1. Section 4: Calculating Enthalpy Change
The enthalpy change for a reaction can be calculated using Hess’s law. K
What I Know W
What I Want to Find Out L
What I Learned

2. 11(C) Use thermochemical equations to calculate energy changes that occur in chemical reactions and classify reactions as exothermic or endothermic.
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Calculating Enthalpy Change

3. Essential Questions
How is Hess’s law applied to calculate the enthalpy change for a reaction?
What is the basis for the table of standard enthalpies of formation?
What is the enthalpy change for a reaction using standard enthalpies of formation data?
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Calculating Enthalpy Change

4. Vocabulary Review New allotrope Hess’s law
standard enthalpy (heat) of formation
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Calculating Enthalpy Change

5. Hess's Law
Hess’s law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction.
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Calculating Enthalpy Change

6. H2O2 (aq) → H2 (g) + O2 (g) ΔH = 188 kJ
HESS’S LAW
KNOWN
a. 2H2(g) + O2(g) → 2 H2O(l) ΔH = –572 kJ
b. H2(g) + O2(g) → H2O2(l) ΔH = –188 kJ
UNKNOWN
ΔH = ? kJ
Use with Example Problem 5.
Problem
Use thermochemical Equations a and b below to determine ΔH for the decomposition of hydrogen peroxide (H2O2), a compound that has many uses ranging from bleaching hair to powering rocket engines.
2H2O2(l) → 2H2O(l) + O2(g)
2H2(g) + O2(g) → 2 H2O(l) ΔH = –572 kJ
b. H2(g) + O2(g) → H2O2(l) ΔH = –188 kJ
SOLVE FOR THE UNKNOWN
H2O2 is a reactant.
Reverse Equation b and change the sign of ΔH.
H2O2 (aq) → H2 (g) + O2 (g) ΔH = 188 kJ
Two moles of H2O2 are needed.
Multiply the reversed Equation b by two to obtain Equation c.
c. 2 H2O2 (aq) → 2 H2 (g) + 2 O2 (g)
Response
ANALYZE THE PROBLEM
You have been given two chemical equations and their enthalpy changes. These two equations contain all the substances found in the desired equation.
Calculating Enthalpy Change
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7. HESS’S LAW EVALUATE THE ANSWER
The two equations produce the desired equation. All values are accurate to the ones place, so ΔH is correctly stated.
SOLVE FOR THE UNKNOWN (continued)
Multipy 188 kJ by two to obtain ΔH for Equation c.
ΔH for Equation c = (188 kJ)(2) = 376 kJ
Write Equation c and ΔH.
c. 2H2O2(aq) → 2H2(g) + 2O2(g), ΔH = 376 kJ
Add Equations a and c, canceling any terms common to both sides of the combined equation. Add the enthalpies of Equations a and c.
Write Equation a.
a. 2H2(g) + O2(g) → 2H2O(l), ΔH = -572 kJ
Write Equation c.
c. 2H2O2(l) → 2H2(g) + 2O2(g), ΔH = 376 kJ
Add Equations a and c. Add the enthalpies.
2H2O2 (l) → 2H2O(l) + O2(g), ΔH = -196 kJ
Calculating Enthalpy Change
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8. Standard Enthalpy (Heat) of Formation
The standard enthalpy (heat) of formation is defined as the change in enthalpy that accompanies the formation of one mole of the compound in its standard state from its elements in their standard states.
Elements in their standard states have a
The formation of compounds are placed above or below elements in their standard states.
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Calculating Enthalpy Change

9. Standard Enthalpy (Heat) of Formation
Standard enthalpies of formation can be used to calculate the enthalpies for many reactions under standard conditions by using Hess’s law.
The summation equation:
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Calculating Enthalpy Change

10. ENTHALPY CHANGE FROM STANDARD ENTHALPIES OF FORMATION
KNOWN
UNKNOWN
ΔH f ₒ (CO2) = –394 kJ
ΔH rxn ₒ = ? kJ
ΔH f ₒ (H2O) = –286 kJ
ΔH f ₒ (CH4) = –75 kJ
ΔH f ₒ °(O2) = 0.0 kJ
Use with Example Problem 6.
Problem
Use standard enthalpies of formation to calculate ΔH rxn ₒ for the combustion of methane.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
SOLVE FOR THE UNKNOWN
Use the formula ΔH rxn ₒ =Σ ΔH rxn ₒ (products) – Σ ΔH f ₒ (reactants). Expand the formula to include a term for each reactant and product. Multiply each term by the coefficient of the substance in the balanced chemical equation.
Substitute CO2 and H2O for the products, CH4 and O2 for the reactants. Multiply H2O and O2 by two.
ΔH rxn ₒ = [ ΔH f ₒ (CO2) + (2) ΔH f ₒ (H2O)] – [ ΔH f ₒ (CH4) + (2) ΔH f ₒ (O2)]
Response
ANALYZE THE PROBLEM
You are given an equation and asked to calculate the change in enthalpy. The formula ΔH rxn ₒ = Σ ΔH rxn ₒ (products) – Σ ΔH f ₒ (reactants) can be used with data from Table R-11 (on the next slide).
Calculating Enthalpy Change
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11. Calculating Enthalpy Change
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Calculating Enthalpy Change

12. ENTHALPY CHANGE FROM STANDARD ENTHALPIES OF FORMATION
SOLVE FOR THE UNKNOWN (continued)
Substitute ΔH f ₒ (CO2) = –394 kJ, ΔH f ₒ (H2O) = –286 kJ, ΔH f ₒ (CH4) = –75 kJ, and ΔH f ₒ (O2) = 0.0 kJ into the equation.
ΔH rxn ₒ = [(–394 kJ) + (2)(–286 kJ)] – [(–75 kJ) + (2)(0.0kJ)]
ΔH rxn ₒ = [– 966 kJ] – [–75 kJ] = –966 kJ+75 kJ = –891 kJ
The combustion of 1 mol CH4 releases 891 kJ.
EVALUATE THE ANSWER
All values are accurate to the ones place. Therefore, the answer is correct as stated. The calculated value is the same as that given in Table 3 (on the next slide). You can check your answer by using the stepwise procedure shown previously.
Calculating Enthalpy Change
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13. ENTHALPY CHANGE FROM STANDARD ENTHALPIES OF FORMATION
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Calculating Enthalpy Change

14. Review Essential Questions Vocabulary
How is Hess’s law applied to calculate the enthalpy change for a reaction?
What is the basis for the table of standard enthalpies of formation?
What is the enthalpy change for a reaction using standard enthalpies of formation data?
Vocabulary
Hess’s law
standard enthalpy (heat) of formation
Calculating Enthalpy Change
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