LiI(s) has a heat of formation of -272 kJ / mol and a lattice energy of -753 kJ / mol. The ion

LiI(s) has a heat of formation of -272 kJ / mol and a...

$\operatorname{LiI}(s)$ has a heat of formation of $-272 \mathrm{~kJ} / \mathrm{mol}$ and a lattice energy of $-753 \mathrm{~kJ} / \mathrm{mol}$. The ionization energy of $\mathrm{Li}(g)$ is 520 . $\mathrm{kJ} / \mathrm{mol}$, the bond energy of $\mathrm{I}_{2}(g)$ is $151 \mathrm{~kJ} / \mathrm{mol}$, and the electron affinity of $\mathrm{I}(g)$ is $-295 \mathrm{~kJ} / \mathrm{mol}$. Use these data to determine the heat of sublimation of $\operatorname{Li}(s)$.

Key Concepts

Born-Haber Cycle

A thermochemical cycle that relates the lattice energy of an ionic solid to the enthalpy of formation and the other energetic steps involved in forming that solid from its constituent elements. It uses Hess's law to combine various energy changes, such as ionization energy, electron affinity, sublimation energy, and bond dissociation energy, to determine unknown values.

Heat of Sublimation

The energy required to convert a substance from the solid phase to the gaseous phase. It is a key step in generating gaseous atoms from a solid element in the Born-Haber cycle, making it an important concept in thermochemical calculations.

Enthalpy of Formation

The change in enthalpy when one mole of a compound is formed from its elements in their standard states. This value serves as the overall energy change for the formation process and is central to calculating individual steps in the Born-Haber cycle.

Ionization Energy

The energy required to remove an electron from a gaseous atom, forming a cation. This concept is crucial in the formation of ionic compounds, as it represents one of the necessary steps for ion creation in the Born-Haber cycle.

Electron Affinity

The energy change that occurs when an electron is added to a gaseous atom, resulting in the formation of an anion. This concept complements ionization energy in the cycle and helps determine the overall energy balance in forming ionic compounds.

Lattice Energy

The energy released when gaseous ions combine to form an ionic solid. It is typically a large exothermic value and plays a significant role in stabilizing the ionic compound within the Born-Haber cycle.

Bond Energy

The energy required to break a chemical bond in a molecule, such as the bond in a diatomic molecule, to produce individual atoms. In the context of the Born-Haber cycle, it is used to account for the energy needed to dissociate molecular forms of elements into gaseous atoms.

Transcript

00:02 Okay, so in this problem we are asked to find the sublimation, the heat of sublimation for lithium.

00:10 And for solving this problem it's useful to look at the reaction of formation for the lithium iodide salt or solid.

00:20 So this is the reaction that we're going to use.

00:22 We start with solid lithium, we have iodine gas, and they will react to form the salt or the ionic solid, which is lithium iodide.

00:33 And we are also given the delta h or the enthalpy for the reaction, which is negative 272 kilojoules per mole.

00:42 So it is useful to think of this reaction as a series of steps that have to be accomplished to go from the reactants to the final product.

00:51 So let's look at what steps we need to accomplish for that.

00:55 The first thing that we need to do is turn that lithium solid into a gas.

01:00 Why? because the iodine is also in a gas phase, so they both need to be in the same phase, in the gas phase, for them to react.

01:08 And that first reaction, turning the lithium solid to the lithium gas, is basically the heat of sublimation that we are going for the problem, we're just going to represent by a delta h sub.

01:19 And this is actually the value that we need to find for the problem.

01:24 So we don't know the answer for this value right now.

01:26 The second step is we would have to take that lithium gas and ionize it, because remember that the final product is an ionic solid.

01:36 So we need to have a positive lithium ion and a negative iodide ion, an ion in this case, for them to merge and form the solid.

01:47 So we need to ionize that lithium gas neutral atom into a positive ion.

01:54 And that's basically the ionization energy for lithium, which is given by the problem.

02:01 And it's 520 kilojoules per mole.

02:05 This is the ionization energy for lithium.

02:09 The third step, or a separate step that we need to accomplish, is we need to turn that gas, which has a covalent bond, into a single iodine atom...

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