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Ch.14 - Chemical Kinetics
All textbooksBrown 14th EditionCh.14 - Chemical KineticsProblem 122
Chapter 14, Problem 122

The rates of many atmospheric reactions are accelerated by the absorption of light by one of the reactants. For example, consider the reaction between methane and chlorine to produce methyl chloride and hydrogen chloride: Reaction 1: CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g). This reaction is very slow in the absence of light. However, Cl2(g) can absorb light to form Cl atoms: Reaction 2: Cl2(g) + hv → 2 Cl(g). Once the Cl atoms are generated, they can catalyze the reaction of CH4 and Cl2, according to the following proposed mechanism: Reaction 3: CH4(g) + Cl(g) → CH3(g) + HCl(g). Reaction 4: CH3(g) + Cl2(g) → CH3Cl(g) + Cl(g). The enthalpy changes and activation energies for these two reactions are tabulated as follows: Reaction ΔH (kJ/mol) Ea (kJ/mol); 3: +4, 17; 4: -109, 4. (c) By using bond enthalpies, estimate where the reactants, CH4(g) + Cl2(g), should be placed on your diagram in part (b). Use this result to estimate the value of Ea for the reaction CH4(g) + Cl2(g) → CH3(g) + HCl(g) + Cl(g).

Verified step by step guidance
1
Identify the bonds broken and formed in the reaction CH4(g) + Cl2(g) → CH3(g) + HCl(g) + Cl(g). The bonds broken are one C-H bond in CH4 and one Cl-Cl bond in Cl2. The bonds formed are one C-Cl bond in CH3Cl and one H-Cl bond in HCl.
Use bond enthalpy values to calculate the total energy required to break the bonds in the reactants. For example, the bond enthalpy for a C-H bond is approximately 413 kJ/mol and for a Cl-Cl bond is approximately 242 kJ/mol.
Calculate the total energy released by forming the bonds in the products. The bond enthalpy for a C-Cl bond is approximately 328 kJ/mol and for an H-Cl bond is approximately 431 kJ/mol.
Determine the overall enthalpy change (ΔH) for the reaction by subtracting the total energy of the bonds formed from the total energy of the bonds broken. This will give you an estimate of the enthalpy change for the reaction.
Use the enthalpy change and the activation energy of the individual steps to estimate the activation energy (Ea) for the overall reaction. Consider that the activation energy is the energy barrier that must be overcome for the reaction to proceed, and it is typically higher than the enthalpy change for an endothermic step.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Bond Enthalpy

Bond enthalpy refers to the amount of energy required to break a bond between two atoms in a molecule. It is a crucial concept in thermochemistry, as it helps predict the stability of molecules and the energy changes during chemical reactions. By calculating the bond enthalpies of the reactants and products, one can estimate the overall energy change (ΔH) for a reaction, which is essential for understanding reaction mechanisms and kinetics.
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Enthalpy of Formation

Activation Energy (Ea)

Activation energy is the minimum energy required for a chemical reaction to occur. It represents the energy barrier that reactants must overcome to transform into products. In the context of the given reactions, understanding Ea helps in estimating how light absorption by Cl<sub>2</sub> lowers the energy barrier, thus accelerating the reaction with methane. A lower Ea typically indicates a faster reaction rate.
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Reaction Mechanism

A reaction mechanism is a step-by-step description of the pathway taken during a chemical reaction, detailing the sequence of elementary steps that lead to the formation of products. In the provided example, the mechanism involves the generation of Cl atoms from Cl<sub>2</sub> and their subsequent reactions with CH<sub>4</sub> and Cl<sub>2</sub>. Understanding the mechanism is vital for predicting reaction rates and the influence of light on the overall process.
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Related Practice
Textbook Question

The gas-phase reaction of NO with F2 to form NOF and F has an activation energy of Ea = 6.3 kJ>mol. and a frequency factor of A = 6.0 * 108 M-1 s-1. The reaction is believed to be bimolecular: NO1g2 + F21g2 ¡ NOF1g2 + F1g2 (e) Suggest a reason for the low activation energy for the reaction.

Textbook Question

The mechanism for the oxidation of HBr by O2 to form 2 H2O and Br2 is shown in Exercise 14.74. (a) Calculate the overall standard enthalpy change for the reaction process.

Textbook Question

The mechanism for the oxidation of HBr by O2 to form 2 H2O and Br2 is shown in Exercise 14.74. (c) Draw a plausible Lewis structure for the intermediate HOOBr. To what familiar compound of hydrogen and oxygen does it appear similar?

Textbook Question

The rates of many atmospheric reactions are accelerated by the absorption of light by one of the reactants. For example, consider the reaction between methane and chlorine to produce methyl chloride and hydrogen chloride:

Reaction 1: CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g)

This reaction is very slow in the absence of light. However, Cl2(g) can absorb light to form Cl atoms:

Reaction 2: Cl2(g) + hv → 2 Cl(g)

Once the Cl atoms are generated, they can catalyze the reaction of CH4 and Cl2, according to the following proposed mechanism:

Reaction 3: CH4(g) + Cl(g) → CH3(g) + HCl(g)

Reaction 4: CH3(g) + Cl2(g) → CH3Cl(g) + Cl(g)

The enthalpy changes and activation energies for these two reactions are tabulated as follows:

Reaction ΔH° (kJ/mol) Ea (kJ/mol)

3 +4 17

4 -109 4 

(b) By using the data tabulated here, sketch a quantitative energy profile for the catalyzed reaction represented by reactions 3 and 4.

Textbook Question

Many primary amines, RNH2, where R is a carboncontaining fragment such as CH3, CH3CH2, and so on, undergo reactions where the transition state is tetrahedral. (a) Draw a hybrid orbital picture to visualize the bonding at the nitrogen in a primary amine (just use a C atom for 'R').