Acetylene (C₂H₂) and nitrogen (N₂) both contain a triple bond, but they differ greatly in their chemical properties. (e) Both N₂ and C₂H₂ possess triple bonds with quite high bond enthalpies (Table 8.3). Calculate the enthalpy of hydrogenation per mole for both compounds: acetylene plus H₂ to make methane, CH₄; nitrogen plus H₂ to make ammonia, NH₃.

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Identify the chemical reactions for the hydrogenation of acetylene and nitrogen. For acetylene: \( \text{C}_2\text{H}_2 + 2\text{H}_2 \rightarrow \text{CH}_4 \). For nitrogen: \( \text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3 \).

Determine the bond enthalpies for each bond involved in the reactions. Use a table of bond enthalpies to find the values for C≡C, C-H, N≡N, and N-H bonds.

Calculate the total bond enthalpies for the reactants in each reaction. For acetylene, consider the C≡C bond and the H-H bonds. For nitrogen, consider the N≡N bond and the H-H bonds.

Calculate the total bond enthalpies for the products in each reaction. For acetylene, consider the C-H bonds in methane. For nitrogen, consider the N-H bonds in ammonia.

Use the formula for enthalpy change: \( \Delta H = \text{Total bond enthalpies of reactants} - \text{Total bond enthalpies of products} \) to find the enthalpy of hydrogenation per mole for both reactions.

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Key Concepts

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

Triple Bonds

A triple bond is a type of chemical bond where three pairs of electrons are shared between two atoms, resulting in a strong interaction. In acetylene (C2H2), the triple bond between carbon atoms contributes to its reactivity and unique properties. In contrast, nitrogen (N2) has a triple bond that makes it very stable and inert under standard conditions, affecting its chemical behavior.

Enthalpy of Hydrogenation

The enthalpy of hydrogenation refers to the heat change that occurs when a compound reacts with hydrogen to form a saturated compound. This value is crucial for understanding the stability of the original compound; a more negative enthalpy indicates a more exothermic reaction and thus a more stable product. For acetylene and nitrogen, calculating this enthalpy helps compare their reactivity and the energy changes involved in their hydrogenation.

Bond Enthalpy

Bond enthalpy is the measure of the strength of a chemical bond, defined as the energy required to break one mole of bonds in a gaseous substance. High bond enthalpies, like those found in triple bonds, indicate strong bonds that are less likely to break during reactions. Understanding bond enthalpy is essential for calculating the enthalpy changes during reactions, such as the hydrogenation of acetylene and nitrogen.

Related Practice

Textbook Question

A common form of elemental phosphorus is the tetrahedral P₄ molecule, where all four phosphorus atoms are equivalent:

(b) How many P-P bonds are there in the molecule?

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Textbook Question

Acetylene (C₂H₂) and nitrogen (N₂) both contain a triple bond, but they differ greatly in their chemical properties. (c) Write balanced chemical equations for the complete oxidation of N₂ to form N₂O₅(g) and of acetylene to form CO₂(g) and H₂O(g). Write a balanced chemical equation for the complete oxidation of acetylene to form CO₂(g) and H₂O(g).

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Textbook Question

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Draw a Lewis structure for a linear P₄ molecule that satisfies the octet rule. Does this molecule have resonance structures?

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Textbook Question

Acetylene (C₂H₂) and nitrogen (N₂) both contain a triple bond, but they differ greatly in their chemical properties. (b) By referring to Appendix C, look up the enthalpies of formation of acetylene and nitrogen. Which compound is more stable?

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Textbook Question

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