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

11(C) Use thermochemical equations to calculate energy changes that occur in chemical reactions and classify reactions as exothermic or endothermic. Copyright © McGraw-Hill Education Calculating Enthalpy Change

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? Copyright © McGraw-Hill Education Calculating Enthalpy Change

Vocabulary Review New allotrope Hess’s law standard enthalpy (heat) of formation Copyright © McGraw-Hill Education Calculating Enthalpy Change

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. Copyright © McGraw-Hill Education Calculating Enthalpy Change

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 Copyright © McGraw-Hill Education

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 Copyright © McGraw-Hill Education

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. Copyright © McGraw-Hill Education Calculating Enthalpy Change

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: Copyright © McGraw-Hill Education Calculating Enthalpy Change

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 Copyright © McGraw-Hill Education

Calculating Enthalpy Change Copyright © McGraw-Hill Education Calculating Enthalpy Change

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 Copyright © McGraw-Hill Education

ENTHALPY CHANGE FROM STANDARD ENTHALPIES OF FORMATION Copyright © McGraw-Hill Education Calculating Enthalpy Change

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 Copyright © McGraw-Hill Education