Problem 80 - Introductory Chemistry Atoms First

Question

Indicate which bonds are broken and which are formed when each reaction occurs: (a) \(\mathrm{N}_{2}+3 \mathrm{Br}_{2} \rightarrow 2 \mathrm{NBr}_{3}\) (b) \(\mathrm{P}_{4}+6 \mathrm{H}_{2} \rightarrow 4 \mathrm{PH}_{3}\) (c) \(\mathrm{KF}+\mathrm{NaI} \rightarrow \mathrm{KI}+\mathrm{NaF}\) (d) \(\mathrm{CH}_{4}+\mathrm{Cl}_{2} \rightarrow \mathrm{CH}_{3} \mathrm{Cl}+\mathrm{HCl}\)

Step-by-Step Solution

Answer

(a) Broken: 1 N-N triple bond, 3 Br-Br single bonds. Formed: 6 N-Br single bonds. (b) Broken: 6 P-P single bonds, 6 H-H single bonds. Formed: 12 P-H single bonds. (c) Broken: 1 K-F ionic bond, 1 Na-I ionic bond. Formed: 1 K-I ionic bond, 1 Na-F ionic bond. (d) Broken: 1 C-H single bond, 1 Cl-Cl single bond. Formed: 1 C-Cl single bond, 1 H-Cl single bond.

Step 1: Identify reactants and products

The reactants in this reaction are N₂ and Br₂, and the product is NBr₃.

Step 2: Determine broken bonds

The N - N triple bond in N₂ and three Br - Br single bonds in Br₂ are broken.

Step 3: Determine formed bonds

Six N - Br single bonds are formed in the two NBr₃ molecules. (b) \(\mathrm{P}_{4}+6 \mathrm{H}_{2} \rightarrow 4 \mathrm{PH}_{3}\)

Step 4: Identify reactants and products

The reactants in this reaction are P₄ and H₂, and the product is PH₃.

Step 5: Determine broken bonds

Six P - P single bonds in P₄ and six H - H single bonds in H₂ are broken.

Step 6: Determine formed bonds

Twelve P - H single bonds are formed in the four PH₃ molecules. (c) \(\mathrm{KF}+\mathrm{NaI} \rightarrow \mathrm{KI}+\mathrm{NaF}\)

Step 7: Identify reactants and products

The reactants in this reaction are KF and NaI, and the products are KI and NaF.

Step 8: Determine broken bonds

One K - F ionic bond in KF and one Na - I ionic bond in NaI are broken.

Step 9: Determine formed bonds

One K - I ionic bond in KI and one Na - F ionic bond in NaF are formed. (d) \(\mathrm{CH}_{4}+\mathrm{Cl}_{2} \rightarrow \mathrm{CH}_{3} \mathrm{Cl}+\mathrm{HCl}\)

Step 10: Identify reactants and products

The reactants in this reaction are CH₄ and Cl₂, and the products are CH₃Cl and HCl.

Step 11: Determine broken bonds

One C - H single bond in CH₄ and one Cl - Cl single bond in Cl₂ are broken.

Step 12: Determine formed bonds

One C - Cl single bond in CH₃Cl and one H - Cl single bond in HCl are formed.

Key Concepts

Bond Breaking

Bond breaking is a fundamental part of chemical reactions, and it requires energy. When reactions occur, certain bonds in the reactants must break to allow new bonds to form in the products. This process is known as bond dissociation. The energy needed is called the bond dissociation energy.
During a reaction, different types of bonds may be broken, such as:

• Triple bonds like the N-N bond in nitrogen molecules (N₂).
• Single bonds like the Br-Br bond in bromine molecules (Br₂).
• Ionic bonds like K-F in potassium fluoride (KF).

Breaking these bonds involves overcoming the forces between atoms, which differ based on the bond type and strength. For instance, triple bonds are generally stronger and require more energy to break compared to single bonds.

Bond Formation

Bond formation occurs when new bonds are made between atoms, releasing energy in the form of bond formation energy. This is a crucial step because it stabilizes the atoms, resulting in the formation of new chemical substances, or products.
In the context of the given reactions:

• In reaction (a), N-Br single bonds form in the product NBr₃.
• In reaction (b), P-H single bonds form to create PH₃.
• In reaction (c), the formation of new ionic bonds like K-I in KI and Na-F in NaF occurs.
• In reaction (d), a C-Cl single bond is created in CH₃Cl, and an H-Cl bond forms in HCl.

The formation of these bonds often leads to a release of energy, making the process exothermic in many cases. The new bonds stabilize the atoms, creating less reactive products compared to the reactants.

Reactants and Products

Chemical reactions always involve reactants and products. Reactants are the starting substances that undergo change during the reaction. Products are the substances formed, showcasing new properties and bonds.
In the example reactions:

• Reaction (a) starts with N₂ and Br₂ as reactants and forms NBr₃ as the product.
• Reaction (b) has P₄ and H₂ as reactants, resulting in PH₃ as the product.
• Reaction (c) involves the reactants KF and NaI, producing KI and NaF as products.
• In reaction (d), CH₄ and Cl₂ are reactants transforming into CH₃Cl and HCl as products.

Each reaction highlights the transformation from reactants to products, emphasizing changes in bond arrangements. Identifying reactants and products helps in understanding the initial and final states of a reaction, crucial for predicting reaction outcomes.

Types of Chemical Bonds

Chemical bonds are interactions that hold atoms together in molecules. They come in various forms, each defining the stability and properties of a compound. Major types of chemical bonds include:

• Covalent bonds: Sharing of electrons between atoms, as seen in NBr₃ where nitrogen and bromine atoms form single covalent bonds.
• Ionic bonds: Transfer of electrons between atoms, forming charged ions, as seen in reactions involving KF and NaI.

Covalent bonds often occur between nonmetals with similar electronegativities, whereas ionic bonds form between metals and nonmetals with greater electronegativity differences.
Understanding these bond types aids in predicting how atoms interact and what energy changes occur during bond breaking and formation, key for comprehending any chemical reaction's mechanism.