Friday, Oct. 25th: “A” Day
Monday, Oct. 28th: “B” Day
Agenda
 Homework questions/Quick review
 Section 10.2 Quiz: “Using Enthalpy”
 Section 10.3: “Changes in Enthalpy During
Chemical Reactions”
 Calorimetry, calorimeter, adiabatic calorimetry, Hess’s
Law, standard enthalpy of formation
 Homework
Pg. 15 practice worksheet (MUST show work)
Sec. 10.3 review, pg. 357: #1-5
Concept Review: “Changes in Enthalpy During
Chemical Reactions”
Homework Questions/Problems
Pg. 349: #1-8
Section 10.2 Quiz:
“Using Enthalpy”
You can use both your book and your notes.
You’ll need both 10.1 AND 10.2 notes.
May the FORCE be
with you!
#4: M = molar mass
#8: Use the “25 J rule”
Changes in Enthalpy Accompany Reactions
Changes in enthalpy occur during chemical
reactions.
A change in enthalpy during a reaction
depends on many variables, but temperature
is one of the most important variables.
To standardize enthalpies of reaction, data are
presented for reactions in which both
reactants and products have the standard
thermodynamic temperature of 25˚C or
298.15 K.
Chemical Calorimetry
Calorimetry: the measurement of heatrelated constants, such as specific heat or
latent heat.
Calorimeter: a device used to measure the
heat absorbed or released in a chemical or
physical change.
Nutritionists Use Bomb Calorimetry
A bomb calorimeter is used to measure enthalpy
changes caused by combustion reactions.
Adiabatic Calorimetry is Another
Strategy
Instead of using a water bath to absorb the
energy generated in a combustion reaction,
adiabatic calorimetry uses an insulating vessel
that doesn’t allow energy to pass through.
As a result, the temperature of the reaction
mixture will change and can be recorded.
Adiabatic calorimetry is used for reactions
that are not ignited, such as for reactions in
aqueous solution.
Hess’s Law
Hess’s Law: the law that states that the
amount of heat released or absorbed in a
chemical reaction does not depend on the
number of steps in the reaction.
The overall enthalpy change in a reaction is
equal to the sum of the enthalpy changes for
the individual steps in the process.
Standard Enthalpies of Formation
Standard enthalpy of formation: the enthalpy
change in forming 1 mol of a substance from
elements in their standard state.
 By definition, the values of the standard
enthalpies of formation for elements are zero.
Symbol: ΔH˚f
Unit: kJ/mol
Calculating Enthalpy Change for a
Chemical Reaction
Using a list of standard enthalpies of formation,
the enthalpy change of any reaction for which
there is data available can be calculated:
ΔHreaction =
°
ΔHf products
-
°
ΔHf reactants
ΔHreaction is in kJ or Joules (moles cancel out)
Table 2: Standard Enthalpies of Formation
Example
Calculate the enthalpy change for the following
reaction. Use the standard enthalpies of formation
listed in Table A-11 on pg 833-834.
HCl(g) + NH3(g)
NH4Cl(s)
ΔHreaction = ΔHf0products - ΔHf0reactants
ΔHf0product = (1 mol)(-314.4 kJ/mol) = -314/4 kJ
ΔHf0reactants=[(1 mol)(-92.3 kJ/mol)+(1 mol)(-45.9 kJ/mol)]
= -138.2 kJ
ΔHreaction = (-314.4 kJ) – (-138.2 kJ)
-176.2 kJ
(exothermic reaction)
Additional Practice
Calculate the enthalpy change for the following reaction.
Use the standard enthalpies of formation listed in Table
A-11 on pg 833-834.
N2(g) + 3 H2(g)
2 NH3(g)
State whether the reaction is exothermic or endothermic.
ΔHreaction = ΔH f0products - ΔH f0reactants
ΔHf0prod = [(2 mol)(-45.9 kJ/mol) = -91.8 kJ
ΔHf0reactants = [(1 mol)(0 kJ/mol) + (3 mol)(0 kJ/mol)]
= 0 kJ
ΔHreaction = (-91.8 kJ) – (0 kJ) = -91.8 kJ
*Reaction is exothermic because ΔH is negative.*
Calculating a Reaction’s Change in Enthalpy
Sample Prob. E, pg.356
 Calculate the change in enthalpy for the reaction below
using data from Table 2 on pg 355.
2 H2(g) + 2 CO2(g)
2 H2O(g) + 2 CO(g)
State whether the reaction is exothermic or endothermic.
ΔHreaction = ΔH f0products - ΔH f0reactants
ΔHf0prod = [(2 mol)(-241.8 kJ/mol) + (2 mol)(-110.5 kJ/mol)]
= -704.6 kJ
ΔHf0reactants = [(2 mol)(0 kJ/mol) + (2 mol)(-393.5 kJ/mol)]
= -787 kJ
ΔHreaction= (-704.6 kJ) – (-787 kJ) = 82.4 kJ
*Reaction is endothermic because ΔH is positive.*
Homework
Pg. 15 practice workshet
MUST show work!
Section 10.3 review, pg. 357: #1-5
Concept Review: “Changes in Enthalpy During
Chemical Reactions”