Problem 82 - Chemistry The Central Science In Si Units

Question

Without doing any calculations, predict the sign of \(\Delta H\) for each of the following reactions: (a) \(2 \mathrm{NO}_{2}(g) \longrightarrow \mathrm{N}_{2} \mathrm{O}_{4}(g)\) (b) \(2 \mathrm{~F}(g) \longrightarrow \mathrm{F}_{2}(g)\) (c) \(\mathrm{Mg}^{2+}(g)+2 \mathrm{Cl}^{-}(g) \longrightarrow \mathrm{MgCl}_{2}(s)\) (d) \(\operatorname{HBr}(g) \longrightarrow H(g)+B r(g)\)

Step-by-Step Solution

Answer

(a) Negative, (b) Negative, (c) Negative, (d) Positive

Step 1: Understanding Reaction Type for (a)

Reaction (a) is the dimerization of nitrogen dioxide to form dinitrogen tetroxide, \[2 \mathrm{NO}_{2}(g) \longrightarrow \mathrm{N}_{2} \mathrm{O}_{4}(g)\]. This process is an example of a synthesis reaction, where two molecules are coming together to form a larger, more complex molecule. Such reactions tend to have negative enthalpies (\(\Delta H < 0\)) as they typically release energy when forming new bonds.

Step 2: Analyzing Bond Formation for (b)

Reaction (b) involves the formation of a diatomic molecule from its separate atoms, \[2 \mathrm{~F}(g) \longrightarrow \mathrm{F}_{2}(g)\]. Since bonds between atoms are being formed, energy is usually released in such processes, resulting in a negative enthalpy change (\(\Delta H < 0\)).

Step 3: Understanding Ionic Compound Formation for (c)

Reaction (c) shows the formation of an ionic solid from its gaseous ions, \[\mathrm{Mg}^{2+}(g)+2 \mathrm{Cl}^{-}(g) \longrightarrow \mathrm{MgCl}_{2}(s).\] The creation of an ionic lattice structure liberates a significant amount of energy as oppositely charged ions attract each other, making it an exothermic process with \(\Delta H < 0\).

Step 4: Identifying Decomposition Reaction for (d)

Reaction (d) involves the dissociation of a hydrogen bromide molecule into separate atoms, \[\operatorname{HBr}(g) \longrightarrow H(g)+Br(g)\]. Breaking chemical bonds requires energy, often absorbed from the surroundings, leading to an endothermic process with \(\Delta H > 0\).

Key Concepts

Synthesis Reaction

In chemistry, a synthesis reaction (also known as a combination reaction) involves two or more simple substances combining to form a more complex substance. An example from the exercise is the reaction involving nitrogen dioxide to dinitrogen tetroxide:\[ 2 \mathrm{NO}_{2}(g) \longrightarrow \mathrm{N}_{2} \mathrm{O}_{4}(g) \]In this reaction, two molecules of nitrogen dioxide (\(\mathrm{NO}_{2}\)) come together to create one molecule of dinitrogen tetroxide (\(\mathrm{N}_{2}\mathrm{O}_{4}\)). Typically, these types of reactions are energetically favorable and release energy, resulting in a negative change in enthalpy \((\Delta H < 0)\). This is because forming one complex molecule tends to stabilize the system, making it energetically efficient. In synthesis reactions:

• Two or more reactants form one complex product.
• Energy is usually released during the process.
• Common in the formation of new compounds.

These characteristics make synthesis reactions crucial for building complex compounds in various chemical processes, including industrial and biological systems.

Bond Formation

Bond formation is a fundamental concept in chemistry that involves the association of atoms to form molecules. In reaction (b):\[ 2 \mathrm{~F}(g) \longrightarrow \mathrm{F}_{2}(g) \]Two fluorine atoms combine to form a single fluorine molecule \((\mathrm{F}_{2})\). The process of bond formation between atoms involves the release of energy, typically resulting in a negative change in enthalpy \((\Delta H < 0)\). This is due to the fact that bonded atoms are in a more stable energy state compared to individual, unbonded atoms.In bond formation:

• Bonds between atoms are formed.
• Energy is released leading to a stable configuration.
• The reaction is often exothermic as the system sheds energy.

This exothermic nature helps dictate the stability and existence of molecular compounds and is a key process in chemical reactions involving diatomic molecules.

Ionic Compound Formation

Ionic compound formation is a process where ions come together to form a structured compound with a lattice arrangement. In reaction (c):\[ \mathrm{Mg}^{2+}(g) + 2 \mathrm{Cl}^{-}(g) \longrightarrow \mathrm{MgCl}_{2}(s) \]The magnesium ion (\(\mathrm{Mg}^{2+}\)) and chloride ions (\(\mathrm{Cl}^{-}\)) combine to form solid magnesium chloride (\(\mathrm{MgCl_{2}}\)). When oppositely charged ions come into contact, they attract each other strongly, releasing substantial amounts of energy as they form an ionic lattice. The energy given off when the lattice structure forms generally results in a negative enthalpy change \((\Delta H < 0)\), indicating an exothermic reaction, meaning that:

• Ions combine to form a solid lattice.
• Significant energy is released in the process.
• This energy release makes the reaction exothermic.

This stability through energy release and strong ionic bonds is why ionic compounds tend to have high melting and boiling points and are fundamental to the structure of many materials.

Decomposition Reaction

A decomposition reaction involves a single compound breaking down into two or more simpler substances, often requiring the input of energy. This can be seen in reaction (d):\[ \operatorname{HBr}(g) \longrightarrow H(g) + Br(g) \]Here, hydrogen bromide gas breaks apart into hydrogen and bromine atoms. This process requires energy to overcome the strong bond between hydrogen and bromine, resulting in a positive enthalpy change \((\Delta H > 0)\), indicating an endothermic reaction. Decomposition reactions are characterized by:

• A single substance breaking down into simpler components.
• The absorption of energy is required to break bonds.
• These reactions are generally endothermic.

Understanding decomposition is important for fields like environmental science and chemical manufacturing, where breaking down complex substances into simpler forms is essential.