Combustion Of Alcos Essay

This essay has a total of 2300 words and 14 pages.

Combustion Of Alcos


Investigating The Combustion Of Alcohols - Planning

This investigation involves burning alcohol in the air. Key science- Chemistry by Eileen
Ramsden says that " an alcohol is a series of organic, homologous compounds, with the
general formula Cn H2n 1OH". The alcohol reacts with the oxygen in the air to form the
products water and carbon dioxide:


Cn H2n 1OH (n n/2)-1O2 ? nH2O nCO2

The structure of the molecules in this reaction is:

H H | | H - C - C - O- H 3[O=O] ? ½[O=C=O] 3[H-O-H] | | H H

This reaction is exothermic, as heat is given out. This is because the amount reactant
energy is more than the product energy the difference between this is ?H, therefore some
energy has been given out in the form of heat.The energy is given out when forming the
bonds between the new water and carbon dioxide molecules. This can be shown in an energy
level diagram: Reaction co-ordinate ?H is the heat content, which is the enthalpy, which
is negative in exothermic reactions as the diagram shows that energy is ‘lost' as heat.
Enthalpy is defined as the energy of reaction, or the heat energy associated with a
chemical change. Chemical Principles By Master & Slowinski says that "For any reaction
carried out directly at a constant pressure, the heat flow is exactly equal to the
difference between enthalpy of products and that of the reactants", or: Qp = Hp - Hr = ?H
Where Qp is the heat flow at constant pressure, Hp is heat energy of products, and Hr is
the heat energy of the reactants.


To measure ?H given off, we must use this energy to heat something, this will be water.
This is assuming that all the heat produced by combustion of fuel (?H) will equal the
amount of heat absorbed by the water (q). So I will measure the amount of energy required
to do so. This can be worked out by useing the formula: q = mass X specific heat capacity
X temperature rise 1000 Where q is the quantity of heat. The specific heat capacity is the
amount of energy required to heat the substance, and is calculated using the formula q =
MC?, where q is the enthalpy, is the specific heat capacity and ? is the temperature rise.
I chose to use water as it is safe, easily obtainable, and has a constant, reliable
specific heat capacity of 4.2J/?C.


The bonds which are made in a exothermic reaction "are forces of attraction between the
atoms or ions in a substance" according to Key science- Chemistry by Eileen Ramsden. These
can be of two types: covalent, in which the atoms share electrons. Examples of this are
water and carbon dioxide, which has a double covalent bond because it shares two pairs of
electrons are shared. The other type of bonds are ionic, where a metal is involved. This
is where electrons are transferred from one ion to another, so there is an electrostatic
force between the ions.


The variables that must be controlled are:



* Mass of water

* Amount of wick on burner

* Type of alcohol

* Height of can above flame

* Type of can

* Time of burning



The alcohols used in this experiment will be from methanol, to hexanol, their formulas and
predicted enthalpy changes are:


Substance Formula - Predicted enthalpy change (KJ/mole)

Alcohol CnH2n 1OH

Methanol CH3OH -730

Ethanol C2H5OH -1370

Propanol C3H7OH -2010

Butanol C4H9OH -2670

Pentanol C5H11OH -3320

Hexanol C6H13OH 3980 N.B. The precicted enthalpy changes come form the book of data by
Nufield science. As the table shows, each alcohol reaction increases each time by :


CH2 11/2O 2 ? CO2 H2O.



Investigating The Combustion Of Alcohols - Method

I did some preliminary results, the results of which are shown in the table below:

Mass water (g) Initial temperature (C) Final temperature (C) Temperature rise (C)

50 19 42 23

100 20 38 18

150 20 31 11

This experiment was done to see which mass of water would be best. The 50g mass was too
large a rise as this caused too much heat to be lost to the environment, and 150 was to
small. Therefore the 100g value was used, as this temperature was right for the enthalpy
calculations. Also the amount of wick was investigated, and I found that if it were too
large, there was more heat lost to the environment, and if it were to small, most of the
heat given out is lost in the can, so 6mm is the optimum wick length. This will be kept
constant throughout, as well as the mass of water, and the temperature will be kept
constant, to ensure that only the type of alcohol is being investigated, so that this can
be a fair test.


As well as this, methods of reducing heat lost to the environment were investigated. I
found that by placing a hardboard draught excluder around the experiment, and a cardboard
lid with a hole for the thermometer on top of the can, the heat lost was significantly
reduced to make this experiment more accurate. Stirring the water means that there is
uniform temperature in the can, and monitoring the temperature rise to ensure uniform
heating. The can is copper as copper is a good conductor of heat, so more is transferred
to the water. The height of the can above the flame is also a factor, so this needs too be
kept constant just touching the can.


I decided to burn the alcohol for three minutes to make sure that enough energy had been
transferred for a accurate experiment. The can was kept the same as different cans have
different conduction properties The heat is transferred from the flame by vibrating air
particles in the flame, caused by the exothermic reaction, which then cause the molecules
in the can to vibrate, and so the molecules in the water vibrate.


Method:



* Measure out 100cm3 of water in to copper can using measuring cylinder.

* Place can so flame just touches bottom of can

* Stir and record initial temperature of water

* Record initial mass of alcohol plus burner

* Start clock and light burner at the same time

* Stir water

* Burn for three minutes, then extinguish flame

* Record final temperature of water

Continues for 7 more pages >>