chapter 9 chemical quantities in reactions
DESCRIPTION
Cold packs use an endothermic reaction. Chapter 9 Chemical Quantities in Reactions. 9.5 Energy in Chemical Reactions. Heat of Reaction. The heat of reaction , is the amount of heat absorbed or released during a reaction at constant pressure - PowerPoint PPT PresentationTRANSCRIPT
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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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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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Summary
Reaction Energy Change Heat Sign of
ΔH
Endothermic Heat absorbed Reactant +
Exothermic Heat released Product ─
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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 kJ
3) 45 kJ
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Solution
STEP 1 List given and needed data for the equation.
Given: 15.0 g of NO produced
ΔH = 180 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 moles of NO kJ
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Solution (continued)STEP 3 Write the conversion factors
including heat of reaction.
2 mol of NO = 180 kJ
180 kJ and 2 mol NO
2 mol NO 180 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 180 kJ = 45 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 = 30.01 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 x 32.00 g O2 = 92.0 g of O2 (1)
890 kJ 1 mol O2