# Developing Fuels Revision Notes

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• Created on: 28-12-12 20:07

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Developing Fuels Revision Notes
Developing Fuels
All gases take up the same volume under the same conditions. AT room temperature and
room pressure this volume is 24 dm3.
in dm3
N umber of moles = volume 24
Balanced equations can also be used to work out gas volumes.
In the equations: 2Na + 2H2O 2NaOH + H2, where 15g of sodium is reacted, you can work
out the volume of gas from working out the number of moles in 15g Na = 0.65 mol. Then you
see that 2 mol of Na 1mol of H2, Thus 0.65 mol Na 0.325 mol H2. 0.325 x 24 = 7.8dm3.
Entropy tells you how much disorder there is in a substance. It is a measure of ways the
particles can be arranged. Solids hare ordered so have low entropy, liquids have some
disorder, so more entropy, and gases are random and so the entropy is high.
Substances like disorder. Particles will naturally move to give a substance the maximum
possible entropy.
Increasing the number of particles increases the entropy as they are a greater number of
ways in which the particles can be arranged. SA mixture of two different types of particles
also has higher entropy than the equal number of one type of particle.
Enthalpy Change ( H) is the heat energy transferred in a reaction at constant pressure. The
units of H are in kJ mol-1. All chemical reactions have enthalpy changes.
The reactions can be exothermic or endothermic. Exothermic reactions give out energy, and
thus the H is negative. Endothermic reactions absorb energy, thus H is positive.
Combustion is exothermic and thermal decomposition is endothermic.
Enthalpy changes can be calculated using average bond enthalpies. To break bonds, energy
is needed, thus H is positive. To make bonds, energy is released, thus H is negative.
The enthalpy change for a reaction is the overall effect of these two changes. If you need more
energy to break bonds then is released when bonds are made, H is positive. If it's less, H is
negative.
Enthalpy Change of Reaction = T otal Energy absorbed to break bonds - T otal Energy released to make bonds
Bond enthalpy is related to the length of a bond. The distance between two nuclei is the
distance where the attractive and repulsive forces balance each other, this distance is bond
length. The stronger the attraction between the atoms, the higher the bond enthalpy and the
shorter the bond length, and vice versa.
A C=C bond has a greater bond enthalpy and is shorter than a C-C bond. Four electrons are
shared in C=C and only two in C-C, so the electron density between the two carbon atoms is
greater in C=C.
There are different types of H depending on the reactions: Standard enthalpy change of
reaction; standard enthalpy change of formation; and standard enthalpy change of
combustion.
Standard enthalpy change of reaction ( Hr) is the enthalpy changes when the reaction occurs
in the molar quantities shown in the chemical equation, under standard conditions in their
standard state.
Standard enthalpy change of formation ( Hf) is the enthalpy change when 1 mole of a
compound is formed from its elements in their standard states under standard conditions.
Standard enthalpy change of combustion ( Hc) is the enthalpy change when 1 mole of a
substance is completely burned in oxygen, under standard conditions.

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Developing Fuels Revision Notes
Hess's Law states that the total enthalpy change of a reaction is always the same, no matter
what route is taken.
Enthalpy Changes can also be found out by using calorimetry. In calorimetry you find how
much heat is given out by a reaction by measuring the temperature and then using the
equations: q=mc T where q = heat lost or gained, m = mass of water, c = specific heat

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Developing Fuels Revision Notes
Alkanes are saturated hydrocarbons. All carbon atoms in alkanes have four single bonds with
other atoms. It is impossible for carbon to make more than four bonds, so alkanes are
saturated.
Alkenes are unsaturated hydrocarbons. Alkene molecules all have at lease on C=C double
covalent bond, thus all alkenes are unsaturated.
Benzene has a ring of delocalised electrons. Benzene is like a cyclic alkene with 6 carbons and

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Developing Fuels Revision Notes
Molecular formula shows you the actual number of atoms of each element in a molecule, with
any functional groups indicated.
Structural formula sows the atoms carbon by carbon, with the attached hydrogens and
functional groups.
Displayed formula shows how all the atoms are arranged, and all the bonds between them
Skeletal formula shows the bonds of the carbon skeleton only, with any functional groups.
The hydrogen and carbon atoms aren't shown. Each end and bend stands for a carbon atom.…read more

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Developing Fuels Revision Notes
Heavy fractions can be cracked to make smaller molecules. Cracking involves breaking
long-chain alkanes into smaller hydrocarbons (which can include alkenes). It involves breaking
the C-C bonds. There are two main types of cracking: thermal cracking and catalytic cracking.
Thermal cracking takes place at high temperature (1000C) and at a high pressure
(70atmosspheres). It produces a lot of alkenes where are then used to make lots of valuable
products such as polymers.
Catalytic cracking makes mostly motor fuels and aromatic hydrocarbons.…read more

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Developing Fuels Revision Notes
It also produces sulphur dioxide which leads to acid rain. Acid rain is caused by burning fossil
fuels that contain sulphur. The sulphur burns to produce sulphur dioxide gas which then enters
the atmosphere, dissolves in the moisture, and is converted into sulphuric acid. Acid rain
destroys trees and vegetation, as well as corroding buildings and statues and killing fish in
lakes.
Pollution is a serious problem, it can affect on an individual scale and a global scale.…read more