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Chapter 5: Problem 41

Atomic hydrogen (H) is used in welding (AHW). The atoms recombine to hydrogen molecules with a large release of heat according to the following reaction: $$ 2 \mathrm{H}(g) \longrightarrow \mathrm{H}_{2}(g) $$ (a) Using the thermodynamic data in Appendix C, calculate the enthalpy change for this reaction per mole of \(\mathrm{H}_{2}\). (b) Which has the higher enthalpy under these conditions, \(2 \mathrm{H}(g)\) or \(\mathrm{H}_{2}(g) ?\)

Short Answer

Expert verified
The enthalpy change for the given reaction is \(-436 kJ/mol\), and under these conditions, 2 H(g) has a higher enthalpy than H₂(g).

Step by step solution

01

Find the enthalpy change for the reaction

First, we need to find the enthalpy change for the reaction using the thermodynamic data provided in Appendix C. The enthalpy change for the reaction can be written as: \(ΔH_{reaction} = H_{products} - H_{reactants}\) Using the thermodynamic data provided, we can find the enthalpy of formation for each species involved in the reaction: \(H_{H_{2}(g)} = 0 kJ/mol\) (by definition, the standard enthalpy formation of an element in its standard state is zero) \(H_{H(g)} = 218 kJ/mol\)
02

Calculate the enthalpy change

Now, we can substitute the enthalpies of the species into the equation for the enthalpy change of the reaction: \(ΔH_{reaction} = (1 \times H_{H_{2}(g)}) - (2 \times H_{H(g)})\) \(ΔH_{reaction} = (1 \times 0) - (2 \times 218)\) \(ΔH_{reaction} = -436 kJ/mol\)
03

Compare the enthalpies to determine which species has higher enthalpy

Now that we have the enthalpy change, we can determine which species - atomic hydrogen (H) or molecular hydrogen (H₂) - has the higher enthalpy under the given conditions. Since \(ΔH_{reaction} = H_{products} - H_{reactants}\), a negative enthalpy change means that the reactants have a higher enthalpy than the products. In this case, the reactants are 2 H(g) and the products are H₂(g). Therefore, under these conditions, 2 H(g) has a higher enthalpy than H₂(g). The answers are: (a) \(-436 kJ/mol\) (b) 2 H(g) has the higher enthalpy

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Most popular questions from this chapter

An aluminum can of a soft drink is placed in a freezer. Later, you find that the can is split open and its contents have frozen. Work was done on the can in splitting it open. Where did the energy for this work come from?

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