F331 Chemistry

Octane numbers and fuels

Octane number measures the tendancy to auto-ignite/pre-ignite/knock
A cycloalkene/cycloalkane molecule is produced during reforming
Heterogeneous means that the catalyst and reactants are in different states
A catalyst speeds up the reaction giving a different pathway to lower activation energy
Using hydrogen as a fuel only produces water on combustion so no greenhouse gases
The reforming process is endothermic because:
- energy in / endothermic to break bonds
- energy given out / exothermic when bonds form
- less energy given out than taken in
During a cracking reaction:
- larger hydrocarbons are split
- to form shorter alkanes or alkenes

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An atomic absorption spectra has a visable spectra background and black lines to indicate where the particles have been absorbed.
An atomic emission spectra is the opposite, has a black background and visible spectra lines.
Why is an elements absorption spectra unique?
- Energy absorbed allows electrons to move up energy levels
- Energy levels are unique to each independent element
- Energy absorbed is related to frequency ( Δ E = hv)
The main stages in a time of flight mass spec:
- Sample is ionised
- All ions are accelerated in magnetic field to the same kinetic energy
- Move into the drift region
- Heavier ions move more slowly

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Adding an oxygenate to fuels increases octane number and lowers the chance of knocking
Calculating energy density: eg: The energy change of combustion of C8H16 is -5400kJmol-1
- Work out relative atomic mass of C8H16 = 112
- Moles in 1kg : 112 / 1000 = 8.93
- kJ per kg = 8.93 x 5400 = 47329
A structural isomer is molecule with the same molecular formula but different structural formulae

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Energy transferred to water:
- vol of water x shc x temp change
Empirical formula: (eg: Cerium forms an oxide containing 81.4% by mass of cerium, Ar Ce: 140.1)
- Moles Ce = 81.4 / 140.1 = 0.58
- Moles O = 18.6 /16.0 = 1.16
- Emperical formula = CeO2

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