Enthalpy – Combustion of alcohols (Determination of the heat capacity of the apparatus)

In this experiment, the aim is to determine the heats of combustion of alcohols, specifically butan-1-ol. The experiment involves burning a measured mass of an alcohol in a spirit lamp and transferring the heat energy released to a calorimeter containing water. From the resulting temperature rise, the heat of combustion can be calculated. The apparatus used must be calibrated using an alcohol with a known heat of combustion due to factors such as the heat energy lost to the surrounding air and the heat capacity of the calorimeter. Safety precautions must also be taken due to the flammability and potential toxicity of the alcohols. The experiment provides an opportunity to calculate the specific heat capacity of the calorimeter and determine the enthalpy change of butan-1-ol. Calculations can be made to determine the percentage difference between experimental values and data book values and to explain why there may be a difference.

Aim

The purpose of this experiment is to determine the heats of combustion of the alcohol butan-1-ol.

Introduction

In this experiment, you burn a measured mass of an alcohol in a spirit lamp and transfer the heat energy released to a calorimeter containing water. From the resulting temperature rise you can calculate the heat of combustion.

In some earlier calorimetric experiments, you assumed that all the heat energy released in a chemical reaction was absorbed by the contents of the calorimeter. You cannot make that assumption in this experiment for two reasons.

• The heat energy is released in a flame, and although the apparatus is designed to transfer most of the energy to the calorimeter, a significant quantity is lost to the surrounding air.
• The heat capacity of the calorimeter itself is not so small as to be insignificant compared with the heat capacity of its contents. You can take account of both these factors by calibrating the apparatus using an alcohol with known heat of combustion.

Requirements

• safety glasses
• heat of combustion apparatus (copper can)
• spirit burner
• wood block
• retort stand (with 2 clamps and bosses)
• water (at room temperature)
• Propan-1-ol, C₃H₇OH
• Butan-1-ol, C₄H₉OH
• Bunsen burner and protective mat
• wood splints
• tweezers (not plastic-tipped)
• balance (capable of weighing to 0.01 g)
• thermometer -5 to -50°C (in 0.1°C)

Hazard warning

The alcohols you will be using are very flammable and can be poisonous by absorption through the skin and lungs. Therefore, you MUST:

• KEEP THE STOPPERS ON THE BOTTLES WHEN NOT IN USE
• KEEP THE BOTTLES AWAY FROM FLAMES
• WASH YOUR HANDS AFTER USE
• WEAR SAFETY GLASSES

Procedure

Determination of the heat capacity of the apparatus

1. Arrange the heat of combustion apparatus, as shown in the diagram below. Adjust the height of the calorimeter so that the top of the spirit lamp is level with the bottom of the calorimeter.
2. Use water at room temperature (not direct from the tap) to fill the calorimeter to about 1.5cm below the rim. Record this volume.
3. Stand the spirit lamp away from the calorimeter, and use a wood splint to light it. Adjust the height of the wick, using metal tweezers, to obtain a flame about 1 cm high.
4. Weigh the spirit lamp, including the cap, as accurately as possible and record the mass in a Results Table.
5. Stir the water in the calorimeter and record its temperature, to the nearest 0.1°C.
6. Put the lamp under the calorimeter and light it.
7. Slowly and continuously stir the water in the calorimeter and watch the thermometer. When the temperature has risen by about 10°C, extinguish the flame and immediately replace the cap. Continue stirring and record the maximum temperature of the water.
8. Re-weigh the spirit lamp and cap and record the mass.
9. Without removing the calorimeter from the stand, and holding both together carefully, pour away the water.
10. If you have time, repeat the experiment to increase the accuracy of your calibration. This second run should be much quicker because you should not need to make any adjustments (i.e. start at step 2).
11. Before doing any calculations, repeat the experiment using butan-1-ol and record the mass and temperature change in your results table.

Results Table – Calibration of calorimeter

	Trial 1	Trail 2
Molar mass of propan-1-ol, Mr			g mol- l
Initial mass of spirit lamp + alcohol, m1			g
Final mass of spirit lamp + alcohol, m2			g
Mass of alcohol burned m1-m2			g
Amount of alcohol burned,n = (ml-m2) /M			mol
Initial temperature of calorimeter			°C
Final temperature of calorimeter			°C
Temperature change			°C

ΔH_c [propan-I-ol] (given) = 2017 kJ mol^-1

	Trial 1	Trial 2
Heat released during the experiment, ΔHc [propan-I-ol] (given) x amount of moles burned = -2017 kJ mol-1 x n			kJ
Heat required for a rise in temperature of 1K = ΔH/ΔT = c (c=specific heat capacity)			kJ K-1
Average value of c for your apparatus			kJ K-1

Results Table – Enthalpy change of butan-1-ol

	Trial 1	Trial 2
Molar mass of alcohol			g mol- l
Initial mass of spirit lamp + alcohol, m1			g
Final mass of spirit lamp + alcohol, m2			g
Mass of alcohol burned m1-m2			g
Amount of alcohol burned,n = (ml-m2) /M			mol
Initial temperature of calorimeter			°C
Final temperature of calorimeter			°C
Temperature change			°C

Calculations

Energy transferred: Q=cΔT

• m=mass of water (g)
• c= Specific heat capacity of your calorimeter (J g^-1 K^-1)
• ΔT= Change in temperature (K)

Enthalpy change ΔH=Q/n

• n=number of moles of fuel burned.

Questions

1. Calculate the percentage difference between the values of ΔH_c for butan-1-ol, using both your experimental values and data book values.
2. Explain why there is a difference between the experimental value and data book value.