1 / 12

Communicating Enthalpy Change

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.

xanti
Download Presentation

Communicating Enthalpy Change

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Communicating Enthalpy Change

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. 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°= ?

  7. 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

  8. 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.

  9. 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.

  10. 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

More Related