperature and pressure.
(24 dm^3 at rtp)
What is a catalyst?
A substance that speeds up the rate of reaction but can be recovered chemically unchanged. It speeds up the reaction by giving an alternative pathway with a lower activation energy.
What is a heterogeneous catalyst?
A catalyst that is a different state to the reactants
How does a heterogeneous catalyst work?
- reactants are adsorbed onto the surface of the catalyst.
- this weakens and breaks bonds in the reactants
- new bonds form between the reactants, held close together on the surface to form the products
- this weakens the bonds to the catalyst surface and the product molecules are released
What is catalyst poisoning?
In a heterogeneous catalyst the ‘poison’ molecules are adsorbed more strongly onto the catalyst surface than the reactant molecules. The catalyst cannot catalyse a reaction of the poison and so becomes inactive with poison molecules blocking the active site on its surface
