Section 10.3
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Transcript Section 10.3
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”