Topic 13 – Energetics II
In order to develop their practical skills, students should be encouraged to carry out a range of practical experiments related to this topic. Possible
experiments include measuring enthalpy change of solution, investigating enthalpy and entropy changes in reactions such as neutralisations.
Mathematical skills that could be developed in this topic include calculating the missing value from a Born-Haber cycle using algebraic expressions,
using natural logarithms when calculating an equilibrium constant from ΔG. Within this topic, students can consider how chemists evaluate
theoretical models by comparing the real and ideal properties of chemicals, for example in the study of theoretical and experimental lattice energies.
The study of entropy shows students how chemists use formal, abstract thinking to answer fundamental questions about the stability of chemicals
and the direction of chemical change.
Topic 13A: Lattice Energy
1. be able to define lattice energy as…
What is the definition of lattice energy?
Lattice energy is the energy change when one mole of an ionic solid is formed from its gaseous ions under standard
conditions of 298K and 100kPa.
2. be able to define the terms:
i enthalpy change of atomisation, ΔatH
Define ‘enthalpy change of atomisation’.
Enthalpy change of atomisation is the energy change when one mole of gaseous atoms is formed from
the element in its standard state – e.g. 0.5Cl2 (g) -> Cl (g).
ii electron affinity
What is the definition of electron affinity?
The energy change when one mole of a gaseous atom gains one mole of electrons to form one mole of
gaseous 1- ions (for 1st electron affinity).
3. be able to construct Born-Haber cycles and carry
out related calculations – FAIL acronym
Calculate the lattice energy for NaCl.
Elattice = -(-349) – 496 – 107 – 122 + (-411) = -
787kJ/mol
Given the energies of the steps needed, calculate the lattice energy of MgCl 2.
Enthalpy of atomisation of magnesium +150
Enthalpy of atomisation of chlorine +107
1st ionisation energy of magnesium +737
2nd ionisation energy of magnesium +1449
1st electron affinity of chlorine -314
Enthalpy of formation of MgCl2 -367
Elattice = –2( –314) – 1449 – 737 – 2(107) – 150 – 367 = -2289kJ/mol
, 4. know that lattice energy provides a measure of ionic bond strength
What does lattice energy measure?
The lattice energy for an ionic bond represents its ionic bond strength. The more exothermic a lattice energy is, the
stronger the ionic bond.
Place the following compounds in order of decreasing magnitude of lattice energy, and explain your reasoning:
magnesium oxide, aluminium oxide, calcium oxide.
Al2O3 has the highest lattice energy as Al3+ has the highest charge stated (3+). The cation also has a smaller ionic
radius than Mg/Ca2+. MgO has the next highest lattice energy, and CaO has the lowest lattice energy as Ca2+ has
more electron shells than Mg2+, causing more electron shielding and giving it a larger ionic radius than Mg2+.
5. understand that a comparison of the experimental lattice energy value (from a Born-Haber cycle) with the
theoretical value (obtained from electrostatic theory) in a particular compound indicates the degree of
covalent bonding
From the table below, comment on the Compoun Experimental Lattice Energy Theoretical Lattice Energy
theoretical and experimental lattice energy d
values, giving reasons for any differences and NaCl -777kJ/mol -766kJ/mol
similarities. MgI2 -2293kJ/mol -1944kJ/mol
NaCl has very similar experimental and theoretical lattice energy values, so is almost 100% ionic (very little covalent character). MgI 2 has a
large difference between the two values, so has some covalent character.
This is because the iodide ion is more polarisable than the chloride ion, as the iodide ion is larger. Also because Mg 2+
is smaller and more highly charged than Na+, so Mg2+ is more polarising than Na+.
6. understand the meaning of polarisation as applied to ions
State the difference between polarising power and polarisability.
Polarising power is the ability of a cation to distort the electron cloud of an anion.
Polarisability is the ability of an anion to have its electron cloud distorted by a cation.
7. know that the polarising power of a cation depends on its radius and charge
What makes a cation more polarising?
Having a small radius and a very positive charge – e.g. Al3+.
8. know that the polarisability of an anion depends on its radius and charge
What makes an anion more polarisable?
Having a large radius and a very negative charge – e.g. O2-.
9a. be able to define the term ‘enthalpy change of solution, ΔsolH’
What is the definition of enthalpy change of solution?
ΔHsolution is the enthalpy change when one mole of a compound dissolves to form an infinitely dilute solution – e.g.
NaCl(s) + aq -> Na+(aq) + Cl-(aq).
An infinitely dilute solution is one where there is so much water that there is no further energy change if more water
is added.
2
In order to develop their practical skills, students should be encouraged to carry out a range of practical experiments related to this topic. Possible
experiments include measuring enthalpy change of solution, investigating enthalpy and entropy changes in reactions such as neutralisations.
Mathematical skills that could be developed in this topic include calculating the missing value from a Born-Haber cycle using algebraic expressions,
using natural logarithms when calculating an equilibrium constant from ΔG. Within this topic, students can consider how chemists evaluate
theoretical models by comparing the real and ideal properties of chemicals, for example in the study of theoretical and experimental lattice energies.
The study of entropy shows students how chemists use formal, abstract thinking to answer fundamental questions about the stability of chemicals
and the direction of chemical change.
Topic 13A: Lattice Energy
1. be able to define lattice energy as…
What is the definition of lattice energy?
Lattice energy is the energy change when one mole of an ionic solid is formed from its gaseous ions under standard
conditions of 298K and 100kPa.
2. be able to define the terms:
i enthalpy change of atomisation, ΔatH
Define ‘enthalpy change of atomisation’.
Enthalpy change of atomisation is the energy change when one mole of gaseous atoms is formed from
the element in its standard state – e.g. 0.5Cl2 (g) -> Cl (g).
ii electron affinity
What is the definition of electron affinity?
The energy change when one mole of a gaseous atom gains one mole of electrons to form one mole of
gaseous 1- ions (for 1st electron affinity).
3. be able to construct Born-Haber cycles and carry
out related calculations – FAIL acronym
Calculate the lattice energy for NaCl.
Elattice = -(-349) – 496 – 107 – 122 + (-411) = -
787kJ/mol
Given the energies of the steps needed, calculate the lattice energy of MgCl 2.
Enthalpy of atomisation of magnesium +150
Enthalpy of atomisation of chlorine +107
1st ionisation energy of magnesium +737
2nd ionisation energy of magnesium +1449
1st electron affinity of chlorine -314
Enthalpy of formation of MgCl2 -367
Elattice = –2( –314) – 1449 – 737 – 2(107) – 150 – 367 = -2289kJ/mol
, 4. know that lattice energy provides a measure of ionic bond strength
What does lattice energy measure?
The lattice energy for an ionic bond represents its ionic bond strength. The more exothermic a lattice energy is, the
stronger the ionic bond.
Place the following compounds in order of decreasing magnitude of lattice energy, and explain your reasoning:
magnesium oxide, aluminium oxide, calcium oxide.
Al2O3 has the highest lattice energy as Al3+ has the highest charge stated (3+). The cation also has a smaller ionic
radius than Mg/Ca2+. MgO has the next highest lattice energy, and CaO has the lowest lattice energy as Ca2+ has
more electron shells than Mg2+, causing more electron shielding and giving it a larger ionic radius than Mg2+.
5. understand that a comparison of the experimental lattice energy value (from a Born-Haber cycle) with the
theoretical value (obtained from electrostatic theory) in a particular compound indicates the degree of
covalent bonding
From the table below, comment on the Compoun Experimental Lattice Energy Theoretical Lattice Energy
theoretical and experimental lattice energy d
values, giving reasons for any differences and NaCl -777kJ/mol -766kJ/mol
similarities. MgI2 -2293kJ/mol -1944kJ/mol
NaCl has very similar experimental and theoretical lattice energy values, so is almost 100% ionic (very little covalent character). MgI 2 has a
large difference between the two values, so has some covalent character.
This is because the iodide ion is more polarisable than the chloride ion, as the iodide ion is larger. Also because Mg 2+
is smaller and more highly charged than Na+, so Mg2+ is more polarising than Na+.
6. understand the meaning of polarisation as applied to ions
State the difference between polarising power and polarisability.
Polarising power is the ability of a cation to distort the electron cloud of an anion.
Polarisability is the ability of an anion to have its electron cloud distorted by a cation.
7. know that the polarising power of a cation depends on its radius and charge
What makes a cation more polarising?
Having a small radius and a very positive charge – e.g. Al3+.
8. know that the polarisability of an anion depends on its radius and charge
What makes an anion more polarisable?
Having a large radius and a very negative charge – e.g. O2-.
9a. be able to define the term ‘enthalpy change of solution, ΔsolH’
What is the definition of enthalpy change of solution?
ΔHsolution is the enthalpy change when one mole of a compound dissolves to form an infinitely dilute solution – e.g.
NaCl(s) + aq -> Na+(aq) + Cl-(aq).
An infinitely dilute solution is one where there is so much water that there is no further energy change if more water
is added.
2
