Using the grid to solve for desired quanties Recap An exothermic change is a reaction that releases energy energy can be thought of as a product An endothermic change is one in which the energy must be added for the reaction to occur energy can be thought of as a reactant ID: 622056
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Slide1
Thermochemical Equations
Using the grid to solve for desired
quanties
Slide2
Recap
An
exothermic
change is a reaction that releases energy: energy can be thought of as a product
An
endothermic
change is one in which the energy must be added for the reaction to occur: energy can be thought of as a reactant
Enthalpy (H)
is the energy (heat) content of a system at constant pressure
.
You cannot measure the actual energy or enthalpy of a substance, but you can measure the change in enthalpy ∆
H
rxn
.
Slide3
Complete the table:
Type of Reaction
Sign of ∆
H
rxn
Which has more energy: reactants or products?Exothermic Endothermic
-
+
reactants
products
Exothermic
EndothermicSlide4
Chemistry problems involving
enthalpy
changes are similar to the dimensional analysis problems.
The amount of energy that is absorbed or released in a reaction depends on the number of moles of reactants involved.
Example Problem:
How much heat will be released if 1.0 gram of hydrogen peroxide H2O2 decomposes? 2H2O2 2H2O + O2 +190 kJ
1.0 gram H2O2
______ gram H
2
O2
34.02
1 mole H2O2
__mole gram H
2
O
2
2
________ kJ
190
energy @ end of
rxn
= exothermic
-
=
-
2.7924kJ
=
-
2.8 kJSlide5
1. How much heat is absorbed during photosynthesis when 9.22 grams of
glucose
C
6
H
12O6 is produced? 6CO2 + 6H2O +2803 kJ C6H12O6 + 6O2 9.22 grams C6H12O6_______ grams C6H12O6180.181 mole C6H12O6____ mole C6H12O
6
1
______ kJ
2803
energy @ beggining of
rxn = endothermic
+
=143.4324564 kJ
=143 kJ Slide6
2. How many moles of NO
2(g)
is dissolved in excess water when 177 kJ of energy is released?
3NO
2 + H2O 2HNO3 + NO + 138 kJ 177 kJ_______kJ138
energy @ end of rxn = exothermic
-
3 mole NO
2
=
-
3.84783 mole NO
2
=
-
3.85 mole NO
2