5th year Chem

exothermic reaction

a reaction in which heat is released e.g fuel burning

endothermic

a reaction in which heat is taken in e.g photosynthesis

heat of reaction

this is the heat change which occurs when a reaction takes place according to a given balanced chemical equation

when is ΔH positive

when heat is gained in the reaction (endothermic reaction)

when is ΔH (delta H) negative

when heat is lost in the reaction (exothermic reaction)

heat of combustion

when 1 mole of a substance is burned in an excess of oxygen

kilogram calorific value

the heat energy produced when 1kg of the fuel is completely burned in oxygen

bond energy

the energy required to break one mole of covalent bonds and separate the neutral atoms completely from each other

heat of neutralisation

this is the heat change when one mole of an OH+ ion from an acid reacts with one mole of an OH- ion from a base

heat of formation

this is the heat change which occurs when one mole of a compound is formed from its elements in their standard states

(standard state)

the elements normal form at 25 degrees and at one atmospheric pressure

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

catalytic cracking

the breaking down of long chained hydrocarbon molecules by the action of heat and catalysts into short chained molecules.

fractional distillation

the heating of crude oil and separating the different mixtures based on their boiling points

characteristics of oil with small molecules

• low boiling point

• light in colour

• easy to light

• runny

characteristics of oils with large molecules

• high boiling point

• dark in colour

• hard to light

• thick

how does fractional distillation occur

• the oil enters the column partially vapourised

• the tower is 50cm

• the temp drops as it moves up the tower

• substances with large molecules come off as liquids at the bottom of the column (large hydrocarbons)

• substances with low boiling points come off as gases at the top of the column (small hydrocarbon)

what is knocking

when in the running of an engine the explosion between petrol and air occurs too early

what is auto ignition

the early explosion of the petrol- air mixture before normal ignition by a spark takes place

what are the reasons for auto ignition

• straight chained molecules ignite more easily e.g heptane, octane

• branched chains like 2,2,4 trimethylpentane don’t auto ignite

what is the octane number

the measure of the tendency of a fuel to resist knocking

what is an example of a molecule with a low octane number

heptane which is prone to auto ignition and has an octane number of 0

what is an example of a molecule with a high octane number

2,2,4 trimethylpentane which has a low tendency to auto-ignite and has an octane rating of 100

what is said about octane numbers and chain length

the shorter the alkane chain the higher the octane number

how do you increase the octane number of a fuel

1. isomerisation

2. adding oxygenates

3. catalytic cracking

4. (Reforming) dehydrocyclization

what is isomerisation

changing straight chained molecules into their branched equivalents

how does isomerisation occur

• the alkanes are heated in the presence of a catalyst to break the chains apart and then they join together again resulting in a more branched product

what is an example of isomerisation

pentane - 2 Methylbutane

what is refinery gas used for

domestic gas

what is naphtha used for

medecine,plastics, fibres

what is kerosene used for

jet fuel

what are mercaptans

sulfur compounds that give an unpleasant smell to detect gas leaks

what compound has an octane number of 100

2,2,4 trimethylpentane

what does tetraethyl lead do

reduce the amount of knocking and increases the octane number of a fuel

what is dehydrocyclization

using catalysts to form ring compounds

what is used to measure heat of combustion

bomb calorimetre