Chapter 17: Measuring and Expressing
Enthalpy Changes
Calorimetry
 Calorimetry measures heat flow into or out of a
system for chemical or physical processes
 The heat released by a system is equal to the heat
absorbed by the surroundings
 The heat absorbed by the system is equal to the heat
released by the surroundings
 The device itself is called a calorimeter
Constant-Pressure Calorimeters
 The heat content of a system at constant pressure is
the same as a property called enthalpy (H)
 The amount of heat released or absorbed from a
chemical reaction at constant pressure is referred to as
a change in enthalpy (ΔH) of the system
Heat and enthalpy change are the same thing
q = ΔH
Same formula from before…
• When you measure the temperature of the water
in the calorimeter before and after the chemical
reaction, enthalpy can be calculated as follows:
qsurr = m x C x ΔT
 This translates to: 'heat absorbed by the
surroundings'
 Since the surroundings is water, the known values
of C is used (4.18 J/g·°C) in calculations involving
the heat transfer
The opposite for the system…
 Conversely, if you are measuring the heat lost or
gained by the system, the enthalpy change is written as
follows:
qsys = ΔH = -qsurr = -m x C x ΔT
The sign for an exothermic reaction (heat
released) is negative and positive for an
endothermic reaction (heat absorbed)
Bomb Calorimeters
 A sample is burned in a constant-pressure chamber in
the presence of oxygen at high pressure
 The heat released by the reaction warms the water
 By measuring the increase in temperature, it is
possible to calculate the amount of heat released
during the combustion
Example
 Remember to write down all that is known in the
example.
 Add up any volumes (or masses). Look for
conversions.
 Rely on known constants.
Complete questions 12 – 13, pg. 513
Thermochemical Equations
 IN A CHEMICAL EQUATION, THE ENTHALPY
CHANGE FOR THE REACTION CAN BE WRITTEN
AS EITHER A REACTANT OR A PRODUCT –
depending on whether it is an exothermic or
endothermic reaction
Ex. CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2kJ
 A chemical equation that includes the enthalpy
change is called a thermochemical equation
Heat of Reaction
 The enthalpy change for the chemical equation exactly
as it is written
Exothermic:
CaO(s) + H2O(l) → Ca(OH)2(s)
ΔH = -65.2kJ
Endothermic:
2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g)
ΔH = 129kJ
Refer to page 17.7, p. 515
This relates to stoichiometry
(remember stoichiometry?)
 The amount of moles factors into the amount of
enthalpy released or absorbed
 The states of matter are important as well
 This is because different physical states of the same
substance can have different ΔH values
 Ex. Water is 4.184J/g∙˚C yet ice is 2.01J/g∙˚C
Example of Heat of Reaction – page 516
 Read through the question to identify what is known
 This is a simple conversion question
 You know how much heat is absorbed when 2 moles of
sodium bicarbonate is heated. However, the amount
you want to investigate is 2.24 mol
Questions 14 – 15, p. 516
Heat of Combustion
 Refers to the amount of heat released when one mole
of substance is burned.
Ex. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
ΔH=-890kJ
Unless otherwise indicated, standard conditions are being
used in these equations. This means the temperature is
25°C and pressure is 101.3kPa.
Review
1) Select a substance from 17.2. Write the
thermochemical equation.
2) Why is the ΔH negative?
3. Draw an enthalpy diagram for this reaction.
4. Is heat flowing into or out of the system?
5. What must the total energy (potential and kinetic)
equal in any chemical or physical process?
For the remainder of class:
 Work on guided reading – section 17.2
 Questions 16 – 20 need to be completed to check for
understanding
 Section review 17.2
 Practice Problems ?