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Ch.14 - Chemical Kinetics
All textbooksTro 4th EditionCh.14 - Chemical KineticsProblem 102a
Chapter 14, Problem 102a

Consider the two reactions:
O + N2 → NO + N Ea = 315 kJ/mol
Cl + H2 → HCl + H Ea = 23 kJ/mol
a. Why is the activation barrier for the first reaction so much higher than that for the second?

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Identify the two reactions and their activation energies: Reaction 1: O + N_2 \rightarrow NO + N with E_a = 315 \text{ kJ/mol}, Reaction 2: Cl + H_2 \rightarrow HCl + H with E_a = 23 \text{ kJ/mol}.
Understand that activation energy (E_a) is the minimum energy required for a reaction to occur. It represents the energy barrier that must be overcome for reactants to transform into products.
Consider the bond strengths involved in each reaction. In Reaction 1, breaking the triple bond in N_2 requires a significant amount of energy, contributing to a higher activation energy.
In Reaction 2, the bond between H_2 is weaker compared to the N_2 bond, and the formation of HCl is highly exothermic, resulting in a lower activation energy.
Conclude that the higher activation energy in Reaction 1 is due to the strong N_2 bond, which requires more energy to break, compared to the weaker H_2 bond in Reaction 2.

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Key Concepts

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

Activation Energy (E<sub>a</sub>)

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. A higher activation energy indicates that the reaction is less likely to occur at a given temperature, as fewer molecules will have sufficient energy to surpass this barrier.
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Reaction Mechanism

A reaction mechanism is a step-by-step description of the pathway taken during a chemical reaction. It includes the sequence of elementary steps that lead to the formation of products from reactants. The complexity of the mechanism can influence the activation energy, as more steps may involve higher energy transitions.
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Bond Strength and Stability

The strength of bonds in reactants and products affects the activation energy of a reaction. Stronger bonds require more energy to break, leading to higher activation energies. In the given reactions, the nature of the bonds formed and broken (e.g., O-N vs. Cl-H) contributes to the differences in activation barriers, as stronger bonds typically correlate with higher energy requirements.
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Related Practice
Textbook Question

The reaction 2 N2O5 → 2 N2O4 + O2 takes place at around room temperature in solvents such as CCl4. The rate constant at 293 K is found to be 2.35⨉10-4 s-1, and at 303 K the rate constant is found to be 9.15⨉10-4 s-1. Calculate the frequency factor for the reaction.

Textbook Question

This reaction has an activation energy of zero in the gas phase: CH3 + CH3 → C2H6 a. Would you expect the rate of this reaction to change very much with temperature?

Textbook Question

Consider the two reactions:

O + N2 → NO + N Ea = 315 kJ/mol

Cl + H2 → HCl + H Ea = 23 kJ/mol

b. The frequency factors for these two reactions are very close to each other in value. Assuming that they are the same, calculate the ratio of the reaction rate constants for these two reactions at 25 °C.

Textbook Question

Anthropologists can estimate the age of a bone or other sample of organic matter by its carbon-14 content. The carbon-14 in a living organism is constant until the organism dies, after which carbon- 14 decays with first-order kinetics and a half-life of 5730 years. Suppose a bone from an ancient human contains 19.5% of the C-14 found in living organisms. How old is the bone?

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