M3L13 Hess_ Law Pt2

Mean bond enthalpy is defined as:

• The average energy required to break one mole of a particular type of bond in a gaseous molecule.

• The energy released when a bond is formed in a solid state.

• The total energy of all bonds in a molecule.

• The energy required to break all bonds in a liquid molecule.

Which of the following best describes the enthalpy profile for an exothermic reaction?

• The products have lower enthalpy than the reactants.

• The products have higher enthalpy than the reactants.

• The enthalpy of reactants and products is the same.

• The enthalpy increases throughout the reaction.

Calculate the enthalpy change of reaction for the following: a. C₂H₄(g) + H₂(g) → C₂H₆(g)

• -124 kJmol⁻¹ (calculated using the given bond enthalpies)

• +124 kJmol⁻¹ (calculated using the given bond enthalpies)

• 0 kJmol⁻¹ (calculated using the given bond enthalpies)

• -52 kJmol⁻¹ (calculated using the given bond enthalpies)

Calculate the enthalpy change of reaction for the following: b. C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)

• -2219 kJmol⁻¹ (calculated using the given bond enthalpies)

• -890 kJmol⁻¹

• +2219 kJmol⁻¹

• 0 kJmol⁻¹

Calculate the enthalpy change of reaction for the following: c. N₂(g) + 3H₂(g) → 2 NH₃(g)

• -92 kJmol⁻¹ (calculated using the given bond enthalpies)

• +92 kJmol⁻¹ (calculated using the given bond enthalpies)

• -46 kJmol⁻¹ (calculated using the given bond enthalpies)

• +46 kJmol⁻¹ (calculated using the given bond enthalpies)

What is the average enthalpy change when breaking 1 mol of a covalent bond in the gaseous state?

• The average enthalpy change when breaking 1 mol of a covalent bond in the gaseous state is called bond enthalpy (or bond dissociation enthalpy).

• It is called lattice energy.

• It is called ionization energy.

• It is called electron affinity.

Which of the following are correct features of an energy profile diagram for an exothermic reaction?

• A) Axes are correctly labelled.

• B) Reactants are higher than products, reactants on right, products on left.

• C) Activation curve is drawn and Ea is labelled.

• D) ΔH is labelled as a downward arrow.

Calculate the enthalpy change (ΔrH) for the following reaction using the given bond energies: Reactants (C=C) 612 + (C-H) (4x413) + (H-H) 436 = 2700 Products (C-C) 347 + (C-H) (6x413) = 2825 ΔrH = _______ kJmol⁻¹

• -125

• +125

• -225

• +225

Calculate the enthalpy change (ΔcH) for the following reaction using the given bond energies: Reactants (C-C) (2x347) + (C-H) (8x413) + (O=O) (5x497) = 6483 Products (C=O) (6x805) + (H-O) (8x463) = 8534 ΔcH = _______ kJmol⁻¹

• -2051

• +2051

• -1051

• +1051

Calculate the enthalpy change (ΔrH) for the following reaction using the given bond energies: Reactants (N≡N) 945 + (H-H) (3x436) = 2253 Products (N-H) (6x391) = 2346 ΔrH = _______ kJmol⁻¹

• -93

• +93

• -603

• +603