Question

Acetylene \(\left(\mathrm{C}_{2} \mathrm{H}_{2}\right)\) and nitrogen \(\left(\mathrm{N}_{2}\right)\) both contain a triple bond, but they differ greatly in their chemical properties. (a) Write the Lewis structures for the two substances. (b) By referring to Appendix \(C\), look up the enthalpies of formation of acetylene and nitrogen and compare their reactivities. (c) Write balanced chemical equations for the complete oxidation of \(\mathrm{N}_{2}\) to form \(\mathrm{N}_{2} \mathrm{O}_{5}(g)\) and of acetylene to form \(\mathrm{CO}_{2}(g)\) and \(\mathrm{H}_{2} \mathrm{O}(g)\). (d) Calculate the enthalpy of oxidation per mole of \(\mathrm{N}_{2}\) and \(\mathrm{C}_{2} \mathrm{H}_{2}\) (the enthalpy of formation of \(\mathrm{N}_{2} \mathrm{O}_{5}(\mathrm{~g})\) is \(11.30 \mathrm{~kJ} / \mathrm{mol}\) ). How do these comparative values relate to your response to part (b)? Both \(\mathrm{N}_{2}\) and \(\mathrm{C}_{2} \mathrm{H}_{2}\) possess triple bonds with quite high bond enthalpies (Table 8.4). What aspect of chemical bonding in these molecules or

Short answer

In short, Acetylene (C2H2) is more reactive than Nitrogen (N2) due to the type of bonds in the molecules and their oxidation products. Acetylene, with its carbon-hydrogen and carbon-carbon bonds, releases a large amount of energy upon forming carbon-oxygen and oxygen-hydrogen bonds in CO2 and H2O. Meanwhile, Nitrogen, with its triple-bonded nitrogen atoms, is relatively stable and releases much less energy when oxidized to N2O5 because nitrogen-oxygen bonds are not as energetically favorable. The Lewis structures of these molecules are H-C≡C-H for Acetylene and N≡N for Nitrogen. The complete oxidation equations are 2 N2 + 5 O2 → 2 N2O5 for Nitrogen and 2 C2H2 + 5 O2 → 4 CO2 + 2 H2O for Acetylene. Enthalpy of oxidation calculations show -2511.2 kJ/mol for Acetylene and 22.60 kJ/mol for Nitrogen, demonstrating that Acetylene is much more reactive than Nitrogen.

Step-by-step solution

Acetylene Lewis Structure

The molecule C2H2, or Acetylene, has 2 Carbon atoms with a triple bond connecting them, and 1 Hydrogen atom bonded to each Carbon with a single bond. The Lewis structure would look like this: H-C≡C-H

Nitrogen Lewis Structure

The molecule N2, or Nitrogen gas, has 2 Nitrogen atoms connected with a triple bond. The Lewis structure would look like this: N≡N #b: Enthalpies of formation and reactivities#

Enthalpies of formation

By referring to Appendix C or other sources, we can find the enthalpies of formation for these molecules: Acetylene (C2H2): +227 kJ/mol and Nitrogen (N2): 0 kJ/mol.

Reactivities comparison

Since Acetylene has a positive enthalpy of formation while Nitrogen has an enthalpy of formation equal to 0, Acetylene is more reactive than Nitrogen, which is relatively inert. #c: Balanced chemical equations#

Nitrogen oxidation

The balanced equation for the complete oxidation of Nitrogen (N2) to Nitrogen pentoxide (N2O5) is: 2 N2 + 5 O2 → 2 N2O5

Acetylene oxidation

The balanced equation for the complete oxidation of Acetylene (C2H2) to Carbon dioxide (CO2) and Water (H2O) is: 2 C2H2 + 5 O2 → 4 CO2 + 2 H2O #d: Enthalpy of oxidation and reactivity#

Enthalpy of oxidation

For Nitrogen: ΔH(N2 oxidation) = [2 mol x 11.30 kJ/mol (N2O5)] - [0 kJ/mol (N2)] = 22.60 kJ/mol For Acetylene: ΔH(C2H2 oxidation) = [4 mol x (-393.5 kJ/mol CO2) + 2 mol x (-285.8 kJ/mol H2O)] - [2 mol x (+227 kJ/mol C2H2)] = -2511.2 kJ/mol

Relation to chemical reactivity

The calculated enthalpies of oxidation show that Acetylene releases much more energy when oxidized than Nitrogen, which is consistent with our observation from part b that Acetylene is much more reactive than Nitrogen.

Bonding and chemical reactivity

The difference in chemical reactivities can be attributed to the type of bonds in the molecules and their oxidation products. Acetylene, with its carbon-hydrogen and carbon-carbon bonds, releases a large amount of energy upon forming carbon-oxygen and oxygen-hydrogen bonds in CO2 and H2O. On the other hand, Nitrogen gas, with its triple-bonded nitrogen atoms, is relatively stable and releases much less energy when oxidized to N2O5 because nitrogen-oxygen bonds are not as energetically favorable. The high bond enthalpy of the N≡N triple bond, as well as the partial double bond character of the N-O bonds in N2O5, contributes to the lower reactivity of Nitrogen compared to Acetylene.

Key concepts

Lewis Structures

Drawing Lewis Structures is an essential skill in understanding molecular geometry and chemical bonding. A Lewis Structure represents the valence electrons of atoms within a molecule. Each element's symbol is used to indicate the atom, while dots are placed around it to reflect the valence electrons. Lines between atoms show shared electron pairs, or covalent bonds, in the molecule.

When constructing the Lewis Structure for acetylene (\(\mathrm{C}_{2}\mathrm{H}_{2}\)), we illustrate the carbon-carbon triple bond with three lines between the two carbon atoms. Each hydrogen is connected by a single bond to a carbon, forming H-C≡C-H.

For nitrogen gas (\(\mathrm{N}_{2}\)), the structure is simpler, as it consists of two nitrogen atoms linked by a triple bond: N≡N. This shared triple bond accounts for the strong bond between the nitrogen atoms, resulting in molecular nitrogen's relative inertness under normal conditions. Constructing Lewis Structures helps chemists predict shapes and identify the reactivity of molecules based on electron configurations.

Enthalpies of Formation

Enthalpy of formation is a key thermodynamic concept that refers to the change in energy when one mole of a substance forms from its elements in their standard states. It is typically expressed in kilojoules per mole (\( \text{kJ/mol} \)).

For acetylene (\(\mathrm{C}_{2}\mathrm{H}_{2}\)), the enthalpy of formation is +227 kJ/mol. This positive value indicates that forming acetylene from its elements requires energy, which correlates with its higher reactivity. Acetylene is combustible and used in welding because it can release significant energy.

In contrast, nitrogen (\(\mathrm{N}_{2}\)) has an enthalpy of formation of 0 kJ/mol. This value suggests high stability, given nitrogen’s prevalence and stability in its gaseous form as a diatomic molecule. The lack of energy change when forming \(\mathrm{N}_{2}\) reflects its strong triple bond and inertness under normal atmospheric conditions.

• Positive enthalpies denote energy-absorbing (endothermic) processes.
• Zero or negative values indicate stable compounds or exothermic processes.

Chemical Equations

Balanced chemical equations serve as representations of chemical reactions. They indicate the reactants, the products, and their respective stoichiometric coefficients which ensure the conservation of mass and atoms.

In the oxidation of nitrogen (\(\mathrm{N}_{2}\)) to form nitrogen pentoxide (\(\mathrm{N}_{2}\mathrm{O}_{5}\)), the balanced equation is:
2 \( \mathrm{N}_{2} \)
+ 5 \( \mathrm{O}_{2} \) → 2 \( \mathrm{N}_{2}\mathrm{O}_{5} \).

For acetylene (\(\mathrm{C}_{2}\mathrm{H}_{2}\)), its complete oxidation to carbon dioxide (\(\mathrm{CO}_{2}\)) and water (\(\mathrm{H}_{2}\mathrm{O}\)) is shown by this equation:
2 \(\mathrm{C}_{2}\mathrm{H}_{2} \) + 5 \(\mathrm{O}_{2} \) → 4 \(\mathrm{CO}_{2} \) + 2 \(\mathrm{H}_{2}\mathrm{O} \).

Balanced chemical equations are crucial in chemical reactions, as they guide mole ratios and predict the outcomes and yields of reactions. Consistency in these equations ensures the calculation of reactants needed and the products formed, making them essential tools in both laboratory and industrial chemistry.

Oxidation Reactions

Oxidation reactions involve the transfer of electrons, typically accompanied by the release of energy. In many cases, such reactions involve the addition of oxygen to a compound.

Acetylene's oxidation is a highly exothermic reaction, releasing significant energy as it converts into carbon dioxide and water, producing a total enthalpy change of -2511.2 kJ/mol. This high energy release reflects acetylene's utility in energy-heavy applications, such as oxy-acetylene welding torches.

Conversely, the oxidation of molecular nitrogen to nitrogen pentoxide is less energetic, with an enthalpy change of only 22.60 kJ/mol. Due to its strong \(\mathrm{N} \equiv \mathrm{N} \) bond, nitrogen's oxidation also requires more energy input to proceed, thereby imparting its characteristic inertness.

• Exothermic reactions (\(\Delta H < 0\)) release energy, making them good energy sources.
• Endothermic reactions (\(\Delta H > 0\)) require energy, correlating with stability and often lower reactivity.