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Communicating Enthalpy Change. Method 1: Molar Enthalpies of Reaction, Δ r H m. When reactants and products are in their standard state , they are at a pressure of 100 kPa, an aqueous concentration of 1.0 mol/L. and liquids and solids are in their pure state.
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Method 1: Molar Enthalpies of Reaction, ΔrHm When reactants and products are in their standard state, they are at a pressure of 100 kPa, an aqueous concentration of 1.0 mol/L. and liquids and solids are in their pure state. To communicate a molar enthalpy, both the substance and the reaction must be specified.
Δf Hm° = –239.2 kJ/mol Formation Reaction CH3OH When 1 mol of methanol is formed from its elements when they are in their standard states at SATP, 239.2 kJ of energy is released. Combustion Reaction Δc Hm° = –725.9 kJ/mol CH3OH The complete combustion of 1 mol of methanol releases 725.9 kJ of energy. Note that the above reactions are balanced for one mole of the compound.
Method 2: Enthalpy Changes, ΔrH Write an enthalpy change (Δr H) beside the chemical equation. CO(g) + 2 H2(g) → CH3OH(l) Δr H = –725.9 kJ The enthalpy change is not a molar value, so does not require the “m” subscript and is not in kJ/mol. Δc H° = –98.9 kJ Δc H° = –197.8 kJ When 2 moles of sulfur dioxide are burned, twice as much heat energy is released as when 1 mole of sulfur dioxide is burned.
Sulfur dioxide and oxygen react to form sulfur trioxide. The standard molar enthalpy of combustion of sulfur dioxide, in this reaction, is -98.9 kJ/mol. What is the enthalpy change for this reaction? 2 SO2(g) + O2 (g) 2 SO3(g) Then get the chemical amount of sulfur dioxide from its coefficients in the balanced equation and use ΔcH° = n ΔcH°m ΔcH° = n ΔcH°m = 2 mol x (-98.9 kJ)/1 mol = -197.8 kJ Finish it off by communicating the enthalpy next to a balanced equation 2 SO2(g) + O2 (g) 2 SO3(g) ΔcH° = -197.8 kJ
Another example... ΔcH= n ΔcH° = 2 mol x -518.0 kJ/mol = -1 036.0 kJ 2 H2S (g) + 3 O2(g) 2 H2O (g) + 2 SO2(g) ΔcH°= -1 036.0 kJ Wild natural gas wells are sometimes lit on fire to eliminate the very toxic hydrogen sulfide gas. The standard molar enthalpy of combustion of hydrogen sulfide is -518.0 kJ/mol. Express this value as a standard enthalpy change for the following: 2 H2S (g) + 3 O2(g) 2 H2O (g) + 2 SO2(g) ΔcH°= ?
Method 3: Energy Terms in Balanced Equations Forendothermicreactions, the energy is listed along with thereactants. reactants +energy→ products Forexothermicreactions, the energy is listed along with theproducts. reactants → products +energy
Method 4: Chemical Potential Energy Diagrams During an exothermic reaction, the enthalpy of the system decreases. Heat flows out of the system and into the surroundings and we observe a temperature increase.
Method 4: Chemical Potential Energy Diagrams During an endothermic reaction, the enthalpy of the system increases. Heat flows into the system from the surroundings and we observe a temperature decrease.
Homework: • Read pgs. 495 – 500 • Read over the Communication Example Problem 4 on page 500 • pg. 501 Section 11.3 Questions #’s 1 – 7 • Handout