Problem 72 - Chemistry The Central Science

Question

Use Table 8.4 to estimate the enthalpy change for each of the following reactions: (a) \(\mathrm{H}_{2} \mathrm{C}=\mathrm{O}(g)+\mathrm{HCl}(g) \longrightarrow \mathrm{H}_{3} \mathrm{C}-\mathrm{O}-\mathrm{Cl}(g)\) (b) \(\mathrm{H}_{2} \mathrm{O}_{2}(g)+2 \mathrm{CO}(g) \longrightarrow \mathrm{H}_{2}(g)+2 \mathrm{CO}_{2}(g)\) (c) \(3 \mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2}(g) \longrightarrow \mathrm{C}_{6} \mathrm{H}_{12}(g)\) (the six carbon atoms form a six-membered ring with two \(\mathrm{H}\) atoms on each \(\mathrm{C}\) atom \()\)

Step-by-Step Solution

Answer

Using Table 8.4 to find the bond enthalpies, we can calculate the enthalpy changes for each of the given reactions: (a) ΔH(a) = Σ Bond enthalpies of bonds formed in the products - Σ Bond enthalpies of bonds broken in the reactants (b) ΔH(b) = Σ Bond enthalpies of bonds formed in the products - Σ Bond enthalpies of bonds broken in the reactants (c) ΔH(c) = Σ Bond enthalpies of bonds formed in the products - Σ Bond enthalpies of bonds broken in the reactants The estimated enthalpy changes for each reaction are represented by ΔH(a), ΔH(b), and ΔH(c).

Step 1: Identify the bonds broken and formed in each reaction.

Look at each chemical reaction and identify the initial bonds that are broken in the reactants and the new bonds that are formed in the products. (a) $\mathrm{H}_{2} \mathrm{C}=\mathrm{O}(g)+\mathrm{HCl}(g) \longrightarrow \mathrm{H}_{3} \mathrm{C}-\mathrm{O}-\mathrm{Cl}(g)$ Bonds broken: H-C, C=O, and H-Cl Bonds formed: H-C, C-O, and O-Cl (b) $\mathrm{H}_{2} \mathrm{O}_{2}(g)+2 \mathrm{CO}(g) \longrightarrow \mathrm{H}_{2}(g)+2 \mathrm{CO}_{2}(g)$ Bonds broken: H-O, O-O, 2 C≡O Bonds formed: H-H, and 4 C=O (c) $3 \mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2}(g) \longrightarrow \mathrm{C}_{6} \mathrm{H}_{12}(g)$ (the six carbon atoms form a six-membered ring with two \(\mathrm{H}\) atoms on each \(\mathrm{C}\) atom \()\) Bonds broken: 6 H-C, 3 C=C Bonds formed: 12 H-C, 6 C-C

Step 2: Consult Table 8.4 to find the bond enthalpies.

Consult Table 8.4 to find the bond enthalpies for each bond broken and formed in the reaction.

Step 3: Calculate the total enthalpy changes.

For each reaction, calculate the total enthalpy change by subtracting the total bond enthalpy of the bonds broken from the total bond enthalpy of the bonds formed: (a) ΔH(a) = Σ Bond enthalpies of bonds formed in the products - Σ Bond enthalpies of bonds broken in the reactants (b) ΔH(b) = Σ Bond enthalpies of bonds formed in the products - Σ Bond enthalpies of bonds broken in the reactants (c) ΔH(c) = Σ Bond enthalpies of bonds formed in the products - Σ Bond enthalpies of bonds broken in the reactants

Step 4: Report the enthalpy changes for each reaction.

After calculating the enthalpy changes for each reaction using the data from Table 8.4, report the values for ΔH(a), ΔH(b), and ΔH(c). These values represent the estimated enthalpy change for each reaction.

Key Concepts

Bond Enthalpies

Bond enthalpies, also known as bond dissociation energies, are crucial to understanding enthalpy changes in chemical reactions. They represent the energy required to break a specific chemical bond in a molecule. This energy is often expressed in units of kilojoules per mole (kJ/mol). When calculating the enthalpy change of a reaction, we focus on the bonds that are broken and formed during the transformation of reactants into products. To accurately estimate these changes:

• Identify all the bonds in the reactants that must be broken.
• Determine the new bonds formed in the products.
• Use tables, such as Table 8.4, to find standard bond enthalpies for each bond type.

By assessing the difference between the total energy needed to break the bonds in the reactants and the energy released when forming bonds in the products, we can calculate the overall enthalpy change for the reaction. This step is key in predicting whether a reaction is endothermic (absorbs energy) or exothermic (releases energy).

Chemical Reactions

Chemical reactions involve the rearrangement of atoms to form new substances. During this process, molecular bonds are broken and new ones are formed. Each reaction has its unique set of initial and final bonds, which impacts the overall energy change. Here are some important points to grasp:

• Bonds like C=O in molecules are typically stronger, requiring more energy to break.
• Breaking bonds generally requires energy input, while forming bonds releases energy.
• The difference between energy absorbed in breaking bonds and energy released in making new bonds determines if a reaction is endothermic or exothermic.

Assessing a chemical reaction entails not just knowing which bonds are involved but also how shifts in atomic arrangements influence energy transitions. Understanding these principles helps predict the outcome and feasibility of reactions, important in fields like synthesis and thermodynamics.

Thermochemistry

Thermochemistry is the branch of chemistry concerned with the heat energy involved in chemical reactions and changes of state. This field focuses on understanding how thermal energy, typically measured as enthalpy, varies during a reaction.Key concepts in thermochemistry include:

• Enthalpy (H): A measure of total energy in a system, including internal and pressure-volume work. Enthalpies of reactions (\(\Delta H\)) indicate heat absorbed or released.
• Endothermic reactions: Absorb energy, resulting in a positive \(\Delta H\). Heat is absorbed from the surroundings.
• Exothermic reactions: Release energy, leading to a negative \(\Delta H\). Heat is expelled into the surroundings.

By using bond enthalpies to estimate changes, we can gauge the thermal characteristics of reactions. This knowledge is instrumental in designing chemical processes that are energy-efficient, eco-friendly, and economically feasible.