[9.4] hess's law and enthalpy cycles

Question 1

what is hess’s law?

Answer 1

• it allows us to determine enthalpy changes indirectly
• it states that if a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route

Question 2

enthalpy cycle for standard enthalpy changes of formation

Answer 2

• write equation
• write elements at the bottom
• arrows point upwards
• B = enthalpy change of reactants
• C = enthalpy change of products
• A + B = C (so A = C - B)

Question 3

enthalpy cycle for standard enthalpy changes of combustion

Answer 3

• write elements (C and H₂) on the left
• thing that is combusting on the right of the equation
• combustion products (carbon dioxide and water) on the bottom
• arrows point downwards
• B = enthalpy change of reactants
• C = enthalpy change of (combustion) products
• C + A = B (so A = B - C)

Question 4

summary of how to calculate using enthalpy changes of formation (but always use ABC)

Answer 4

ΔᵣH = ∑ΔfH products - ∑ΔfH reactants

Question 5

summary of how to calculate using enthalpy changes of combustion (but always use ABC)

Answer 5

ΔᵣH = ∑Δ꜀H reactants - ∑Δ꜀H products

Question 6

why does ∑Δ꜀H (C) and ∑ΔfH (CO₂) have the same value?

Answer 6

• ∑Δ꜀H (C) is for combustion of 1 mol C with 1 mol O₂ to form 1 mol CO₂
• ∑ΔfH (CO₂) is for formation of 1 mol CO₂ from 1 mol (C) and 1 mol O₂
• both processes represent the same reaction and equation so enthalpy changes will have the same value