Question
Which step(s) is/are endothermic in the Born-Haber cycle for the formation of LiCl?
A. \(\frac{1}{2}{\text{C}}{{\text{l}}_{\text{2}}}{\text{(g)}} \to {\text{Cl(g)}}\) and \({\text{Li(s)}} \to {\text{Li(g)}}\)
B. \({\text{Cl(g)}} + {{\text{e}}^ – } \to {\text{C}}{{\text{l}}^ – }{\text{(g)}}\) and \({\text{Li(g)}} \to {\text{L}}{{\text{i}}^ + }{\text{(g)}} + {{\text{e}}^ – }\)
C. \({\text{L}}{{\text{i}}^ + }{\text{(g)}} + {\text{C}}{{\text{l}}^ – }{\text{(g)}} \to {\text{LiCl(s)}}\)
D. \(\frac{1}{2}{\text{C}}{{\text{l}}_2}{\text{(g)}} \to {\text{Cl(g)}}\) and \({\text{Cl(g)}} + {{\text{e}}^ – } \to {\text{C}}{{\text{l}}^ – }{\text{(g)}}\)
▶️Answer/Explanation
A
Endothermic stages include sublimation, dissociation and ionization energy.
In A, first reaction is bond dissociation and second is sublimation.
Question
Which ionic compound has the most endothermic lattice enthalpy?
A. NaCl
B. KCl
C. NaF
D. KF
▶️Answer/Explanation
C
The lattice energy trend rises as the ion charge rises and the distance or radius falls. More specifically, it rises through the bottom to top groups and from left to right throughout periods. Na and F being smaller in size, hence, NaF has most endothermic lattice enthalpy.
Question
Which is a correct definition of lattice enthalpy?
A. It is the enthalpy change that occurs when an electron is removed from 1 mol of gaseous atoms.
B. It is the enthalpy change that occurs when 1 mol of a compound is formed from its elements.
C. It is the enthalpy change that occurs when 1 mol of solid crystal changes into a liquid.
D. It is the enthalpy change that occurs when 1 mol of solid crystal is formed from its gaseous ions.
▶️Answer/Explanation
D
The lattice enthalpy is the enthalpy change when 1 mole of solid crystal is formed from its separated gaseous ions. Lattice formation enthalpies are always negative.
Question
Which equation corresponds to the lattice enthalpy for silver iodide, AgI?
A. \({\text{AgI(s)}} \to {\text{Ag(s)}} + {\text{I(g)}}\)
B. \({\text{AgI(s)}} \to {\text{Ag(s)}} + \frac{1}{2}{{\text{I}}_2}{\text{(g)}}\)
C. \({\text{AgI(s)}} \to {\text{A}}{{\text{g}}^ + }{\text{(aq)}} + {{\text{I}}^ – }{\text{(aq)}}\)
D. \({\text{AgI(s)}} \to {\text{A}}{{\text{g}}^ + }{\text{(g)}} + {{\text{I}}^ – }{\text{(g)}}\)
▶️Answer/Explanation
D
The lattice dissociation enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions. Lattice dissociation enthalpies are always positive. Here, in D, AgI is converted into its gaseous ions.
Question
Which ionic compound has the greatest lattice enthalpy?
A. MgO
B. CaO
C. NaF
D. KF
▶️Answer/Explanation
A
The lattice enthalpy of magnesium oxide is much greater than that of sodium fluoride or potassium fluoride. That’s because in magnesium oxide, 2+ ions are attracting 2- ions; in sodium fluoride or potassium fluoride, the attraction is only between 1+ and 1- ions.
The lattice enthalpy of magnesium oxide is also increased relative to calcium oxide because magnesium ions are smaller than calcium ions, That means that the ions are closer together in the lattice, and that increases the strength of the attractions.
Question
Which change will not increase the entropy of a system?
A. Increasing the temperature
B. Changing the state from liquid to gas
C. Mixing different types of particles
D. A reaction where four moles of gaseous reactants changes to two moles of gaseous products
▶️Answer/Explanation
D
Entropy increases when temperature increases. Entropy of Solid<Liquid<Gas. Entropy also increases when different types of particles are mixed.