Problem 124 - Chemistry An Atoms First Approach

Question

For each of the following, write an equation that corresponds to the energy given. a. lattice energy of \(\mathrm{NaCl}\) b. lattice energy of \(\mathrm{NH}_{4} \mathrm{Br}\) c. lattice energy of \(\mathrm{MgS}\) d. \(O=O\) double bond energy beginning with \(O_{2}(g)\) as a reactant

Step-by-Step Solution

Answer

a. Lattice energy of NaCl: \[Na^+_{(g)} + Cl^-_{(g)} \rightarrow NaCl_{(s)}\] b. Lattice energy of NH4Br: \[NH_4^+_{(g)} + Br^-_{(g)} \rightarrow NH_4Br_{(s)}\] c. Lattice energy of MgS: \[Mg^{2+}_{(g)} + S^{2-}_{(g)} \rightarrow MgS_{(s)}\] d. O=O double bond energy beginning with O2(g) as a reactant: \[O_2 (g) \rightarrow 2O (g)\]

Step 1: a. Lattice Energy of NaCl

To write the equation for the lattice energy of NaCl, first, we need to convert the reactants (Na and Cl) into their gaseous ions and then form the ionic solid NaCl. So, the equation is: \[Na _{(g)} + e^- \rightarrow Na^+_{(g)}\] \[Cl_{(g)} \rightarrow Cl^-_{(g)} + e^-\] Now, combine these ions to form NaCl (solid): \[Na^+_{(g)} + Cl^-_{(g)} \rightarrow NaCl_{(s)}\]

Step 2: b. Lattice Energy of NH4Br

To write the equation for the lattice energy of NH4Br, follow a similar process as in part a: \[NH_4^+_{(g)} + Br^-_{(g)} \rightarrow NH_4Br_{(s)}\]

Step 3: c. Lattice Energy of MgS

To write the equation for the lattice energy of MgS, follow a similar process as in part a: \[Mg^{2+}_{(g)} + S^{2-}_{(g)} \rightarrow MgS_{(s)}\]

Step 4: d. O=O double bond energy beginning with O2(g) as a reactant

For the formation of the O=O double bond, we start with O2 gas as a reactant and break the O=O bond. The bond dissociation energy is the energy required to break a bond. \[O_2 (g) \rightarrow 2O (g)\] For each of these energy equations, the energy term (lattice energy or bond energy) could be added or subtracted depending on whether energy is released or absorbed in the reaction. But in these examples, the primary goal was to construct the equations representing these energies based on the given compounds and bonds.

Key Concepts

Ionic Bonding

Ionic bonding is a type of chemical linkage that involves the electrostatic attraction between oppositely charged ions. These ions are typically a metal donating one or more electrons to become a positively charged cation, and a non-metal accepting those electrons to become a negatively charged anion.

For instance, when sodium (Na), a metal, reacts with chlorine (Cl), a non-metal, sodium donates one electron to chlorine. This creates a sodium cation (Na^+_{(g)}) and a chloride anion (Cl^-_{(g)}), which are then attracted to each other due to opposite charges, forming sodium chloride (NaCl_{(s)}) through an ionic bond. The lattice energy associated with forming the crystalline structure of NaCl_{(s)} from these gaseous ions is a measure of the strength of this ionic bond.

Lattice Formation and Ionic Bonding

If we delve into the process of lattice formation, we can better understand the concept of ionic bonding. As the ions attract each other, they organize into a repeating three-dimensional pattern called a lattice. This structure maximizes the attractive forces between the ions while minimizing repulsive forces, leading to a stable compound. The amount of energy released when this lattice forms is reflected in the lattice energy, emphasizing the bond strength within the solid ionic compound.

Bond Dissociation Energy

The bond dissociation energy is a specific type of bond energy; it is the energy required to break a chemical bond in a molecule under standard conditions to form separated atoms in the gas phase. This is always a positive value, as energy must be expended to overcome the attraction between the bonded atoms.

Take, for example, the diatomic molecule oxygen (O_2). The molecule contains a double bond, denoted as O=O. To break this bond and form individual oxygen atoms (2O (g)), a certain amount of energy must be provided. This quantity is the bond dissociation energy, which is unique to each type of bond.

Measurement of Bond Energy

Bond dissociation energy is typically measured in kilojoules per mole (kJ/mol) or electronvolts (eV). A high bond dissociation energy indicates a strong bond, as more energy is needed to break it. Conversely, a low bond dissociation energy implies a weaker bond. Thus, bond dissociation energy is an essential concept when considering the stability of molecules and the amount of energy involved in chemical reactions.

Chemical Reaction Equations

Chemical reaction equations are symbolic representations of chemical reactions, displaying the reactants (substances initially present) and the products (substances generated by the reaction). To ensure mass balance, the same number of atoms for each element must be presented on both sides of the equation, abiding by the law of conservation of mass.

Considering the lattice energy examples provided, chemical equations illustrate the process of forming ionic solids such as NaCl_{(s)}, NH_4Br_{(s)}, and MgS_{(s)} from their respective gaseous ions. These equations serve a dual purpose by also highlighting energy changes—either absorbed or released—during the ion combination.

Significance of Reaction Equations in Energy Concepts

Chemical reaction equations not only provide details about the substances involved and their stoichiometry but also give insights into the energy changes occurring during the reaction. By including terms such as lattice energy or bond dissociation energy, we can further connect the equation to thermodynamical aspects. Therefore, understanding how to interpret these equations is vital for grasping the energy landscape of chemical processes.