IB Chemistry HL - Born-Haber Definitions

Which chemical equation represents the enthalpy of formation?

• C(s) + O₂(g) → CO₂(g)

• Na(s) + ½ Cl₂(g) → NaCl(l)

• 2 Na(s) + Cl₂(g) → 2 NaCl(s)

• 2 C(s) + O₂(g) → 2 CO(g)

Answer explanation

The enthalpy of formation is defined as the heat change when one mole of a compound is formed from its elements in their standard states. The equation C(s) + O₂(g) → CO₂(g) represents this process for CO₂.

Which chemical equation represents the enthalpy of formation?

• H₂(g) + ½ O₂(g) → H₂O(l)

• 2 H₂(g) + O₂(g) → 2 H₂O(l)

• H₂(l) + ½ O₂(g) → H₂O(l)

• H₂(g) + O₂(g) → 2 H₂O(l)

Answer explanation

The enthalpy of formation is defined for one mole of a compound from its elements in their standard states. The correct equation, H₂(g) + ½ O₂(g) → H₂O(l), represents this, forming one mole of water from its gaseous elements.

Which equation represents the enthalpy of combustion of methane?

• CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(l)

• CH₄(g) + 1½ O₂(g) → CO₂(g) + H₂O(l)

• CH₄(g) + O₂(g) → CO₂(g) + 2 H₂O(l)

• CH₄(g) + 2 O₂(g) → CO₂(g) + H₂O(g)

Answer explanation

The correct equation for the enthalpy of combustion of methane is CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(l). This shows complete combustion, producing carbon dioxide and liquid water, which is typical for combustion reactions.

Which equation represents the enthalpy of neutralisation?

• HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

• 2 HCl(aq) + NaOH(aq) → NaCl(aq) + 2 H₂O(l)

• HCl(g) + NaOH(aq) → NaCl(aq) + H₂O(l)

• HCl(aq) + NaOH(aq) → NaCl(s) + H₂O(l)

Answer explanation

The correct equation for the enthalpy of neutralisation is HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l), as it shows the reaction between aqueous hydrochloric acid and sodium hydroxide producing aqueous sodium chloride and liquid water.

Which equation represents the first ionisation energy?

• Mg(g) → Mg⁺(g) + e⁻

• Mg(s) → Mg⁺(g) + e⁻

• Mg(g) → Mg²⁺(g) + 2 e⁻

• 2 Mg(g) → 2 Mg⁺(g) + 2 e⁻

Answer explanation

The first ionisation energy is represented by the equation Mg(g) → Mg⁺(g) + e⁻, which shows the removal of one electron from a gaseous magnesium atom, forming a magnesium ion.

Which equation represents the second ionisation energy?

• Mg⁺(g) → Mg²⁺(g) + e⁻

• Mg(g) → Mg²⁺(g) + 2 e⁻

• Mg²⁺(g) → Mg³⁺(g) + e⁻

• 2 Mg⁺(g) → 2 Mg²⁺(g) + 2 e⁻

Answer explanation

The second ionisation energy is represented by the equation Mg⁺(g) → Mg²⁺(g) + e⁻, which shows the removal of an electron from a singly charged ion to form a doubly charged ion.

Which equation represents the first electron affinity?

• Cl(g) + e⁻ → Cl⁻(g)

• Cl₂(g) + e⁻ → 2 Cl⁻(g)

• Cl(g) + e⁻ → Cl⁻(s)

• 2 Cl(g) + 2 e⁻ → 2 Cl⁻(g)

Answer explanation

The first electron affinity is defined as the process of adding one electron to a neutral atom in the gas phase. The correct equation is Cl(g) + e⁻ → Cl⁻(g), which shows chlorine gaining an electron to form a negative ion.

Which equation represents the enthalpy of atomisation?

• ½ Br₂(l) → Br(g)

• Br₂(l) → 2 Br(g)

• Br₂(g) → 2 Br(g)

• 2 Br₂(l) → 4 Br(g)

Answer explanation

The enthalpy of atomisation refers to the energy required to convert one mole of a substance into its gaseous atoms. The correct equation, ½ Br₂(l) → Br(g), shows the formation of one mole of gaseous bromine from liquid bromine.

Which equation represents hydration enthalpy?

• Na⁺(g) + (aq) → Na⁺(aq)

• Na⁺(s) + (aq) → Na⁺(aq)

• Na⁺(g) + H₂O(l) → Na⁺(aq)

• Na⁺(g) → Na⁺(aq)

Answer explanation

The correct equation for hydration enthalpy shows the transition of a gaseous ion to an aqueous state, represented as Na⁺(g) + H₂O(l) → Na⁺(aq). This reflects the process of solvation, where water molecules surround the ion.

Which equation represents the enthalpy of solution?

• NaCl(s) → Na⁺(aq) + Cl⁻(aq)

• NaCl(s) → Na⁺(g) + Cl⁻(g)

• 2 NaCl(s) → 2 Na⁺(aq) + 2 Cl⁻(aq)

• NaCl(s) → Na⁺(aq) + ½ Cl⁻(aq)

Answer explanation

The equation NaCl(s) → Na⁺(aq) + Cl⁻(aq) correctly represents the enthalpy of solution, showing the dissolution of solid NaCl into its aqueous ions, which is the process of solvation.