Problem 106 - Introductory Chemistry Atoms First

Question

In each reaction, indicate which bonds are broken and which bonds are formed: (a) \(\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightarrow 2 \mathrm{NH}_{3}\) (b) \(\mathrm{PCl}_{5} \rightarrow \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\) (c) \(\mathrm{H}_{2}+2 \mathrm{ICl} \rightarrow 2 \mathrm{HCl}+\mathrm{I}_{2}\) (d) \(4 \mathrm{HBr}+\mathrm{O}_{2} \rightarrow 2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{Br}_{2}\)

Step-by-Step Solution

Answer

(a) Bonds broken: 1 N-N bond, 3 H-H bonds; Bonds formed: 6 N-H bonds (b) Bonds broken: 5 P-Cl bonds; Bonds formed: 3 P-Cl bonds, 1 Cl-Cl bond (c) Bonds broken: 1 H-H bond, 2 I-Cl bonds; Bonds formed: 2 H-Cl bonds, 1 I-I bond (d) Bonds broken: 4 H-Br bonds, 1 O=O bond; Bonds formed: 4 O-H bonds, 2 Br-Br bonds

Step 1: (a) Reaction: N2 + 3H2 -> 2NH3

In the reactant side, we have one N-N bond in the N2 molecule and three H-H bonds in the three H2 molecules. On the product side, we have two NH3 molecules, which have one N-H bond each. Bonds broken: 1. One N-N bond in N2 molecule 2. Three H-H bonds in H2 molecules Bonds formed: 1. Six N-H bonds in 2 NH3 molecules

Step 2: (b) Reaction: PCl5 -> PCl3 + Cl2

In the reactant side, we have one PCl5 molecule, which has five P-Cl bonds. On the product side, we have one PCl3 molecule, which has three P-Cl bonds, and one Cl-Cl bond in the Cl2 molecule. Bonds broken: 1. Five P-Cl bonds in PCl5 molecule Bonds formed: 1. Three P-Cl bonds in PCl3 molecule 2. One Cl-Cl bond in Cl2 molecule

Step 3: (c) Reaction: H2 + 2ICl -> 2HCl + I2

In the reactant side, we have one H-H bond in the H2 molecule and two I-Cl bonds in the two ICl molecules. On the product side, we have two HCl molecules (each with an H-Cl bond) and one I-I bond in the I2 molecule. Bonds broken: 1. One H-H bond in H2 molecule 2. Two I-Cl bonds in ICl molecules Bonds formed: 1. Two H-Cl bonds in 2 HCl molecules 2. One I-I bond in I2 molecule

Step 4: (d) Reaction: 4HBr + O2 -> 2H2O + 2Br2

In the reactant side, we have four H-Br bonds in the four HBr molecules and one O=O bond in the O2 molecule. On the product side, we have two H2O molecules (each with two O-H bonds) and two Br-Br bonds in the two Br2 molecules. Bonds broken: 1. Four H-Br bonds in HBr molecules 2. One O=O bond in O2 molecule Bonds formed: 1. Four O-H bonds in 2 H2O molecules 2. Two Br-Br bonds in 2 Br2 molecules

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, transform into different substances, called products. This transformation involves changing the arrangement of atoms and involves breaking old bonds and forming new ones. During a chemical reaction, the total mass and number of atoms are conserved, which means that what goes into a reaction as reactants always comes out as products. In essence, chemical reactions rearrange the molecular structure without losing or gaining atoms from the system.

• In the example of the Haber process, nitrogen (\(N_2\)) and hydrogen (\(H_2\)) are reactants which transform into ammonia (\(NH_3\)), showcasing bond breaking and formation.

Bond Energy

Bond energy is a measure of the strength of a chemical bond. It is the amount of energy required to break one mole of a type of bond in a molecule in the gaseous state. When bonds are formed, energy is released, while breaking bonds requires an input of energy.

• In the reaction \(N_2 + 3H_2 \rightarrow 2NH_3\), we need energy to break the N-N and H-H bonds and when new N-H bonds are formed, energy is released.
• This difference in energy is crucial for determining whether a reaction is endothermic (absorbs energy) or exothermic (releases energy).

Understanding bond energies helps predict the energy changes that accompany reactions and enables the calculation of reaction enthalpy.

Stoichiometry

Stoichiometry refers to the calculation of reactants and products in chemical reactions based on the balanced chemical equation. It provides the proportion in which substances react and form products. Knowing stoichiometry allows chemists to scale reactions up or down, predicting the amounts of substances consumed and produced.

• For example, in the reaction \(N_2 + 3H_2 \rightarrow 2NH_3\), the stoichiometry tells us that one molecule of nitrogen reacts with three molecules of hydrogen to form two molecules of ammonia.
• This understanding is crucial for practical applications, such as in industrial synthesis of chemicals where precise quantities of chemicals are necessary.

Molecular Structure

Molecular structure refers to the arrangement of atoms within a molecule and determines the molecule's properties and reactivity. The way atoms are bonded in a molecule defines its geometry and affects how it interacts with other molecules.

• In the case of \(NH_3\), its structure with N-H bonds contributes to its properties as a polar molecule.
• Understanding molecular structure is vital as it influences physical properties like boiling point and chemical reactivity.

Recognizing the importance of molecular structure helps in predicting how molecules will behave in different conditions and when involved in chemical reactions.