Practical 6 - Determination of an enthalpy change of combustion

Question 1

What are the two solutions we determined the enthalpy changes of combustion for?

Answer 1

Methanol and ethanol

Question 2

What are the hazards involved with methanol?

Answer 2

Highly flammable
Toxic

Question 3

What are the hazards involved with ethanol?

Answer 3

Highly flammable

Question 4

What type of spirits are used industrially?

Answer 4

Methylated spirits

Question 5

What’s methylated spirit and why does it exist?

Answer 5

Spirits with methanol added to discourage human consumption, as the methanol makes it poisonous

Question 6

What’s the method for this practical?

Answer 6

1. Add 100cm^3 of deionised water to a 250cm^3 conical flask
2. Clamp the conical flask to a stand at a suitable height so that the spirit burner can be placed below it
3. Weigh a spirit burner and lid containing methanol using a mass balance and record the mass
4. Record the initial temperature of the water using the thermometer
5. Place a spirit burner under the conical flask on a heatproof mat and light the wick
6. Allow the flame to heat the water by about 40 degrees Celsius
7. Replace the cap to distinguish the flame
8. Record the final water temperature
9. Re-weigh the spirit burner and cap and record in your table
10. Calculate the mass of alcohol used
11. Repeat steps 1-10 for ethanol
12. Determine the energy released by each alcohol + calculate the enthalpy change of combustion

Question 7

What type of water is added and where?

Answer 7

Deionised water to a conical flask

Question 8

How do we distinguish the flame of the wick?

Answer 8

Replace the cap

Question 9

Which was the most exothermic - methanol or ethanol and why?

Answer 9

Ethanol, as more bonds need to be broken (comparing their chemical formulae) and so more bonds are formed in the products so more energy is released

Question 10

What was wrong with our final value in this practical?

Answer 10

It was massively underestimated

Question 11

Why was our final value massively underestimated and what could we do to stop this?

Answer 11

Significant amounts of heat loss to surroundings - need to better insulate the apparatus (e.g - draft shield)
Glass is an insulator, making it a poor conductor of heat - use a metal container

Question 12

How could we better insulate our apparatus?

Answer 12

Using a draft shield

Question 13

Which material is a better conductor of heat compared to glass to use?

Answer 13

Metal

Question 14

What did the significant amount of heat loss mean for our final value?

Answer 14

It was massively underestimated