HEAT OF COMBUSTION
The purpose of this experiment is to measure the amount of energy released when alcohol burns.
As the alcohol burns, it releases energy as heat, which you will use to warm some water. You will
calculate the amount of heat captured in the water using the equation q = mc∆T, where q is heat, m is the
mass of water, c is a constant, and ∆T is the change in the temperature of the water. The unit of energy is
joule (J). A joule is a very small amount of energy, so we usually express the energy of a chemical
reaction in kilojoules (kJ). One kJ equals 1000 J.
The energy produced by the burning alcohol is part of the chemical reaction, so we can include it in the
chemical equation. Reactions that release energy are called exothermic processes; the energy is added to
the right side of the balanced equation, as a product. Reactions that absorb energy are called endothermic
processes; the energy is added to the left side of the balanced equation, as a reactant.
Procedure
1. Weigh a 50 mL flask and add about 40 mL of water. Weigh the flask + water and subtract to
determine the mass of water. Weigh a microburner filled with ethanol.
2. Clamp the flask above a microburner, so that the flask bottom is about one inch above the burner. .
Record the temperature of the water, then light the microburner , and slide the burner under the flask.
3. Stir the water gently with a thermometer and allow the ethanol to burn until the temperature of the
water has risen by about 10 to 12 °C. Keep stirring the water gently as you slide the burner out from
under the beaker, and extinguish the microburner. Record the highest temperature the water reaches.
4. Weigh the microburner and subtract to determine the mass of ethanol burned.
Data
Mass of flask empty
Mass of flask and water
Mass of water
Mass of burner before
Mass of burner after
Mass of ethanol burned
Temperature of water before heating
Temperature of water after heating
Temperature change
Analysis
1. Calculate q, the amount of energy gained by the water, using the equation
q  mwater c T 
mwater is the mass of the water (after subtracting the beaker), c is 4.184 J/g °C, and ∆Twater is the final
temperature of the water minus the initial temperature of the water. Divide your answer by 1000 to
convert the value of q from J to kJ.
2. The formula for ethanol is C2H5OH. Convert the grams of ethanol burned into moles, and calculate the
energy released in kJ per mole ethanol –– that is, the amount of energy in kJ divided by the number of
moles that burned.
3. Calculate the grams of CO2 produced when the ethanol burns. (This is a three step question.) Balance
the equation first.
C2H5OH +
O2

CO2
+
H2O
Discussion
1. Write the balanced thermochemical equation for the combustion of ethanol and indicate whether the
process is exothermic or endothermic.
2. The accepted value for the amount of energy released by this reaction is 1400 kJ/mol C2H5OH.
Calculate the percent error in your result.
3. This lab assume that all the mass lost was from the burning of the ethanol, are there any other places
where mass could have been lost and how would that have affected the results?