1 / 11

Bond enthalpies and mean bond enthalpies

Bond enthalpies and mean bond enthalpies. Connector: What type of energy change happens when a chemical bond is: Broken endothermic Made exothermic

bethany-duffy
Download Presentation

Bond enthalpies and mean bond enthalpies

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. Bond enthalpies and mean bond enthalpies • Connector: • What type of energy change happens when a chemical bond is: • Broken endothermic • Made exothermic • How is the enthalpy change of a chemical reaction calculated? Write an equation to show this. What are the units of enthalpy change? • ΔH = H (products) – H (reactants) • units kJ mol-1 (kJ/mol) Crowe2010

  2. Using bond enthalpy data

  3. Bond enthalpy Specific to a particular bond in a diatomic molecule e.g H2, HBr H2(g) 2H (g) bond dissociation energy = +436 Kj/mol HBr (g) H (g) + Br (g) bond dissociation energy = +366 Kj/mol

  4. Mean bond enthalpy e.g. methane As H atoms are removed from the methane molecule the environment of the others left behind changes and so the strength of the bond changes. So, here we have to rely on the mean bond enthalpy. ΔH = + 1662 kJ mol-1 So, mean bond enthalpy = +1662/4 = + 415.5 kJ mol-1

  5. Calculations involving bond enthalpies

  6. Method: • Construct a Hess cycle, showing all the bonds to be broken, and the products of the broken bonds. • Put in the bond enthalpies

  7. What is needed to complete the Hess cycle? (Hint: What is the definition of the heat of formation? What is the definition of the heat of atomisation?) • Put in the given values. • Now find two routes around the cycle without going against the flow of an arrow.

More Related