Module 4.2 Enthalpy and Hess' Law

Consider the imaginary molecules A₂, B₂ and B₂A.

The molar enthalpy of formation of the A-A bond is -20 kJ mol^-1. The molar enthalpy of formation of the B-B bond is -10 kJ mol^-1. The molar enthalpy of formation of the A-B bond is -15 kJ mol^-1.

What is the enthalpy change of the reaction described in the diagram?

• -15 kJ mol^-1

• -20 kJ mol^-1

• +15 kJ mol^-1

• +20 kJ mol^-1

Tin reacts with oxygen to form two different compounds, SnO and SnO₂:

2Sn + O₂ → 2SnO ΔH = -562 kJ mol^-1

Sn + O₂ → SnO₂ ΔH = -578 kJ mol^-1

Consider the chemical reaction:

2SnO + O₂ → 2SnO₂

Which value is closest to the enthalpy change for this reaction?

• -1000 kJ mol^-1

• -500 kJ mol^-1

• 0 kJ mol^-1

• +500 kJ mol^-1

Nitrogen gas (N₂) reacts exothermically with hydrogen gas (H₂) to form ammonia gas (NH₃). The structural formulae of these molecules and a table of average bond energies are shown.

Based on this information, what is the value of the enthalpy change for the following reaction?

N₂(g) + 3H₂(g) ↔ 2NH₃(g)

• +93 kJ mol^-1

• -93 kJ mol^-1

• +1080 kJ mol^-1

• -1080 kJ mol^-1

Three equations and their ΔH values are shown.

N₂H₄(l) + O₂(g) → N₂(g) + 2H₂O(l) ΔH = -622.3 kJ

H₂(g) + ¹/₂O₂(g) → H₂O(l) ΔH = -285.8 kJ

H₂(g) + O₂(g) → H₂O₂(l) ΔH = -187.8 kJ

Using this information and Hess' law, what is the ΔH for the following reaction?

N₂H₄(l) + 2H₂O₂(g) → N₂(g) + 4H₂O(l)

• -532.5 kJ

• -818.3 kJ

• -1005.5 kJ

• -1568.9 kJ

As a chemical reaction proceeds from reactants to products, the enthalpy change is constant, regardless of the number of steps.

Which of the following generally represents this concept?

• Avogadro's Law

• Hess' Law

• Gibbs free energy

• Charles' Law

Bromine gas reacts with fluorine gas to produce bromine trifluoride.

Br₂(g) + 3F₂(g) → 2BrF₃(g)

The average bond energies for this reaction are shown in the table.

What is the heat of reaction at 298 K for this reaction?

• -516 kJ

• -410 kJ

• -611 kJ

• -665 kJ

Hydrogen gas reacts with chlorine gas to form hydrogen chloride gas.

H₂(g) + Cl₂(g) → 2HCl(g) ΔHº = –184 kJ mol^–1

What is the Cl-Cl bond enthalpy for this reaction?

• 1114 kJ mol^–1

• 652 kJ mol^–1

• 610 kJ mol^–1

• 242 kJ mol^–1

Examine the three redox reactions shown.

1. 1. O₂(g) + 2H₂O(l) + 4e^- ↔ 4OH^-(aq) E^ο = +0.40 V

2. 2. 2H⁺(aq) + 2e^- ↔ H₂(g) E^ο = 0.0 V

3. 3. 2H₂O(l) + 2e^- ↔ H₂(g) + 2OH^-(aq) E^ο = -0.83 V

4. Using any of these three equations, what is the E^ο for the following reaction?

5. 2H₂(g) + O₂(g) → 2H₂O(l)

• 0.43 V

• 1.23 V

• 2.06 V

• 4.06 V

Using the above information, which of the following represents the bond energy for the H-Cl bond?

• 92 kJ mol^-1

• 123 kJ mol^-1

• 384 kJ mol^-1

• 430 kJ mol^-1

Fermentation can be represented by:

C₆H₁₂0₆(s) → 2C₂H₅OH(l) + 2CO₂(g)

glucose ethanol

The equations for the enthalpies of formation of these three chemicals are shown.

Which of the following is the correct enthalpy change for fermentation based on Hess's law?

• -71.4 kJ

• +206.3 kJ

• +322.1 kJ

• -2613.4 kJ