Comparing the enthalpy of isopropanol and ethanol

References:

R1.1.4 The standard enthalpy change for a chemical reaction, ΔH⦵, refers to the heat transferred at constant pressure under standard conditions and states. It can be determined from the change in temperature of a pure substance.

R1.2.3 Standard enthalpy changes of combustion, ΔH_c⦵, data can be used in thermodynamic calculations

T3 Mathematical skills

Aim

To calculate and compare the enthalpy of combustion between two alcohols using a spirit burner.

Introduction

When a fuel is burned in a spirit burner, the energy released as heat is transferred to water placed in a copper cup directly above the spirit burner. Assuming that all the energy is transferred as heat to the water, and using the heat capacity of water c = 4.18 J g⁻¹ K⁻¹, the amount of energy released by the spent fuel can be calculated. This can then be converted to the amount of energy given out per mole of fuel in order to find, the molar enthalpy change of combustion of each fuel.

Pre-lab questions

1. The water is placed in a metal can. Why is a metal can used and not a glass beaker or other similar container?

2. Ethanol and isopropanol were chosen because they are members of the same family of compounds that contain the same functional group. Name the compound family, the functional group, and another member of this family.

3. One of the biggest causes of error in this experiment is heat loss. Does this lead to systematic or random errors? Suggest where these heat losses occur and how to improve the experiment to minimize these errors.

Please note

· A full risk assessment should be carried out prior to commencing this experiment.

· Personal safety equipment should be worn.

· Chemicals should be disposed of safely and with due regard to any environmental considerations.

Risk assessment

Environmental risks

Ethical risks

Equipment list

Method

1. Set up your apparatus as shown in the diagram.

2. Put 75 cm³ of cold water into the copper can. Record the temperature of the water.

3. Support the copper can over a spirit burner containing the fuel that you are going to burn. It should be arranged so that when the wick is lit, the flame just touches the bottom of the can.

4. Weigh the spirit burner and fuel.

5. Record the initial temperature (T₁) on the thermometer or start data collection if you are using a temperature probe with a data-logger. It may take several seconds for the temperature probe to reach the temperature of the water in the can.

Do not leave the thermometer/temperature probe resting on the base of the can or it will not register the correct temperature of the water.

6. Replace the burner under the can and light the wick. Use a glass rod to gently stir the water all the time that the fuel is being burned.

7. Continue heating, and stirring with the glass rod, until the temperature of the water has risen by about 20°C.

8. Extinguish the flame and weigh the burner.

9. Continue stirring the water until there is no further rise in temperature. Note the final reading (T₂) on the thermometer or terminate the trial by stopping data collection on the data-logger.

10. Repeat using the other fuel. Do not forget to use fresh cold water in the copper can.

11. If time allows, repeat the experiment for each alcohol three times and average the results.

Analysis

Use the readings obtained to work out:

· the mass of water used

· the mass of fuel used

· the temperature change of the water in each case.

Carry out the following calculations for each fuel

Assume that all the energy released from the burning fuel is transferred to the water:

energy transferred to water = (mass of water × temperature change × 4.18) J = q

Calculate the energy released per mole of fuel by completing the following:

(Show uncertainties and propagation of uncertainties)

· formula of fuel = __________________________

· mass of 1 mole of fuel =  __________________________ g

· mass of fuel burned =  __________________________ g

· number of moles of fuel burned =  _________________________

· energy transferred by this number of moles of fuel = −q = ____________________ J

(q is the quantity calculated in step 1)

· energy transferred by 1 mole of fuel =  __________________________ J

· therefore enthalpy change of combustion =  _________________________ kJ mol^–1

Reflection

· Look up the accepted values for the enthalpies of combustion of your fuels using Section 14 in the data booklet and calculate the percentage difference between the values you obtained and the quoted values. For which of your fuels was your experimental value most accurate? Why do you think this was?

· Suggest ways in which the accuracy of your experiment could be improved.

· The next member of the homologous series of alcohols is propanol. Predict the expected enthalpy of combustion for propanol.

· Explain why you would expect a (roughly) similar increase in enthalpy of combustion between members of a homologous series