Molar Heat of Combustion of Alcohols

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Molar Heat of Combustion of Alcohols

Task:

To find the molar heat of combustion of propanol, ethanol and butanol and to calculate the theoretical values using average bond enthalpies. Then to compare these to the practical values obtained from the experiments, also to predict the theoretical and practical molar heat of combustion of pentanol.

Planning

Background Information

Alcohols:

Alcohols are part of a family of compounds that are all based on a chain of carbon atoms. They can be considered derivatives of water in which the hydrocarbon atom has been replaced by a hydrocarbon chain. Alcohols all have the general formula CnH2n+1OH and which burn cleanly in oxygen or a plentiful supply of air to produce carbon dioxide and water vapour. If the air supply is reduced then they will burn to produce carbon monoxide, soot and water vapour.

All Alcohols contain an oxygen atom in a hydroxyl group, meaning that the oxygen atom is joined to the hydrogen atom as well as the carbon atom. All alcohols contain at least one oxygen atom joined to a carbon atom by a single bond.

Equations for the Three Alcohols:

This is the formula for all alcohols:

Cn H2n+1 OH

The molar heat of combustion is the heat liberated when one mole of alcohol is burnt in a plentiful supply of air.

These are the formulas for the combustion of the three alcohols.

Butanol + Oxygen → Carbon Dioxide + Water

C4H9OH + 602 → 4CO2 + 5H2O

(One Mole)

Propanol + Oxygen → Carbon Dioxide + Water

C3H7OH + 4.5O2 → 3CO2 + 4H2O

(One Mole)

Ethanol + Oxygen → Carbon Dioxide + Water

C2H5OH + 3O2 → 2CO2 + 3H2O

(One Mole)

Bond Enthalpies:

Bond enthalpy is the amount of energy needed to break a chemical bond. Bond enthalpies differ with each type of bond and the exact enthalpy for a particular bond can differ. Average bond enthalpies can be found on the Internet and in data books.

We can use bond enthalpies to work out the energy given out by a reaction by comparing the energy needed to break the bonds (endothermic) to the energy given out when new bonds are made (exothermic). We can use these figures to work out the theoretical molar heat of combustion of the alcohols, which I will do later.

Average Bond Enthalpies:

Preliminary Experiments

Firstly, I plan to find the practical enthalpy of combustion by using a spirit burner with the alcohols ethanol, propanol and butanol to heat a container with 25 ml of water. We are testing a test-tube, a copper calorimeter and a glass beaker to find out which container transfers heat to the water the best. We chose these three vessels to test because they are the most commonly used for heating liquids so we think it is likely that they will all transfer the heat to the water relatively efficiently. We are going to heat for two minutes and use equations (using the figures from how much the water was heated and how much alcohol was burnt) to work out how much energy was liberated during the reaction. We are heating for two minutes because we want the water to increase about 40-50 degrees in temperature. We decided that two minutes would be the best for this as the water boiled in three and did not change as much as we would have liked in two. I think that the copper calorimeter will work the best because metal is the best conductor. I will weigh the alcohol before burning and afterburning. I will take the temperature of the water after heating for two minutes.

The beaker, which holds the 25ml of water, will be held 5cm above the flame by a clamp stand because we think this height will be best to heat the water. We tried heating the water at three consecutive heights, once at 5cm, once at 7cm and once at 3cm and found that 5cm was the most effective height. The second experiment will be surrounded with draft excluder and a covering for the lid, we want to test whether these affect the accuracy of the experiment much. The room temperature will be constant so there should not be many outside factors affecting the experiment.

Preliminary Work:

We wanted to find out whether a lid and draft excluder would work and what container was best to heat the water in. We heated the water in a copper calorimeter, a test tube and a beaker. Once we found out which vessel was the best we then tested this with a lid and draft excluder. We wanted to keep the preliminary tests fair so we used 25 ml of water each time and heated it for two minutes keeping the distance between the flame and the container the same. We also weighed the alcohol before and after to see how much fuel was used, we could then use an equation to work out how much energy was actually used to heat the water. From this you get a much more accurate picture of which container was the best because even if a container was heated a lot more than the others were it may have also  used a lot more fuel.

Prediction

I think that the copper calorimeter will heat the best because it conducts heat better than the other two containers, the test tube will probably be the worst because the surface area near the flame will be much less than the other two, which means the heat conduction will be reduced. The test using the lid and the draft excluder should be the best but they will still be very inaccurate, as most of the heat will be lost through radiation. Much heat will also be lost through convection as the hot air rises past the container. Conduction would be a relatively minor problem. The copper calorimeter will conduct some energy to the clamp stand but the test tube and the beaker are both glass and do not conduct the heat as well.

Radiation could be reduced by using insulation and a lid for the beaker. There is no real way to stop heat loss through convection but it could be reduced by making a foil skirt to fit the container.

In a perfect world we would produce the test in a controlled environment where aspects such as the weather could be emulated so that the humidity would always be the same. This would stop any outside factors altering the test and therefore eliminate day-to-day fluctuations in our results. Using the same burner would be required in an accurate test because two separate burners may not have the same efficiency.

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Specific Heat Capacity of Water

It takes exactly 4.2 joules of energy to heat 1ml of water by 1oC. This is called the specific heat capacity of water.

This formula uses the specific heat capacity of water to work out the energy supplied to an amount of water:

Energy Supplied (J) = Mass of Water (g) * Specific Heat Capacity of Water (J/gOC) * Temp Change (OC)

If we say that the energy supplied is A and the amount of alcohol burnt is B then we can use the formula A/B to work out the amount of ...

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