Combustion of Alcohol Investigation

I will wait until the fuel in the crucible has stopped burning and then I will take the temperature of the water again. I will then weigh the crucible and it's contents. I will make every effort to ensure that all the fuel in the crucible is used (burned) each time. I will also endeavour to make sure that exactly 1.69g of each fuel is used each time. No more and no less.

All results will be repeated to make the results ultimately more accurate.

Theory

When you burn a fuel, in this case an alcohol, energy in the form of heat is given out in an exothermic reaction. This means that heat is given out.

We measure this energy in joules, the more joules the higher the energy and the more heat there is.

The alcohols that I will be using for my experiment are:

Methanol CH OH

Ethanol C H OH

Propanol C H OH

Butanol C H OH

Pentanol C H OH

To work out the amount of energy transferred to the water in each experiment, I am going to use this formula.

Energy transferred = mass of water (g) x 4.2 x temperature change

The number 4.2 is called the specific heat capacity of water, in joules per gram per degree Celsius.

Suppose we have 10g of water and we heat it up by 1 C

Energy transferred = 10 x 4.2 x 1

= 42J

Now if we use an alcohol as an example. Let's say that I have 200g of water and I've used the energy from burning some Ethanol to heat it up by 2 C

Energy transferred = 200 x 4.2 x 2

= 1680J

Now if I wanted to work out how much energy per mole of Ethanol was transferred, here's what I would need to do.

Firstly I would need to work out the mass of fuel used. Let's say I used 2g of Ethanol. The molar mass of Ethanol is as follows:

C H OH = (12 x 2) + (1 x 5) + 16 + 1 = 46g

I already know that the energy transferred to the water was 1680J, so now I have to work out how much energy was transferred per gram of fuel to the water. I used 2g of Ethanol so...

680 2 = 840J

So now I can work out the energy obtained by burning one mole of Ethanol, in this experiment.

Energy obtained

By burning 1 mole = molar mass x Energy transferred to the water per gram

Of Ethanol

= 46 x 840

= 38640 Joules

Bond Energy diagrams can also be used to show what happens in the reaction.

Here is a bond energy diagram for Ethanol.

Input Variables

The input variables that may affect my experiment are:

How much heat escapes by convection.

How hot or cold the room, in which I do my experiment, is.

How much fuel I use.

How much water I heat.

The distance from the crucible to the gauze.

Which fuel I use.

The only variable that I intend to have in my experiment is the type of fuel I use. I will use the same amount of fuel each time and keep the conditions the same to make it a fair test.

Predictions

Here are the molar masses of all the fuels I am going to use:

Methanol CH OH = (12 x 1) + (1 x 3) + 16 + 1 = 32g

Ethanol C H OH = (12 x 2) + (1 x 5) + 16 + 1 = 46g

Propanol C H OH = (12 x 3) + (1 x 7) + 16 +1 = 60g

Butanol C H OH = (12 x 4) + (1 x 9) + 16 +1 = 74g

Pentanol C H OH = (12 x 5) + (1 x 11) + 16 + 1 = 88g

Each of the moles of these five different alcohols is a different size. But they are larger than each other, going up in progressive steps, an extra CH each time.

When alcohols burn, an exothermic reaction occurs. This is because when the alcohol burns, more bonds (between Atoms) are being made than are being broken, so heat is given off.

In a larger molecule, more bonds need to be broken; therefore more bonds are made, in an exothermic reaction that gives off more heat. So a larger molecule, in this case, means that there will be more heat energy given out.

Therefore, I predict that the size of the molecule will be proportional to the amount of energy given out. So the amount of energy given out, when the same amount of alcohol is used each time, should go up in even steps. This is my prediction.