Learning Goal Given the heat of reaction the rate of a reaction calculate the loss or gain of heat for an exothermic or endothermic reaction Almost every chemical reaction involves the loss or gain of energy When cold packs are activated an endothermic reaction ID: 596475
Download Presentation The PPT/PDF document "7.9 Energy in Chemical Reactions" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
7.9 Energy in Chemical Reactions
Learning Goal
Given the heat of reaction the rate of a reaction, calculate the loss or gain of heat for an exothermic or endothermic reaction.
Almost every chemical reaction involves the loss or gain of energy. When cold packs are activated, an endothermic reactiontakes place.Slide2
Energy Units for Chemical Reactions
The SI unit for energy is the
joule
(J). Often the unit of kilojoules (kJ) is used to illustrate the amount of energy lost or gained in a chemical reaction. 1 kilojoule (kJ) = 1000 joules (J) Slide3
Heat of Reaction
The heat of reaction is the amount of heat absorbed or released during a reaction that takes place at constant pressure. The change in energy occurs when
reactants interact.
bonds break apart. products are formed.The heat of reaction or enthalpy change, symbol ΔH, is the difference between the enthalpy of the products and enthalpy of the reactants. ΔH = H
products
−
H
reactants
Slide4
Exothermic Reactions Released
In
an
exothermic reaction,heat is released.the energy of the products is less than the energy of the reactants.heat is a product.
H
2
(
g
)
+
Cl
2
(
g
)
2HCl(
g
) +
185 kJ
Δ
H
= −185 kJSlide5
Exothermic Reactions Released
In an
endothermic
reaction, heat is absorbed.the energy of the products is greater than the energy of the reactants.heat is a reactant (added).N2(
g
)
+
O
2
(
g
)
+
180 kJ
2NO(
g
)
Δ
H
= +180 kJSlide6
Calculating Heat Changes
The value of Δ
H
refers to the heat change for each substance in the balanced equation.For the decomposition reaction,2H2O(l) 2H2(
g
) + O
2
(
g
)
Δ
H =
+572 kJ
2H
2
O(
l
) + 572 kJ
2H
2
(
g
) + O
2(g)We can write the following conversion factors:Core Chemistry Skill Using the Heat of ReactionSlide7
Guide to Calculations Using Heat
of ReactionSlide8
Study Check
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N
2(g) + 3H2(g) 2NH3(g) ΔH = −92.2 kJSlide9
Solution
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N
2(g) + 3H2(g) 2NH3(g) ΔH = −92.2 kJ
STEP 1
State the given and needed quantities.
ANALYZE Given Need
THE PROBLEM
50.0 g of NH
3
heat released (kJ)
Δ
H
= −92.2 kJ
N
2
(
g
) + 3H
2
(
g) 2NH3(g) Slide10
Solution
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N
2(g) + 3H2(g) 2NH3(g) ΔH = −92.2 kJ
STEP 2
Write a plan using the heat of reaction and any molar mass needed.
grams moles
kilojoules
of NH
3
of NH
3
Molar mass
Heat of reactionSlide11
Solution
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N
2(g) + 3H2(g) 2NH3(g) ΔH = −92.2 kJ
STEP 3
Write the conversion factors including heat of reaction.
Slide12
Solution
How much heat in kJ is released when nitrogen and hydrogen react to form 50.0 g of ammonia?
N2(g) + 3H2(g) 2NH3(g) ΔH
= −92.2 kJ
STEP 4
Set up the problem to calculate the heat.
×
×Slide13
Chemistry Link to Health: Cold
Packs and Hot Packs
Inside the cold pack, solid ammonium nitrate (NH4NO3) is separated from the compartment containing water. When the cold pack is hit or squeezed, the compartments break and the NH4NO3 mixes with the water in an endothermic reaction.
NH4NO3(s) + 26 kJ NH4NO3(aq)
H
2
OSlide14
Chemistry Link to Health: Cold
Packs and Hot Packs
Inside the hot pack, solid calcium chloride (CaCl2) is separated from the compartment containing water. When the cold pack is hit or squeezed, the compartments break and the CaCl2 mixes with the water in an exothermic reaction.CaCl
2(s) CaCl2 (aq) + 82 kJ
H
2
OSlide15
Concept Map