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Method 1: Molar Enthalpies of Reaction, Δ r H m

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, Δ r H m

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  1. 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.

  2. 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.

  3. 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.

  4. Method 3: Energy Terms in Balanced Equations For endothermic reactions, the energy is listed along with the reactants. reactants + energy→ products For exothermic reactions, the energy is listed along with the products. reactants → products + energy

  5. 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.

  6. 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.

  7. Homework: • Read pgs. 495 – 500 • pg. 501 Section 11.3 Questions #’s 1 – 7

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