Chemical Energetics (2) Measuring the enthalpy change of combustion

Chemical Energetics (2) Measuring the enthalpy change of combustion. 

Fuels like alcohols are one of the easiest compounds to use to measure the enthalpy of combustion.

Most school and college courses will include this experiment.

At a basic level, the experiment involves using a simple copper calorimeter filled with water,  a spirit burner and a thermometer to measure the temperature rise in the water. 

You might be asked to describe how you might compare two or three alcohols and so provide a fair test of the measurements. 

And at a more sophisticated level, you would first calibrate the apparatus with an alcohol of known enthalpy of combustion.  (see below)

Let’s take the basic approach first:

Here’s a simple diagram of the apparatus you might use or have used.

And yes here’s another case of watch the internet.  As you can see there is no thermometer in the water in the calorimeter and the calorimeter is made of glass not a better conducting material like copper. 

But we have a spirit burner and the calorimeter placed at a constant height above the burner. 

There are also two draught shields either side of the apparatus to prevent convection of heat away from the calorimeter. 

So how do we make this experiment a fair measure of heat of combustion between two or three alcohols?

First, keep the calorimeter at the same height above the burner.

Second, keep the flame the same size (very tricky to do).

Third, keep the wick the same length.

Fourth, use the same volume of water in each experiment, say 300ml.

Fifth, raise the temperature of the water by the same number of degrees in each experiment, say 10.0^oC.  The thermometer should read to 0.1^oC.

Better results come with a higher temperature change and a greater volume of water as the percentage error in each measure is reduced. 

You’ll also need snuffer for the spirit burner. 

Here is a better diagram to follow:

How to carry out this experiment

The procedure goes like this:

• Weigh the burner with snuffer (it prevents evaporation of the alcohol before and after burning so preventing the weight actually burned changing, if not the weight burned  would be higher than it actually is)

• Light the wick and place under calorimeter.

• Stir the water in the calorimeter until the temperature rises 10^oC

• Snuff out the burner and reweigh.

You can then calculate the energy transferred to the water in the calorimeter like this:

Energy transferred (in Joules) E  = m c ΔT

M = the mass of water in the calorimeter (not repeat not the mass of alcohol burned!!)

C = specific heat capacity of water (This is the number of Joules of energy that raise the temperature of 1 g of water by 1^oC i.e. 4.2 J/g/ ^oC)

ΔT  =  the temperature change in ^oC

What you can do with the data:

At this point, there are several things you can do with the data.

You could calculate the energy released per gram of fuel burned like this:

Energy per gram (J/g)      =        energy released (J)  

                                                             mass of fuel burnt (g)

But suppose you wanted to measure the enthalpy of combustion of your alcohol.

Let’s remind ourselves of the definition first:

The standard enthalpy change of combustion (denoted  ΔH^o_c ) is the enthalpy change that occurs when one mole of a compound is completely burned in oxygen.

Given this definition we are not going to get close to the value unless we calibrate the apparatus because the apparatus does not use pure oxygen, and it has many places where the heat is wasted because it does not heat the water in the calorimeter but the surrounding air. 

There are draughts despite the draught shields.

The flame is usually yellow so the alcohol is not burning completely.

And we do not burn one mole of the compound.

We need to calibrate the apparatus with a known alcohol whose enthalpy of combustion is known which will give us a conversion factor (heat capacity of the apparatus) based on using the same apparatus each time. 

Provided all else remains the same then burning a different alcohol and applying the conversion factor we can determine a heat of combustion for a different alcohol. 

Here’s how to use propan-1-ol to calibrate your apparatus:

Look up the molar mass of propan-1-ol and its enthalpy change of combustion

M_r is  60.1 g/mol and ΔH^o_c  is -2021kJ/mol. 

The calibration factor is calculated like this:

First, calculate the energy released using this equation:

Mass propan-1-ol burned/g * ΔH^o_c     =     energy produced Q (kJ) — 1

Molar mass propan-1-ol

Then the heat capacity or conversion factor becomes:

heat capacity of the apparatus c (kJ/K)    =    Energy produced (Q)  — 2

                                                                                 Temperature rise (K)

All you then need to do is use exactly the same set up as before only this time with a different alcohol. 

Then with the heat capacity of the apparatus known you can use the temperature rise to calculate the energy produced from a different alcohol.

This energy produced value will of course be the same as before because the same temperature rise has been measured.

Energy produced (kJ)  =  heat capacity (kJ/K) *  temperature rise (K)  — 3

But what’s different with the second alcohol is that the measured mass of alcohol burned is different.

If the alcohol has more carbon atoms the measured mass burnt should be smaller.

So the enthalpy of combustion can then be calculated like this:

Enthalpy of combustion ΔH^o_c (kJ/mol)   =     Energy produced (from — 3)

                                                                                    Number of moles burned

You should be able to see why this is not the standard enthalpy of combustion: the value is not measured under standard conditions e.g. the pressure is not likely to be 1 atm.