Textbook Question
The concentration of H2O in the stratosphere is about 5 ppm. It undergoes photodissociation according to: H2O(g) → H(g) + OH(g) (b) Using Table 8.3, calculate the wavelength required to cause this dissociation.
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Ch.18 - Chemistry of the Environment
Chapter 18, Problem 84
The standard enthalpies of formation of ClO and ClO2 are 101 and 102 kJ/mol, respectively. Using these data and the thermodynamic data in Appendix C, calculate the overall enthalpy change for each step in the following catalytic cycle: ClO(g) + O(g) → ClO(g) + O(g). What is the enthalpy change for the overall reaction that results from these two steps?
Verified step by step guidance1
Step 1: Identify the given data and the reaction. The problem provides the standard enthalpies of formation for ClO and ClO2 as 101 kJ/mol and 102 kJ/mol, respectively. The reaction given is ClO(g) + O(g) → ClO(g) + O(g).
Step 2: Recognize that the reaction provided is a balanced equation where the reactants and products are the same, indicating no net change in the chemical species involved.
Step 3: Understand that since the reactants and products are identical, the enthalpy change for this reaction is zero. This is because the enthalpy of formation of the reactants is equal to the enthalpy of formation of the products.
Step 4: Consider the concept of Hess's Law, which states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction is carried out in. Since the reaction does not change the chemical species, the enthalpy change remains zero.
Step 5: Conclude that the enthalpy change for the overall reaction, given the identical reactants and products, is zero. This is consistent with the principle that no net chemical change results in no enthalpy change.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enthalpy of Formation
The enthalpy of formation is the heat change that results when one mole of a compound is formed from its elements in their standard states. It is a crucial concept in thermodynamics, as it allows for the calculation of the overall enthalpy change in chemical reactions by using standard enthalpy values for reactants and products.
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Enthalpy of Formation
Thermodynamic Cycle
A thermodynamic cycle is a series of processes that return a system to its initial state, allowing for the calculation of changes in enthalpy or other thermodynamic properties. In the context of chemical reactions, it helps in understanding how the enthalpy changes during different steps of a reaction can be summed to find the overall change for the reaction.
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Hess's Law
Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps or the pathway taken. This principle is essential for calculating the overall enthalpy change in a reaction by summing the enthalpy changes of individual steps, which is particularly useful when direct measurement is difficult.
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Related Practice
Textbook Question
The concentration of H2O in the stratosphere is about 5 ppm. It undergoes photodissociation according to: H2O(g) → H(g) + OH(g)
(c) The hydroxyl radical, OH, can react with ozone, giving the following reactions:
OH(g) + O3(g) → HO2(g) + O2(g)
HO2(g) + O(g) → OH(g) + O2(g)
What overall reaction results from these two elementary reactions? What is the catalyst in the overall reaction? Explain.
Textbook Question
The following data were collected for the desturction of O3 by H (O3 + H → O2 + OH) at very low concentrations (b) Calculate the rate constant