Topic 15: Energetics & Thermochemistry (HL only)

Which reaction has the greatest increase in entropy of the system?

• CH₄(g) + H₂O(g) → CO(g) + 3H₂(g)

• MgCO₃(s) → MgO(s) + CO₂(g)

• Pb(NO₃)₂(aq) + 2KI(aq) → 2KNO₃(aq) + PbI₂(s)

• 2NI₃(s) → N₂(g) + 3I₂(g)

Answer explanation

What is the correct order of increasing (more exothermic) enthalpy change of hydration?

Xⁿ⁺(g) → Xⁿ⁺(aq)

• K⁺, Na⁺, Be²⁺, Li⁺

• K⁺, Na⁺, Li⁺, Be²⁺

• Be²⁺, K⁺, Na⁺, Li⁺

• Be²⁺, Li⁺, Na⁺, K⁺

Answer explanation

Which is correct for this process?

H₂O(l) → H₂O(s)

• Enthalpy decreases and entropy decreases

• Enthalpy increases and entropy decreases

• Enthalpy increases and entropy increases

• Enthalpy decreases and entropy increases

Answer explanation

Which equation represents the standard enthalpy of atomization of astatine, At₂?

• At₂(s) → 2At(s)

• At₂(s) → 2At(g)

• 1/2At₂(s) → At(s)

• 1/2At₂(s) → At(g)

Answer explanation

Which equation represents lattice enthalpy?

• KCl(s) → K⁺(g) + Cl^-(g)

• KCl(s) → K⁺(s) + Cl^-(s)

• KCl(g) → K⁺(g) + Cl^-(g)

• KCl(s) → K⁺(aq) + Cl^-(aq)

Answer explanation

What are the signs for ΔH° and ΔS° for this reaction which is spontaneous at low temperatures and non-spontaneous at very high temperatures?

4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

• ΔH° is positive; ΔS° is positive

• ΔH° is negative; ΔS° is negative

• ΔH° is positive; ΔS° is negative

• ΔH° is negative; ΔS° is positive

Answer explanation

What is the enthalpy of solution for magnesium chloride, MgCl₂(s), in kJ mol^-1?

• -2540 + 1963 + 359

• 2540 - 1963 + 2(-359)

• 2540 - 1963 - 359

• -2540 + 1963 - 2(-359)

Answer explanation

Which value represents the lattice enthalpy (in kJ mol^-1) for magnesium fluoride, MgF₂, as shown on the diagram?

• -(-1124) + 148 + 2(79) + 738 + 1450 + 2(-328)

• 1124 + 148 + 2(79) + 738 + 1450 + 2(328)

• -1124 + 148 + 2(79) + 738 + 1450 + 2(-328)

• -2(-328) - 1450 -738 - 2(79) - 148 + 1124

Answer explanation

Which statements are correct for ionic compounds?

1: Lattice enthalpy increases as ionic radii decrease.

2: Lattice enthalpy increases as the charge on the cation decreases.

3: Solubility in water depends on the relative magnitude of the lattice enthalpy compared to the hydration enthalpy.

• 2 and 3 only

• 1 and 3 only

• 1, 2 and 3

• 1 and 2 only

Answer explanation

The combustion of ethanol is exothermic. Which is correct for this reaction?

C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)

• ΔH° is negative; ΔS° is positive; the reaction is non-spontaneous

• ΔH° is positive; ΔS° is positive; the reaction is non-spontaneous

• ΔH° is negative; ΔS° is positive; the reaction is spontaneous

• ΔH° is positive; ΔS° is negative; the reaction is spontaneous

Answer explanation