Transcript 9.5_Energy_in_Chem_Reaction

Chapter 9
Chemical Quantities in Reactions
9.5
Energy in Chemical Reactions
Cold packs use an endothermic
reaction.
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Heat of Reaction
The heat of reaction,
• is the amount of heat absorbed or
released during a reaction at constant
pressure
• is the difference in the energy of the
reactants and the products
• is shown as the symbol ΔH
ΔH = Hproducts − Hreactants
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Endothermic Reactions
In an endothermic reaction,
• heat is absorbed
• the sign of ΔH is +
• the energy of the products is
greater than the energy of the
reactants
• heat is a reactant
N2(g) + O2 (g) + 181 kJ
2NO(g)
ΔH = +181 kJ (heat added)
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Endothermic Reactions
http://www.youtube.com/watch?v=ubvo5AKzltY
 Mix equal quantities of NaHCO3 (sodium
hydrogen carbonate) with some citric acid
(C6H8O7). Add water and the reaction will start.
The reaction is endothermic, so the mixture and
container cools down. Na3C6H5O7 (sodium
citrate), carbon dioxide and water are the
products, which have a total entropy greater
than that of the reactants and surroundings.
This allows
this reaction to be spontaneous,
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even though it is endothermic.

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Endothermic Reactions
Ba(OH)2(s)+2NH4SCN(aq)
ΔH = +
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Ba(SCN)2(s)+2NH3(g)+H2O(l)
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Exothermic Reactions
In an exothermic reaction,
• heat is released
• the sign of ΔH is • the energy of the products is
less than the energy of the
reactants
• heat is a product
C(s) + O2(g)
CO2(g) + 394 kJ
ΔH = –394 kJ/mol (heat released)
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Exothermic Reaction

2Al (s) + 3CuSO4 (aq)
Al2(SO4)3 (aq) +3Cu (s)
ΔH = ─

http://www.youtube.com/watch?v=AgBvRMFX
His&feature=related
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Summary
Reaction
Energy Change
Heat
Sign of
ΔH
Endothermic Heat absorbed
Reactant
+
Exothermic
Product
─
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Heat released
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Learning Check
Identify each reaction as
(Ex) exothermic or (En) endothermic.
A. N2(g)+ 3H2(g)
2NH3(g) + 92 kJ
B. CaCO3(s) + 556 kJ
CaO(s) + CO2(g)
C. 2SO2(g) + O2(g)
2SO3(g) + heat
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Solution
Identify each reaction as
(Ex) exothermic or (En) endothermic.
(Ex) A. N2(g)+ 3H2(g)
2NH3(g) + 92 kJ
(En) B. CaCO3(s) + 556 kJ
CaO(s) + CO2(g)
(Ex) C. 2SO2(g) + O2(g)
2SO3(g) + heat
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Calculations Using Heat of
Reaction
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Heat Calculations for Reactions
In the reaction
N2(g) + O2 (g)
2NO(g)
ΔH = +181 kJ
181 kJ is absorbed when 1 mol of N2 and 1 mol of O2
react to produce 2 mol of NO.
N2(g) + O2 (g) + 181 kJ
2NO(g)
This can be written as conversion factors.
181 kJ
181 kJ
181 kJ
1 mol N2
1 mol O2
2 mol NO
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Heat Calculations for Reactions
(continued)
N2(g) + O2 (g) + 181kJ
2NO(g)
If 15.0 g of NO is produced, how many kJ was
absorbed?
1) 1400 kJ
2) 90.4 kJ
3) 45.2 kJ
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Solution
STEP 1 List given and needed data for the
equation.
Given: 15.0 g of NO produced
ΔH = 181 kJ/2 mol of NO
Need: kJ absorbed
STEP 2 Write a plan using heat of reaction
and any molar mass needed.
Plan: g of NO
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moles of NO
kJ
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Solution (continued)
STEP 3 Write the conversion factors including
heat of reaction.
2 mol of NO = 181 kJ
181 kJ
and 2 mol NO
2 mol NO
181 kJ
1 mol of NO = 30.01 g of NO
1 mol NO and
30.01 g NO
30.01 g NO
1 mol NO
STEP 4 Set up the problem.
15.0 g NO x 1 mol NO x 181 kJ
= 45.2 kJ (3)
30.01 g NO 2 mol NO
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Learning Check
How many grams of O2 react if 1280 kJ is released
in the following reaction?
CH4(g) + 2O2(g)
CO2(g) + 2H2O(l) ΔH = -890 kJ
1) 92.0 g of O2
2) 46.0 g of O2
3) 2.87 g of O2
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Solution
STEP 1 List given and needed data for the equation.
Given 1280 kJ
Need grams of O2
STEP 2 Write a plan using heat of reaction and any
molar mass needed.
kilojoules
moles of O2
grams of O2
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Solution (continued)
STEP 3 Write the conversion factors including heat
of reaction.
2 mol of O2 = 890 kJ
890 kJ
and 2 mol O2
2 mol O2
890 kJ
1 mol of O2 = 32.00 g of O2
1 mol O2 and
32.00 g O2
32.00 g O2
1 mol O2
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Solution (continued)
STEP 4 Set up the problem.
1280 kJ x 2 mol O2
890 kJ
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x 32.00 g O2
1 mol O2
= 92.0 g of O2 (1)
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