Which ionic compound has the largest value of lattice enthalpy?

A. MgS

B. MgO

C. CaBr₂

D. NaF

B

Which combination of ΔH ^θ and ΔS ^θ will result in a non-spontaneous reaction at all temperatures?

A

Which combination gives the standard hydration enthalpy of ${\mathrm{Na}}^{+} \left(\mathrm{g}\right)$?

A.  $4+359+790$

B.  $4+359-790$

C.  $-4-359+790$

D.  $4-359+790$

B

About 60% of candidates could select values to put in an energy cycle for hydration enthalpy. The higher scoring candidates performed better on this question than lower scoring ones.

Which equation represents the lattice enthalpy of magnesium sulfide?

A. MgS (s) → Mg (g) + S (g)

B. MgS (s) → Mg⁺ (g) + S^– (g)

C. MgS (s) → Mg²⁺ (g) + S^2– (g)

D. MgS (s) → Mg (s) + S (s)

C

The table shows the variation of standard Gibbs energy with temperature for a reversible reaction.

$∆G^{⦵}=∆H^{⦵}\mathit{-}T∆S^{⦵}$

$∆G^{⦵}=\mathit{-}RT \ln K$

What can be concluded about the reaction?

A.  Equilibrium shifts left as temperature increases.

B.  The forward reaction is more spontaneous below 300 K.

C.  Entropy is higher in the products than in the reactants.

D.  K_c decreases as temperature increases.

C

Which value represents the lattice enthalpy, in kJ mol⁻¹, of strontium chloride, SrCl₂?

A.     – (–829) + 164 + 243 + 550 + 1064 – (–698)

B.     –829 + 164 + 243 + 550 + 1064 – 698

C.     – (–829) + 164 + 243 + 550 + 1064 – 698

D.     –829 + 164 + 243 + 550 + 1064 – (–698)

C

In which of the following situations is the forward reaction spontaneous?

A.  The equilibrium constant is greater than one under standard conditions.

B.  The cell potential is negative.

C.  The Gibbs free energy change of the reverse reaction is negative.

D.  The entropy change of the universe for the forward reaction is negative.

A

In which reaction does entropy decrease?

A.  NaCl (s) → NaCl (aq)

B.  Zn (s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

C.  NH₃(g) + HCl (g) → NH₄Cl (s)

D.  CuCO₃(s) → CuO (s) + CO₂(g)

C

Second best answered question in the exam with 85% of candidates identifying the correct answering for reaction in which entropy decreases.

What are the signs of ΔH and ΔS for a reaction that is non-spontaneous at low temperatures but spontaneous at high temperatures?

D

Two-thirds of the candidates related information about the spontaneity of a reaction at different temperatures to the signs of ΔH and ΔS correctly. This question discriminated well between high-scoring and low-scoring candidates.

Which ion’s hydration energy is the most exothermic?

A.     Li⁺

B.     Na⁺

C.     Br^–

D.     I^–

A

Which represents electron affinity?

A.  Al²⁺(g) → Al³⁺(g) + e⁻

B.  C (g) + e⁻ → C⁻(g)

C.  Cl₂(g) → 2Cl (g)

D.  S (s) → S⁺(g) + e⁻

B

Which equation represents the standard enthalpy of atomization of bromine, Br₂?

A. $\frac{1}{2}$Br₂ (l) → Br (g)

B. Br₂ (l) → 2Br (g)

C. Br₂ (l) → 2Br (l)

D. $\frac{1}{2}$Br₂ (l) → Br (l)

A

One G2 comment asked if enthalpy of atomization was on the syllabus. This Topic 15.1 question was answered correctly by 54 % of candidates but did not differentiate well between higher and lower scoring candidates.

What is the enthalpy of solution of MgF₂(s) in kJ mol⁻¹?

Lattice enthalpy of MgF₂(s) = 2926 kJ mol⁻¹

Hydration enthalpy of Mg²⁺(g) = −1963 kJ mol⁻¹

Hydration enthalpy of F⁻(g) = −504 kJ mol⁻¹

A.     2926 − 1963 + 2(−504)

B.     2926 − 1963 − 504

C.     −2926 − (−1963) − (−504)

D.     −2926 − (−1963) − 2(−504)

A

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

Xⁿ⁺ (g) → Xⁿ⁺ (aq)

A.  Ca²⁺, Mg²⁺, K⁺, Na⁺

B.  Na⁺, K⁺, Mg²⁺, Ca²⁺

C.  K⁺, Na⁺, Ca²⁺, Mg²⁺

D.  Mg²⁺, Ca²⁺, Na⁺, K⁺

C

Which compound has the largest value of lattice enthalpy?

A.  Na₂O

B.  K₂O

C.  Na₂S

D.  K₂S

A

A majority of students recognized the ionic compound with the highest lattice enthalpy.

What are the signs of ΔH^Θ and ΔS^Θ for the reaction, which is spontaneous at low temperature and non-spontaneous at very high temperature?

ΔG^Θ = ΔH^Θ − TΔS^Θ

SO₃ (g) + CaO (s) → CaSO₄ (s)

C

Which system has the most negative entropy change, ΔS, for the forward reaction?

A.     N₂(g) + 3H₂(g) $\rightleftharpoons$ 2NH₃(g)

B.     CaCO₃(s) → CaO(s) + CO₂(g)

C.     2S₂O₃²⁻(aq) + I₂(aq) → S₄O₆²⁻(aq) + 2I^–(aq)

D.     H₂O(l) → H₂O(g)

A

Which statements are correct for ionic compounds?

I.   Lattice energy increases as ionic radii increase.
II.  Within the same group, the melting point of salts tends to decrease as the radius of the cation increases.
III. Solubility in water depends on the relative magnitude of the lattice energy compared to the hydration energy.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

C

Which term in the expression ΔG^⦵ = ΔH^⦵ − TΔS^⦵ is an indirect measure of the entropy change of the surroundings when divided by T?

A.  ΔG^⦵

B.  ΔH^⦵

C.  ΔS^⦵

D.  −TΔS^⦵

B

This challenging question raised some debate and several teachers did not think it was suitable for this level. Most candidates chose -TΔS as “an indirect measure of the entropy change of the surroundings when divided by T”. While candidates are not expected to be familiar with the equation ΔS surroundings = -ΔHsystem/T, they could solve the question by recognizing that ΔH is the value that affects the surroundings while ΔS relates to the system. The wording of the question could have been simplified.

Which change is exothermic?

A.  $\frac{1}{2}$Cl₂ (g) → Cl (g)

B.   K (g) → K⁺ (g) + e⁻

C.   KCl (s) → K⁺ (g) + Cl⁻ (g)

D.   Cl (g) + e⁻ → Cl⁻ (g)

D

Which represents the enthalpy change of hydration of the chloride ion?

A

Which statement is correct?

A.     If ΔH < 0, reaction is always spontaneous.

B.     If ΔH > 0, reaction is never spontaneous.

C.     If ΔS < 0, reaction can be spontaneous if temperature is low enough.

D.     If ΔS < 0, reaction can be spontaneous if temperature is high enough.

C

The Born-Haber cycle for potassium oxide is shown below:

Which expression represents the lattice enthalpy in kJ mol^–1?

A.     –361 + 428 + 838 + 612

B.     –(–361) + 428 + 838 + 612

C.     –361 + 428 + 838 – 612

D.     –(–361) + 428 + 838 – 612

B

Which substance has the highest lattice enthalpy?

A.  $\text{KCl}$

B.  ${\text{CaCl}}_2$

C.  $\text{KF}$

D.  ${\text{CaF}}_{\text{2}}$

D

Which equation represents lattice enthalpy?

A. NaCl (g) → Na⁺ (g) + Cl⁻ (g)

B. NaCl (s) → Na⁺ (g) + Cl⁻ (g)

C. NaCl (s) → Na⁺ (aq) + Cl⁻ (aq)

D. NaCl (s) → Na⁺ (s) + Cl⁻ (s)

B

72 % of candidates chose the correct equation that represents lattice enthalpy. Many candidates chose A, where the ionic compound (NaCl) was gaseous, and others chose distractor C, where the ions produced were aqueous. The discrimination index for the question was quite high.

Which equation represents hydration enthalpy?

A.  Na⁺(g) → Na⁺(aq)

B.  Na⁺(aq) → Na⁺(g)

C.  NaCl (s) → NaCl (aq)

D.  NaCl (aq) → NaCl (s)

A

65% of the candidates recognized the equation that represents hydration enthalpy. The most commonly chosen distractor was an enthalpy of solution.

Consider the Born–Haber cycle for the formation of sodium oxide:

What is the lattice enthalpy, in kJ mol⁻¹, of sodium oxide?

A.  414 + 2(108) + 249 + 2(496) − 141 + 790

B.  414 + 2(108) + 249 + 2(496) + 141 + 790

C.  −414 + 2(108) + 249 + 2(496) − 141 + 790

D.  −414 − 2(108) − 249 − 2(496) + 141 − 790

A

What is the standard enthalpy of formation, in kJ mol^–1, of IF (g)?

IF₇ (g) + I₂ (s) → IF₅ (g) + 2IF (g)        ΔH${}^{\theta }$ = –89 kJ

ΔH${}_f^{\theta }$ (IF₇) = –941 kJ mol^–1

ΔH${}_f^{\theta }$ (IF₅) = –840 kJ mol^–1

A. –190

B. –95

C. +6

D. +95

B

Which equation represents enthalpy of hydration?

A.     Na(g) → Na⁺(aq) + e⁻

B.     Na⁺(g) → Na⁺(aq)

C.     NaCl(s) → Na⁺(g) + Cl⁻(g)

D.     NaCl(s) → Na⁺(aq) + Cl⁻(aq)

B

Which is correct for the reaction H₂O (g) → H₂O (l) ?

A. Enthalpy increases and entropy increases.

B. Enthalpy decreases and entropy increases.

C. Enthalpy increases and entropy decreases.

D. Enthalpy decreases and entropy decreases.

D

Higher scoring candidates had more success understanding enthalpy and entropy decreases.

Which change has the greatest increase in entropy?

A. CO₂ (s) → CO₂ (g)

B. CO₂ (g) → CO₂ (l)

C. CO₂ (g) → CO₂ (s)

D. CO₂ (l) → CO₂ (s)

A

94 % of the candidates chose the change with the greatest increase in entropy.

Which reaction becomes more spontaneous as temperature increases?

A.  ${\mathrm{CaCO}}_3 \left(\mathrm{s}\right)\to \mathrm{CaO} \left(\mathrm{s}\right)+{\mathrm{CO}}_2 \left(\mathrm{g}\right)$

B.  ${\mathrm{N}}_2 \left(\mathrm{g}\right)+3{\mathrm{H}}_2 \left(\mathrm{g}\right)\rightleftharpoons 2{\mathrm{NH}}_3 \left(\mathrm{g}\right)$

C.  $3{\mathrm{CO}}_2 \left(\mathrm{g}\right)+4{\mathrm{H}}_2\mathrm{O} \left(\mathrm{g}\right)\to {\mathrm{C}}_3{\mathrm{H}}_8 \left(\mathrm{g}\right)+5{\mathrm{O}}_2 \left(\mathrm{g}\right)$

D.  ${\mathrm{SO}}_2 \left(\mathrm{g}\right)+{\mathrm{H}}_2{\mathrm{O}}_2 \left(\mathrm{l}\right)\to {\mathrm{H}}_2{\mathrm{SO}}_4 \left(\mathrm{l}\right)$

A

The majority of candidates could see that reactions which form more mol of gas become spontaneous at higher temperature.

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

A. HCl (g) + NH₃ (g) → NH₄Cl (s)

B. (NH₄)₂Cr₂O₇ (s) → Cr₂O₃ (s) + N₂ (g) + 4H₂O (g)

C. CaCO₃ (s) → CaO (s) + CO₂ (g)

D. I₂ (g) → I₂ (s)

B

Which change results in the largest negative value of ΔS?

A.  C₂H₅OH (l) + SOCl₂(l) → C₂H₅Cl (l) + SO₂(g) + HCl (g)

B.  CaCO₃(s) → CaO (s) + CO₂(g)

C.  H₂O (l) → H₂O (s)

D.  NH₃(g) + HCl (g) → NH₄Cl (s)

D

The combustion of glucose is exothermic and occurs according to the following equation:

C₆H₁₂O₆ (s) + 6O₂ (g) → 6CO₂ (g) + 6H₂O (g)

Which is correct for this reaction?

A