Investigating the Combustion of Alcohols - Planning

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] ? Ð'Ð...[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 Ð''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 using

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:

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