chapter 9 lecture basic chemistry fourth edition 9.5 energy in chemical reactions learning goal...
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Chapter 9 Lecture
Basic ChemistryFourth Edition
9.5 Energy in Chemical Reactions
Learning Goal Given the heat of reaction (enthalpy change), calculate the loss or gain of heat for an exothermic or endothermic reaction.
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Chapter 9 Chemical Quantities in Reactions
Most reactions involve the loss or gain of energy.
• To determine the change in energy for a reaction, we examine the energy of the reactants and products.
• The SI unit used for representing energy is the joule (J) or kilojoules (kJ).
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Energy Units, Chemical Reactions
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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
Heat of Reaction, Enthalpy Change
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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
H2(g) + Cl2(g) 2HCl(g) + 185 kJ
ΔH = –185 kJ/mol (heat released)
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The energy of the products is less than the energy of the reactants.
Exothermic Reactions
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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)
Endothermic Reactions
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The energy of the products is greater than the energy of the reactants.
Endothermic Reactions
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Exothermic, Endothermic Reactions
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Identify each reaction as exothermic or 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
Learning Check
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Identify each reaction as exothermic or endothermic.
A.N2(g) + 3H2(g) 2NH3(g) + 92 kJ
Exothermic
A.CaCO3(s) + 556 kJ CaO(s) + CO2(g)
Endothermic
C. 2SO2(g) + O2(g) 2SO3(g) + heat
Exothermic
Solution
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Calculations of Heat in Reactions
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In the reactionN2(g) + O2(g) 2NO(g) ΔH = +181 kJhow much heat (kJ) is absorbed when 1.65 grams of nitrogen monoxide gas is produced?Step 1 State the given and needed quantities.
Given: 1.65 grams of NO, ΔH = +181 kJ
Need: heat absorbed in kJ
Calculations of Heat in Reactions
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N2(g) + O2(g) 2NO(g) ΔH = +181 kJ 1.65 g ? kJ
Step 2 Write a plan using the heat of reaction and any molar mass needed.
Learning Check
grams of NO
Molar MassMolar Mass
moles of NO
kilojoules ofenergy
Heat of ReactionHeat of
Reaction
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N2(g) + O2(g) 2NO(g) ΔH = +181 kJ 1.65 g ? kJ
Step 3 Write the conversion factors, including heat of reaction. 1 mole of NO = 30.01 g of NO
Calculations of Heat in Reactions
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N2(g) + O2(g) 2NO(g) ΔH = +181 kJ 1.65 g ? kJ
Step 3 Write the conversion factors, including heat of reaction.
2 moles of NO = +181 kJ
Calculations of Heat in Reactions
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N2(g) + O2(g) 2NO(g) ΔH = +181 kJ 1.65 g ? kJ
Step 4 Set up the problem to calculate the heat.
Calculations of Heat in Reactions
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How many kilojoules of energy are absorbed when 23.0 g of solid ammonium nitrate is dissolved in water? NH4NO3(s) + 26 kJ NH4NO3(aq)
Learning Check
H2O
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Solution
NH4NO3(s) + 26 kJ NH4NO3(aq) 23.0 g ? kJ
Step 1 State the given and needed quantities. Given: 23.0 g of NH4NO3
Need: heat absorbed
H2O
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NH4NO3(s) + 26 kJ NH4NO3(aq) 23.0 g ? kJ
Step 2 Write a plan using the heat of reaction and any molar mass needed.
Solution
grams of NH4NO3
Molar MassMolar Mass
moles of NH4NO3
kilojoulesHeat of ReactionHeat of
Reaction
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NH4NO3(s) + 26 kJ NH4NO3(aq) 23.0 g ? kJ
Step 3 Write the conversion factors,including heat of reaction.
1 mole of NH4NO3 = 80.06 g of NH4NO3
Solution
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NH4NO3(s) + 26 kJ NH4NO3(aq) 23.0 g ? kJ
Step 4 Set up the problem to calculate the heat.
Solution
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Concept Check