Module 3
Lesson 4 – Calorimetry and enthalpy
change of combustion
Objectives
Must
Follow a set of instructions to set up practical
equipment safely and quickly
Should
Use the equipment provided to investigate the
heat given out when a substance burns
Could
Calulate a value for ΔHc.
Equation for ΔHc
Balance the following equations for burning methanol and ethanol in
air and add the correct state symbols for the substances in their
standard states:
__CH3OH + __O2  __CO2 + __H2O
__C2H5OH + __O2  __CO2 + __H2O
In an equation to represent ΔHc you must not put a balancing number
in front of the substance being burnt. If you do, then more than 1 mol
would have been combusted.
AfL – what are standard conditions?
The correct equations
CH3OH(l) + 1½O2(g)  CO2(g) + 2H2O(l)
C2H5OH(l) + 3O2(g)  2CO2(g) + 3H2O(l)
Calorimetry
How does calorimetry work?
• A known volume of cold water is measured
into the beaker/can
• The starting temperature of the water is
recorded
• The water is heated using the flame from the
burning fuel
• The final temperature of the water is recorded
• The spirit burner containing the fuel is
weighed before and after the experiment.
Practical
• Follow the instructions on Activity sheet 26 to
compare the enthalpy of combustion of
methanol and ethanol.
Calculations
The amount of energy transferred from the
burning fuel to the water (the surroundings) in
the calorimeter can be calculated if you know:
• the
• the
Calculations
ΔE = mcΔT
Use this equation to calculate the energy
transferred for each of your experiments.
Energy per gram of fuel
• You can compare fuels by measuring the mass
of fuel burned in the experiment. The best
fuel is likely to release the most energy per
gram of fuel. This is worked out using:
Errors
The figure you have calculated is lower than the
figure found using bond energies (remember
breaking and making bonds).
Why do you think this could be?
Errors
Why do experimental values for ΔHc deviate
from their theoretical counterparts.
• Energy lost to the surroundings
• Incomplete combustion
• Conditions which are not standard (ie not
298K and 1atm pressure)
Examination questions (AS)
Mark scheme
Examination question
Mark scheme
Homework
• Complete the practical write up and
examination paper questions.
• Short test next lesson on enthalpy changes –
learn your definitions. YOU MUST DO THIS OR
YOU WILL NOT UNDERSTAND THE NEXT BIT!