1. WebAssign #14 q=c∙m∙ΔT ΔHchange 693 kJ
How much heat is required to change 213 g of ice from -46.5ᵒC to 0.0ᵒC, melt the ice, warm the water from 0.0ᵒC to 100ᵒC, boil the water, and heat the steam to 173.0ᵒC?
100ᵒ to 173ᵒC
q=c∙m∙ΔT
boil
ΔHchange
melting
0.0ᵒC to 100ᵒC
-46.5ᵒC to 0.0ᵒC
693 kJ

2. Using ∆Hf to calculate enthalpy changes or ∆H⁰rxn
Energy and Chemical Change: Basic Concepts
There are two ways that we will calculate the standard enthalpy of reaction:
Hess’ Law (adding equations)
Comparing products to reactants
both use enthalpies of formation
Using ∆Hf to calculate enthalpy changes or ∆H⁰rxn
The standard enthalpy (heat) of formation (∆H°f) of a compound is the change in enthalpy for the formation of one mole of a compound

3. Calculating Enthalpy Change
Energy and Chemical Change: Basic Concepts
Calculating Enthalpy Change
Hess’s law: If you can add two equations, you can add their enthalpies
Used to find the enthalpy of the overall reaction.
Think of it like you’re doing a puzzle

4. Applying Hess’ Law Identify products and reactants
Hess’ Law Steps:
Identify products and reactants
Match coefficients to overall equation
Add equations (cancel spectators)
Add enthalpies
Whatever you do to the equation, you must also do to its enthalpy
Some problems have 3 equations – follow the same principles

5. Applying Hess’ Law
Ex13) Use the following chemical equations to determine ΔH for this equation: 2H2O2(l) → 2H2O(l) +O2(g)
2H2(g) +O2(g) → 2H2O(l) ΔH = -572 kJ
H2(g) +O2(g) → H2O2(l) ΔH = -188 kJ
ΔH = -196 kJ

6. Determine ΔH for each reaction
Ex14) 2CO(g) + 2NO(g) → 2CO2(g) + N2(g) ∆H = ?
2CO(g) + O2(g) → 2CO2(g) ∆H = kJ
N2(g) + O2(g) → 2NO(g) ∆H = kJ
Ex15) 4Al(s) + 3MnO2(s) → 2Al2O3(s) + 3Mn(s) ∆H = ?
4Al(s) + 3O2(g) → 2Al2O3(s) ∆H = kJ
Mn(s) + O2(g) → MnO2(s) ∆H = -521kJ kJ
-1789 kJ

7. Uses the standard enthalpies of formation for each individual molecule
Energy and Chemical Change: Basic Concepts
A simpler way:
Uses the standard enthalpies of formation for each individual molecule
f reactants
f products
The standard heat of formation of an element in its standard state is zero. Ex) Au, O2; ΔH°f = 0 kJ

8. Standard Enthalpies of formation
Ex 16) Use standard enthalpies of formation to calculate ∆H°rxn for CH4(g) + 2O2(g) → CO2(g) +2H2O(l)
1. Find ∆H°f for known elements
∆H°f (CH4(g)) = -75 kJ
∆H°f (O2(g)) = 0.0 kJ
∆H°f (CO2(g)) = -394 kJ
∆H°f (H2O(g)) = -286 kJ

9. Standard Enthalpies of formation
Ex16) Use standard enthalpies of formation to calculate ∆H°rxn for CH4(g) + 2O2(g) → CO2(g) +2H2O(l)
2. Use formula ∆H°rxn = ∆H°f(prod) - ∆H°f(react)
∆H°rxn = (∆H°f(CO2) + ∆H°f(H2O)) – (∆H°f(CH4) + ∆H°f(O2))
∆H°rxn = [(-394 kJ + 2(-286 kJ)] – [(-75 kJ + 2(0.0 kJ)]
= [-966 kJ] – [-75 kJ]
= -966 kJ + 75 kJ
= -891 kJ
The combustion of
1 mol CH4 releases 891 kJ

10. Practice ∆H°rxn ∆H°rxn = 178.3 kJ ∆H°rxn = 66.36 kJ
Use standard enthalpies of formation to calculate ∆H°rxn for each of the following reactions.
Ex17) CaCO3(s) → CaO(s) + CO2(g)
Ex18) N2(g) + 2O2(g) → 2NO2(g)
Ex19) 4NH3(g) + 7O2(g) → 4NO2(g) + 6H2O(l)
∆H°rxn = kJ
∆H°rxn = kJ
∆H°rxn = kJ