Investigating the enthalpy change of different fuels when combusted.

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Investigating the enthalpy change of different fuels when combusted.

Aim:

The aim of this experiment is to see how the enthalpy changes vary when different alcohols are combusted in standard conditions. The alcohols used will be: Ethanol, Propan-1-ol, Butan-1-ol, Pentan-1-ol and Hexan-1-ol. These alcohols all have the OH functional group on the first Carbon atom of the molecule.

Background Knowledge

The enthalpy change of combustion (also shown as ?Hc) is a measure of energy when one mole of a fuel burns completely in air, at a standard temperature and pressure. This standard temperature is 298 Kelvin and 1ATM pressure. Maintaining these conditions would be complicated; therefore the experiment will be carried out in normal conditions. Any drastic alterations to these conditions will need to be taken care of to maintain a degree of accuracy.

As combustion is exothermic (heat is transferred to the environment) all of the enthalpy changes will be negative. The formula for enthalpy change is:

Energy Transferred= Heat capacity of water x Change in water temp x mass of water.

Or

E=MC?T

Hess's Law.

"the enthalpy change for any chemical reaction is independent of the intermediate stages, provided the initial and final conditions are the saeme for each route."

This basically means that in an enthalpy cycle, detours can be made to reach the final products and the total energy required for the detour will be equal to the direct route ie.

The enthalpy change for ?H1 = ?H2 + ?H3. as we cannot go directly along ?H1, we must take the detour and from this we can calculate the enthalpy change. This can be calculated by the enthalpy changes of combustion for carbon, hydrogen and the alcohol in question.

Bond Enthalpies:

The amount of energy required to break the bonds between atoms is called the disassociation energy. The higher the disassociation energy, the shorter the bond is in length and the stronger the bond is. Single bonds are relatively easy to break as opposed to double or triple bonds.

Endothermic and Exothermic Reactions:

Exothermic reactions are those that give energy into their surroundings in the form of heat, heating the surroundings. This allows us to heat up the water directly above the spirit burner.

An endothermic reaction is totally opposite to an exothermic reaction as endothermic reactions take in energy from the surroundings and cool the surroundings.

Here are some bond energy diagrams for exothermic and endothermic reactions:

This shows the negative enthalpy change as energy is given to the surroundings. The downward pointing arrow shows the energy lost as heat during the reaction.

This diagram shows the positive energy change of an endothermic reaction. The intake of energy from the environment is represented by the smaller of the 2 arrows.

Alcohols

Alcohols are modified alkane chains with a functional hydroxyl group. E.g.

Ethane Ethanol

The only difference between these molecules is the hydroxyl group. The position of this hydroxyl group will determine if the alcohol is primary, secondary, tertiary and so on. Primary alcohols have the functional group on the first carbon atom of the molecule at either end; Secondary alcohols have the hydroxyl group on the second carbon of the molecule and so on. The number of the carbon atoms is determined by counting from the end nearest the carbon atom in question. These are all isomers and all molecules with more than 2 carbon atoms can form an isomer. The isomers have the same molecular formula but have different chemical structures and therefore have slightly different chemical properties. This is because the molecules shape has changed so they may no longer fit together in this isomer form as they did in the straight chain molecules. This may affect the boiling and melting point of the alcohol as the intermolecular forces are affected.

This experiment will use the 2nd, 3rd,4th,5th and 6th alcohols, which will al be primary alcohols where applicable. Methanol will not be used as it produces poisonous carbon monoxide and the combustion of this alcohol must be undertaken in the fume cupboards. This will affect our results as the fume cupboard produces a current of air near the front of the cupboard which has the tendency to blow the flame at an angle, reducing the amount of heat that is transferred to the water
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In the combustion of alcohols, if there is a plentiful supply of oxygen, there will be complete combustion, in which the only waste products are water and carbon dioxide. If there is not a plentiful supply of oxygen, incomplete combustion occurs and the products are water and poisonous carbon monoxide.

Alcohol

Bond Breaking

Bond making

Total

Breaking - Making

Ethanol

C-H= 413x5=2065

C-O= 385x1=385

O-H= 464x1=464

O-O=498x3=1494

C-C= 347x1=347

O-H= 464x6=2784

C=O= 805x4=3220

-1249 kJ/mol

Propan-1-ol

C-H= 413x7=2891

C-O= 385x1=385

...

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