THERMOCHEMISTRY - Enthalpy, Calorimetry and Bond Energies

9 important questions on THERMOCHEMISTRY - Enthalpy, Calorimetry and Bond Energies

What are the three ways to calculate for ΔH?

  1. Hess Law
  2. Standard Enthalpy of formation
  3. Bond Dissociation

What is Hess law?

  • It is the change in enthalpy of a reaction
    • Sum of ΔH of all series of steps in the reaction
    • sum of ΔH of any reaction that summed to give overall reaction
  • Example:

How to solve for Hess Law?

  1. Work backwards from the net reaction
  2. Multiple and reverse reactions needed
  3. Apply relevant values to Δ
  • Higher grades + faster learning
  • Never study anything twice
  • 100% sure, 100% understanding
Discover Study Smart

What is the enthalpy of formation of stable elements?

They have 0 enthalpy of formation.
  1. O2
  2. H2
  3. N2
  4. Cl2
  5. F2
  6. Group 18 (ex: argon)
  7. Br2
  8. I2
  9. C (graphite)
  10. Metals (Example: zinc)
  11. P4
  12. S8

How do you find the enthalpy of formation?

ΔH = nΔHp - nΔHr
products - reactants

What is bond dissociation energy?

It is the energy required to break 1 mole of covalent bonds to form a gaseous species

Increasing bond energy =

  1. Multiple bonds
  2. shorter bond length
  3. higher bond strength

How do you calculate the reaction enthalpy from bond dissociation energies?

ΔH = nD(bonds broken, reactants) - nD(bonds formed, products)

For bond dissociation energies, why can you only use gas phase reactants and products?

It is because bond dissociation energies does not account for intermolecular forces in liquids and solid!

The question on the page originate from the summary of the following study material:

  • A unique study and practice tool
  • Never study anything twice again
  • Get the grades you hope for
  • 100% sure, 100% understanding
Remember faster, study better. Scientifically proven.
Trustpilot Logo