Free response · Unit 2 Beta

Potential energy of a hydrogen bromide molecule

Short free response · 4 points

The question

Name: ______________________   Date: ____________   Potential energy of a hydrogen bromide molecule (4 points)

Graph

Potential energy of an H atom and a Br atom

The graph shows the potential energy of a hydrogen atom and a bromine atom as a function of the distance between their nuclei. The minimum of the curve is marked.

−500−400−300−200−1000100200300050100150200250300350400Distance between the nuclei (pm)Potential energy (kJ/mol)141 pm−366 kJ/mol

  1. (a) Identify the bond length of H–Br, in picometers, and the energy needed to break one mole of H–Br bonds, in kJ/mol, using the graph. [1 point]

  2. (b) Explain, in terms of the forces between the particles, why the potential energy rises steeply when the distance between the nuclei is less than 141 pm. [1 point]

  3. (c) Calculate the energy, in kJ, needed to break all the H–Br bonds in 2.50 g of HBr (molar mass 80.91 g/mol). [1 point]

  4. (d) The bond energy of H–Cl is 431 kJ/mol. Explain why the H–Br bond is weaker than the H–Cl bond. [1 point]