Which of the following processes are exothermic? Which of the following processes are exothermic? Br (g) + e - → Br - (g) Li (s) → Li (g) NaF (s) → Na + (g) + F - (g) Cl2 (g) → 2Cl (g) None of the above are exothermic.

None of the above are exothermic.

Explanation:

Which of the following processes are exothermic?

Br (g) + e⁻ → Br⁻ (g)

When a gaseous atom gains an electron, the energy involved is known as electron affinity (EA). The EA of Br is 325 kJ/mol. The positive sign indicates that the process is endothermic.

Li (s) → Li (g)

When a solid atom sublimates into a gaseous atom, the energy involved is known as heat of sublimation. The heat of sublimation of Li is 159 kJ/mol. The positive sign indicates that the process is endothermic.

NaF (s) → Na⁺ (g) + F⁻ (g)

When a crystalline solid is separated in gaseous ions, the energy involved is known as lattice energy (Note: some authors call lattice energy to that of the opposite process). Using our definition, the lattice energy of NaF is 904 kJ/mol. The positive sign indicates that the process is endothermic.

Cl₂ (g) → 2 Cl (g)

When the bond in Cl₂ is broken to produce to gaseous atoms of Cl, the energy involved is known as dissociation energy. The dissociation energy of Cl₂ is 242 kJ/mol. The positive sign indicates that the process is endothermic.

The correct answer is: None of the above are exothermic.

Explanation:

Energy is the ability to perform a job or to produce heat. Chemical reactions involve a rearrangement of atoms between substances with rupture or formation of chemical bonds and this generates changes in the energy of the system.

An exothermic reaction is that reaction where heat is released. This means that the energy of the molecules of the products is less than the energy of the molecules of the reactants.

An endothermic reaction is that chemical reaction that, when it occurs, consumes energy, that is, the products obtained have greater energy than the initial reagents, since they have taken part of the heat of the environment.

Sublimation is the change of state of a substance from solid to gas directly without going through the liquid state. This process requires energy to be carried out because the strong attractive energies between the solid molecules must be broken. So it is an endothermic process. This is what happens in the following reactions:

Li (s) → Li (g)

NaF (s) → Na + (g) + F - (g)

Dissociation is a process in which complexes, molecules or salts are separated into smaller molecules, ions or radicals, usually reversibly. Within the dissociation is the homolytic rupture, where each atom separates and carries the same charge of electrons. Thus free radicals are formed. To dissociate or break these bonds requires energy, so it is endothermic. This type of reaction occurs in:

Cl2 (g) → 2Cl (g)

Electronic affinity is defined as the energy exchanged when an isolated and fundamental state gaseous atom takes an electron to form an anion (negative ion). When the electronic affinity is greater than zero, the process would be endothermic and when the electroaffinity is less than zero, the process would be exothermic.

This occurs in:

Br (g) + e - → Br - (g)

The highest electronic affinities are for the elements of group 17 of the periodic table, the group to which bromine belongs. This means that the electronic affinity is greater than zero and is endothermic.

The correct answer is: None of the above are exothermic.

Br (g) + e - → Br - (g)