Enthalpy change of combustion of fuels
To calculate the Enthalpy change of combustion of a flammable liquid
When measuring the energy produced from a reaction, water is used to measure the amount of heat; this is because of its specific absorption of energy. In this lab, burning a fuel releases energy which will be absorbed by the water.
Calorimetry is the technique we use in the lab to measure the change in the temperature produced by energy released or absorbed in a chemical reaction. For this technique, a calorimeter is used (in this case, our calorimeter will be an empty soda can).
a) 1 gram of water needs 4.18 J (1 cal) of energy to increase the temperature in 1 °C, so knowing the mass of water and the change in the temperature, we can calculate the amount of energy change. we can represent this change with the following formula:
|Q = m x c x Δt
- Q represents the total energy absorbed or released BY THE WATER
- m is the mass of water in the calorimeter *
- Δt is the change in temperature we measured (always final – initial)
This experiment is very inefficient. This is because not all of the energy is absorbed by the water; some is absorbed by the air, while some is absorbed by the container holding the water. Also, the ethanol may not have entirely reacted and it has a % of water in the mixture as long as Methanol to denature it.
- Spirit burner containing ethanol with lid
- soda can hanging from a glass rod sustained by an iron ring
- 100 mL of water
- Balance with at least 0.01g accuracy (the more decimals you can measure, the better)
- be sure your soda can is empty
Measure exactly 100 mL of distilled water in it. (You can do it by measuring the mass of the empty can and then add water until you get EXACLY 100g more)
- Measure the temperature of the water for at least 3 minutes (so it is stable). RECORD THIS TEMPERATURE AS INITIAL TEMP.
- Measure the mass of the spirit burner containing the ethanol WITH THE LID
- Place the burner underneath the soda can (This should be around 1 inch (2.5 cm) below the can. if not, lots of heat will be lost.
- Take out the lid of the burner and turn it on IMMEDIATELY.
- take the temperature of the water while swirling it with the thermometer. BE SURE YOU SWIRL ALL THE TIME AND THAT THE THERMOMETER DOES NOT TOUCH THE BASE OF THE CAN.
- When the temperature of the water reaches AROUND 60 °C, place the lid on the burner to turn it off, and measure the mass of the burner with the lid IMMEDIATELY.
- Record the maximum temperature reached by the water as FINAL TEMP.
To work out the enthalpy change:
1) Calculate the heat given out by the fuel, this can be done by calculating the amount of energy absorbed by the water,
qwater = m(H2O) c (H2O)Δt (H2O)
qwater = -qalcohol
2) Calculate the number of moles of fuel that has been burnt. This can be done by working out the difference between the mass of fuel before and after the burning.
n =Mass of alcohol burnt
Mr of alcohol
3) Calculate the energy given out by 1 mole of the substance through proportions
ΔHc[alcohol(l)] = Q measured (alcohol)
Number of moles burnt