Define lattice energy .

Lattice energy is the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions. It is always exothermic (large negative value), as strong electrostatic forces of attraction are established between oppositely charged ions.

Why is lattice energy always exothermic when expressed as a formation process?

Lattice energy is always exothermic because forming an ionic lattice from gaseous ions releases a large amount of energy due to the strong electrostatic forces of attraction between the oppositely charged ions. No such forces exist between ions in the gas phase, so the lattice is much more stable than the separated gaseous ions.

The lattice energy of NaCl involves forming .......... mole(s) of ionic solid from .......... Na + (g) and Cl - (g) ions, and its value is a large .......... number.

The lattice energy of NaCl involves forming 1 mole(s) of ionic solid from gaseous Na + (g) and Cl - (g) ions, and its value is a large negative number.

Define standard enthalpy change of atomisation .

Standard enthalpy change of atomisation (Δ H at ꝋ ) is the enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions. It is always endothermic (positive value) because energy must be supplied to break bonds and produce isolated gaseous atoms.

True or False? The first electron affinity of an element is always exothermic.

False. Most elements have an exothermic first electron affinity — energy is released when an electron is added to a neutral gaseous atom. However, group 2 metals (e.g. Be, Mg) and noble gases (e.g. Ne, Ar) have an endothermic first electron affinity, because the added electron must occupy a higher-energy subshell or a new principal quantum level.

Why can Δ H latt ꝋ not be determined directly from a single experiment?

Δ H latt ꝋ cannot be determined directly because it is not possible to carry out an experiment that converts gaseous ions directly into the ionic solid in one measurable step. Instead, multiple experimental values are combined using an energy cycle (Born-Haber cycle) to find the lattice energy indirectly.

In the first electron affinity of chlorine, a gaseous Cl atom gains .......... electron(s) to form .......... (g), releasing energy because the electron is .......... to the atom.

In the first electron affinity of chlorine, a gaseous Cl atom gains one electron(s) to form Cl - (g), releasing energy because the electron is attracted to the atom.

What does a more exothermic Δ H latt ꝋ value indicate about the ionic bonds in a lattice?

A more exothermic Δ H latt ꝋ value indicates that the ionic bonds within the lattice are stronger , meaning more energy is released when the gaseous ions come together to form the solid lattice.

Define Born-Haber cycle .

A Born-Haber cycle is a specific application of Hess's law for ionic compounds that allows the lattice energy, Δ H latt ꝋ , to be calculated indirectly using a series of measurable enthalpy changes.

What does it mean for energy to increase going upward on a Born-Haber cycle diagram?

Energy increases going upward on a Born-Haber cycle diagram means that endothermic processes (such as atomisation and ionisation) are shown with arrows pointing upward, while exothermic processes (such as electron affinity and lattice energy) point downward.

Δ H f ꝋ = Δ H 1 ꝋ + Δ H latt ꝋ , so Δ H latt ꝋ = .......... – .......... .

Δ H f ꝋ = Δ H 1 ꝋ + Δ H latt ꝋ , so Δ H latt ꝋ = Δ H f ꝋ – Δ H 1 ꝋ .

How is Δ H latt ꝋ calculated from a Born-Haber cycle using Hess's law?

Δ H latt ꝋ is calculated by identifying the direct and indirect routes around the cycle and applying Hess's law: Δ H latt ꝋ = Δ H f ꝋ – Δ H 1 ꝋ where Δ H 1 ꝋ is the sum of all enthalpy changes needed to convert the elements in their standard states to gaseous ions.

True or False? The standard enthalpy of formation of an ionic compound is always exothermic.

False. The standard enthalpy of formation, Δ H f ꝋ , can be exothermic (negative) or endothermic (positive) depending on the compound. If Δ H f ꝋ is endothermic, the ionic compound sits above the elements on the Born-Haber energy diagram.

Why must the first electron affinity of chlorine be doubled when constructing the Born-Haber cycle for MgCl 2 ?

The first electron affinity of chlorine must be doubled in the MgCl 2 cycle because 2 moles of chlorine atoms each gain one electron to form 2 moles of Cl - (g) ions. In general, any enthalpy change involving a species present in more than one mole must be multiplied by the appropriate coefficient .

To create gaseous ions from elements in a Born-Haber cycle, the two steps are: (1) .......... the element to gaseous atoms and (2) .......... or remove electrons from the atoms.

To create gaseous ions from elements in a Born-Haber cycle, the two steps are: (1) atomise the element to gaseous atoms and (2) add or remove electrons from the atoms.

In what direction are bond-breaking steps drawn on a Born-Haber cycle diagram, and why?

Bond-breaking steps are drawn with arrows pointing upward because they are endothermic processes, requiring an input of energy. This includes atomisation and ionisation energy steps, both of which involve separating or removing electrons from atoms.

True or False? A Born-Haber cycle can only be used to calculate lattice energy.

False. A Born-Haber cycle can be used to calculate any enthalpy change in the cycle, such as the enthalpy of formation , ionisation energy , or electron affinity , provided all other values are given.

Define polarising power .

Polarising power is the ability of a cation to distort the electron cloud of an adjacent anion , introducing covalent character into an otherwise ionic bond. It increases with greater charge density (small ionic radius and/or high charge).

Why does polarisation of an anion introduce covalent character into an ionic compound?

When a cation polarises an anion, it distorts the anion's electron cloud, pulling electron density towards itself. This results in a degree of electron sharing between the ions, giving the bond covalent character even though the compound is formally ionic.

True or False? A larger anion is more easily polarised than a smaller anion.

True. A larger anion has its outer electrons further from the nucleus and more spread out , making the electron cloud more easily distorted by the electric field of a neighbouring cation.

A cation that is .......... and highly .......... will have the greatest polarising power because it has the highest charge density .

A cation that is small and highly charged will have the greatest polarising power because it has the highest charge density .

Define polarisability (of an anion).

Polarisability is the ease with which an anion's electron cloud can be distorted by a cation. It increases with ionic radius : larger anions are more polarisable because their outer electrons are further from the nucleus and less tightly held.

Why does the measured lattice energy of an ionic compound sometimes differ from the theoretical value?

The theoretical lattice energy assumes a perfect ionic model with purely spherical ions and entirely electrostatic interactions. When significant polarisation of the anion occurs, the bond gains covalent character , and the measured lattice energy (from Born-Haber cycle) is more exothermic than the theoretical value.

True or False? Na + has greater polarising power than Mg 2+ .

False. Mg 2+ has greater polarising power than Na + because, although both have similar ionic radii, Mg 2+ carries a higher charge , giving it a greater charge density and therefore greater ability to distort an anion.

Which has greater polarising power, Li + or K + , and why?

Li + has greater polarising power than K + because it has a smaller ionic radius and therefore a higher charge density . Both ions carry a 1+ charge, so the difference in polarising power is entirely due to the smaller size of Li + .

Define standard enthalpy change of solution .

Standard enthalpy change of solution (Δ H sol ꝋ ) is the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution . It can be exothermic (negative) or endothermic (positive).

True or False? Enthalpy changes of hydration are always exothermic.

True. Hydration enthalpies are always exothermic because energy is released when ion-dipole attractions form between the ions and the polar water molecules as the ions enter solution.

Define standard enthalpy change of hydration .

Standard enthalpy change of hydration (Δ H hyd ꝋ ) is the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution . Hydration enthalpies are always exothermic .

Δ H hyd ꝋ = Δ H latt ꝋ + Δ H sol ꝋ so Δ H sol ꝋ = .......... Δ H latt ꝋ + .......... Δ H hyd ꝋ

Δ H hyd ꝋ = Δ H latt ꝋ + Δ H sol ꝋ so Δ H sol ꝋ = reverse Δ H latt ꝋ + total Δ H hyd ꝋ

How does water interact with positive and negative ions during hydration?

Water is a polar molecule with a δ− oxygen and δ+ hydrogen atoms. The δ− oxygen is attracted to positive ions , while the δ+ hydrogens are attracted to negative ions , forming ion-dipole attractions that stabilise the ions in solution.

True or False? The standard enthalpy change of solution, Δ H sol ꝋ , is always endothermic.

False. Δ H sol ꝋ can be exothermic (negative) or endothermic (positive) depending on the relative magnitudes of the lattice energy and the total hydration enthalpies of the ions.

How is the Δ H hyd ꝋ of one ion calculated from an energy cycle?

Using Hess's law , the energy cycle gives: Δ H hyd ꝋ (unknown ion) = Δ H latt ꝋ + Δ H sol ꝋ − Δ H hyd ꝋ (known ion) All values are substituted with their signs and the equation is rearranged to find the unknown hydration enthalpy .

For MgCl 2 , the total Δ H hyd ꝋ is calculated as Δ H hyd ꝋ [Mg 2+ ] + .......... × Δ H hyd ꝋ [Cl - ] because there are .......... chloride ions per formula unit.

For MgCl 2 , the total Δ H hyd ꝋ is calculated as Δ H hyd ꝋ [Mg 2+ ] + 2 × Δ H hyd ꝋ [Cl - ] because there are two chloride ions per formula unit.

Why does Mg 2+ have a more negative Δ H hyd ꝋ than Ba 2+ ?

Mg 2+ has a smaller ionic radius than Ba 2+ , giving it a higher charge density . The greater charge density of Mg 2+ means it attracts polar water molecules more strongly, releasing more energy and producing a more exothermic hydration enthalpy.

A LevelChemistryEdexcelFlashcards5. Advanced Physical ChemistryEnergetics II: Born-Haber

Energetics II: Born-Haber (Edexcel A Level Chemistry): Flashcards

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Define lattice energy.

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Define lattice energy.
Lattice energy is the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions.
It is always exothermic (large negative value), as strong electrostatic forces of attraction are established between oppositely charged ions.
Why is lattice energy always exothermic when expressed as a formation process?
Lattice energy is always exothermic because forming an ionic lattice from gaseous ions releases a large amount of energy due to the strong electrostatic forces of attraction between the oppositely charged ions.
No such forces exist between ions in the gas phase, so the lattice is much more stable than the separated gaseous ions.
The lattice energy of NaCl involves forming .......... mole(s) of ionic solid from .......... Na⁺ (g) and Cl^- (g) ions, and its value is a large .......... number.
The lattice energy of NaCl involves forming 1 mole(s) of ionic solid from gaseous Na⁺ (g) and Cl^- (g) ions, and its value is a large negative number.
Define standard enthalpy change of atomisation.
Standard enthalpy change of atomisation (ΔH_at^ꝋ) is the enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions.
It is always endothermic (positive value) because energy must be supplied to break bonds and produce isolated gaseous atoms.
True or False?
The first electron affinity of an element is always exothermic.
False.
Most elements have an exothermic first electron affinity — energy is released when an electron is added to a neutral gaseous atom. However, group 2 metals (e.g. Be, Mg) and noble gases (e.g. Ne, Ar) have an endothermic first electron affinity, because the added electron must occupy a higher-energy subshell or a new principal quantum level.
Why can ΔH_latt^ꝋ not be determined directly from a single experiment?
ΔH_latt^ꝋ cannot be determined directly because it is not possible to carry out an experiment that converts gaseous ions directly into the ionic solid in one measurable step.
Instead, multiple experimental values are combined using an energy cycle (Born-Haber cycle) to find the lattice energy indirectly.
In the first electron affinity of chlorine, a gaseous Cl atom gains .......... electron(s) to form .......... (g), releasing energy because the electron is .......... to the atom.
In the first electron affinity of chlorine, a gaseous Cl atom gains one electron(s) to form Cl^- (g), releasing energy because the electron is attracted to the atom.
What does a more exothermic ΔH_latt^ꝋ value indicate about the ionic bonds in a lattice?
A more exothermic ΔH_latt^ꝋ value indicates that the ionic bonds within the lattice are stronger, meaning more energy is released when the gaseous ions come together to form the solid lattice.
Define Born-Haber cycle.
A Born-Haber cycle is a specific application of Hess's law for ionic compounds that allows the lattice energy, ΔH_latt^ꝋ, to be calculated indirectly using a series of measurable enthalpy changes.
What does it mean for energy to increase going upward on a Born-Haber cycle diagram?
Energy increases going upward on a Born-Haber cycle diagram means that endothermic processes (such as atomisation and ionisation) are shown with arrows pointing upward, while exothermic processes (such as electron affinity and lattice energy) point downward.
ΔH_f^ꝋ = ΔH₁^ꝋ + ΔH_latt^ꝋ, so ΔH_latt^ꝋ = .......... – ...........
ΔH_f^ꝋ = ΔH₁^ꝋ + ΔH_latt^ꝋ, so ΔH_latt^ꝋ = ΔH_f^ꝋ – ΔH₁^ꝋ.
How is ΔH_latt^ꝋ calculated from a Born-Haber cycle using Hess's law?
ΔH_latt^ꝋ is calculated by identifying the direct and indirect routes around the cycle and applying Hess's law:
ΔH_latt^ꝋ = ΔH_f^ꝋ – ΔH₁^ꝋ
where ΔH₁^ꝋ is the sum of all enthalpy changes needed to convert the elements in their standard states to gaseous ions.
True or False?
The standard enthalpy of formation of an ionic compound is always exothermic.
False.
The standard enthalpy of formation, ΔH_f^ꝋ, can be exothermic (negative) or endothermic (positive) depending on the compound. If ΔH_f^ꝋ is endothermic, the ionic compound sits above the elements on the Born-Haber energy diagram.
Why must the first electron affinity of chlorine be doubled when constructing the Born-Haber cycle for MgCl₂?
The first electron affinity of chlorine must be doubled in the MgCl₂ cycle because 2 moles of chlorine atoms each gain one electron to form 2 moles of Cl^- (g) ions.
In general, any enthalpy change involving a species present in more than one mole must be multiplied by the appropriate coefficient.
To create gaseous ions from elements in a Born-Haber cycle, the two steps are: (1) .......... the element to gaseous atoms and (2) .......... or remove electrons from the atoms.
To create gaseous ions from elements in a Born-Haber cycle, the two steps are: (1) atomise the element to gaseous atoms and (2) add or remove electrons from the atoms.
In what direction are bond-breaking steps drawn on a Born-Haber cycle diagram, and why?
Bond-breaking steps are drawn with arrows pointing upward because they are endothermic processes, requiring an input of energy.
This includes atomisation and ionisation energy steps, both of which involve separating or removing electrons from atoms.
True or False?
A Born-Haber cycle can only be used to calculate lattice energy.
False.
A Born-Haber cycle can be used to calculate any enthalpy change in the cycle, such as the enthalpy of formation, ionisation energy, or electron affinity, provided all other values are given.
Define polarising power.
Polarising power is the ability of a cation to distort the electron cloud of an adjacent anion, introducing covalent character into an otherwise ionic bond.
It increases with greater charge density (small ionic radius and/or high charge).
Why does polarisation of an anion introduce covalent character into an ionic compound?
When a cation polarises an anion, it distorts the anion's electron cloud, pulling electron density towards itself.
This results in a degree of electron sharing between the ions, giving the bond covalent character even though the compound is formally ionic.
True or False?
A larger anion is more easily polarised than a smaller anion.
True.
A larger anion has its outer electrons further from the nucleus and more spread out, making the electron cloud more easily distorted by the electric field of a neighbouring cation.
A cation that is .......... and highly .......... will have the greatest polarising power because it has the highest charge density.
A cation that is small and highly charged will have the greatest polarising power because it has the highest charge density.
Define polarisability (of an anion).
Polarisability is the ease with which an anion's electron cloud can be distorted by a cation.
It increases with ionic radius: larger anions are more polarisable because their outer electrons are further from the nucleus and less tightly held.
Why does the measured lattice energy of an ionic compound sometimes differ from the theoretical value?
The theoretical lattice energy assumes a perfect ionic model with purely spherical ions and entirely electrostatic interactions.
When significant polarisation of the anion occurs, the bond gains covalent character, and the measured lattice energy (from Born-Haber cycle) is more exothermic than the theoretical value.
True or False?
Na⁺ has greater polarising power than Mg²⁺.
False.
Mg²⁺ has greater polarising power than Na⁺ because, although both have similar ionic radii, Mg²⁺ carries a higher charge, giving it a greater charge density and therefore greater ability to distort an anion.
Which has greater polarising power, Li⁺ or K⁺, and why?
Li⁺ has greater polarising power than K⁺ because it has a smaller ionic radius and therefore a higher charge density.
Both ions carry a 1+ charge, so the difference in polarising power is entirely due to the smaller size of Li⁺.
Define standard enthalpy change of solution.
Standard enthalpy change of solution (ΔH_sol^ꝋ) is the enthalpy change when 1 mole of an ionic substance dissolves in sufficient water to form an infinitely dilute solution.
It can be exothermic (negative) or endothermic (positive).
True or False?
Enthalpy changes of hydration are always exothermic.
True.
Hydration enthalpies are always exothermic because energy is released when ion-dipole attractions form between the ions and the polar water molecules as the ions enter solution.
Define standard enthalpy change of hydration.
Standard enthalpy change of hydration (ΔH_hyd^ꝋ) is the enthalpy change when 1 mole of a specified gaseous ion dissolves in sufficient water to form an infinitely dilute solution.
Hydration enthalpies are always exothermic.
ΔH_hyd^ꝋ = ΔH_latt^ꝋ + ΔH_sol^ꝋ
so ΔH_sol^ꝋ = .......... ΔH_latt^ꝋ + .......... ΔH_hyd^ꝋ
ΔH_hyd^ꝋ = ΔH_latt^ꝋ + ΔH_sol^ꝋ
so ΔH_sol^ꝋ = reverse ΔH_latt^ꝋ + total ΔH_hyd^ꝋ
How does water interact with positive and negative ions during hydration?
Water is a polar molecule with a δ− oxygen and δ+ hydrogen atoms.
The δ− oxygen is attracted to positive ions, while the δ+ hydrogens are attracted to negative ions, forming ion-dipole attractions that stabilise the ions in solution.
True or False?
The standard enthalpy change of solution, ΔH_sol^ꝋ, is always endothermic.
False.
ΔH_sol^ꝋ can be exothermic (negative) or endothermic (positive) depending on the relative magnitudes of the lattice energy and the total hydration enthalpies of the ions.
How is the ΔH_hyd^ꝋ of one ion calculated from an energy cycle?
Using Hess's law, the energy cycle gives:
ΔH_hyd^ꝋ (unknown ion) = ΔH_latt^ꝋ + ΔH_sol^ꝋ − ΔH_hyd^ꝋ (known ion)
All values are substituted with their signs and the equation is rearranged to find the unknown hydration enthalpy.
For MgCl₂, the total ΔH_hyd^ꝋ is calculated as ΔH_hyd^ꝋ[Mg²⁺] + .......... × ΔH_hyd^ꝋ[Cl^-] because there are .......... chloride ions per formula unit.
For MgCl₂, the total ΔH_hyd^ꝋ is calculated as ΔH_hyd^ꝋ[Mg²⁺] + 2 × ΔH_hyd^ꝋ[Cl^-] because there are two chloride ions per formula unit.
Why does Mg²⁺ have a more negative ΔH_hyd^ꝋ than Ba²⁺?
Mg²⁺ has a smaller ionic radius than Ba²⁺, giving it a higher charge density.
The greater charge density of Mg²⁺ means it attracts polar water molecules more strongly, releasing more energy and producing a more exothermic hydration enthalpy.

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