Investigation into the energy produced per mole when burning different alcohols to heat up water.

Investigation into the energy produced per mole when burning

different alcohols to heat up water.

Plan

The aim of this investigation is to find out how much energy is produced per mole of alcohol when burning the alcohol to heat up water to a certain temperature rise.

Equipment

5 Alcohol Burners:

Methanol,

Ethanol,

Propanol,

Butanol,

Pentanol,

Bunsen Burner,

Splints,

Wooden Board,

Safety Glasses,

200ml beaker,

Water,

Thermometer,

Clamp Stand,

Ruler,

Scales,

Measuring Cylinder,

Method

Set up all the equipment as shown above. First weigh the alcohol burner, so that the amount of alcohol that has been burnt can be worked out after the experiment has been carried out. Then fill up the beaker with 100mls of water, using the measuring cylinder for greater accuracy. The water used will always be around room temperature, because it is from the tap. Place the alcohol burner on to the wooden board next to the clamp stand, and then attach the beaker with water in, to the clamp stand, directly above the

alcohol burner and 15cm from the board, and put the thermometer into the water.

Now record the temperature of the water. It should be around 20°C, because that is the normal room temperature. Now light the Bunsen burner. Using the splint, light the first alcohol burner, which is methanol. Let the flame from the alcohol burner, burn until the temperature of the water has risen 40°C from the original temperature. Now put out the flame, and weigh the burner to calculate the amount of alcohol that was burnt during the experiment.

Do this experiment again for each of the alcohols up to pentanol, every time changing the water, and washing off any soot (carbon) on the beaker that may have been caused by incomplete combustion, which is when there is a restricted supply of oxygen, and some of the alcohol does not burn completely, giving carbon and carbon monoxide rather than carbon dioxide.

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Repeat this complete experiment with all the alcohols three times for good accuracy. Now work out how much alcohol was burnt during each separate test, by taking away the amount of alcohol that you were left with from the original quantity of alcohol. Now work out the amount of energy that is going into the water by doing the following equation:

Amount of energy going = Mass of x Rise in x Specific

into the water Water Temperature Heat Capacity

KJ Kg °C 4.2

Then work out the formula mass of each of the alcohols. Then, using the formula masses, work out the number of moles of alcohol burnt by doing this equation:

Number of moles = Mass of alcohol burnt

Formula mass of alcohol

Using the answer to this equation, work out what the amount of energy produced per mole of alcohol burnt is by using this equation:

Amount of energy going into the water

Number of moles of alcohol burnt

Fair Test

To make this investigation fair, I will make sure that I only change one variable throughout the experiment, and keep all the other variables the same. This will make it a controlled investigation.

Variable

Variable that I will be changing

Throughout this investigation, I will be testing the effect that different alcohols have on the amount of energy produced per mole, so the only variable that I will be changing is the type of alcohol that is being used.

Variables that I will be keeping the same

The variables that I will be keeping the same throughout this investigation are:

The rise in temperature of the water. I am keeping this the same, because if I change it, the experiments will be inaccurate because the flame will need to burn for longer if the temperature rise is higher, or if it is a lower temperature rise, the flame will need to burn for a shorter period of time. This will lower the accuracy of the results, and make it an uncontrolled test.
The length between the bottom of the beaker and the wooden board should be kept the same, because otherwise the beaker will be nearer or further away from the flame, making the readings inaccurate.
The mass of the water, because more water would take longer to heat up, making the investigation inaccurate.

Safety

To make this investigation safe, there are some guidelines that I will follow.

I will:

Follow all the usual laboratory rules.
Make sure that I do not go near any electricity with the water.
Be careful with the alcohol burners, because they are very flammable.
If I come into contact with the alcohol, wash it off immediately, so that it doesn’t burn my skin.
Be sensible around the flames.

Prediction

I predict that as the number of bonds, or formula mass of the alcohols increases, the energy produced per mole of alcohol burnt will also increase.

For any reaction to take place bonds must be broken and made, bond breaking requires energy while bond making releases energy. Bonds between different atoms require or release different amounts of energy when broken or made because they are different in strength.

The energy that is produced per mole is the bond energy, or enthalpy. This is the energy that is required to break one particular bond. I predicted that the energy produced per mole would increase as the number of bonds in the alcohol. This is because of the breaking of bonds between the molecules within the alcohols. The minimum energy that the molecules must possess in order for the bonds to break is called the activation energy. If they gain enough energy while they are colliding during the reaction, the process of bond breaking can start. As this process of bond breaking goes on, new bonds start to form. This is the reactant molecule reacting, and changing into the product molecules of the reaction. The activation energy in the reaction is less than the energy that is released from the new bonds. Overall, the more bonds that have to be broken within the alcohol, the more energy will be produced in the reaction.