Problem 83 - Chemistry The Molecular Nature of Matter

Question

Ethyne, more commonly called acetylene, is a gas used in welding torches. It has the Lewis structure \(\mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\). What would you expect the \(\mathrm{H}-\mathrm{C}-\mathrm{C}\) bond angle to be in this molecule?

Step-by-Step Solution

Answer

The H-C-C bond angle in ethyne (acetylene) is expected to be 180 degrees due to its linear geometry and sp hybridization.

Step 1: Identify the hybridization of the carbon atoms

The first step is to determine the hybridization of the carbon atoms in acetylene. Each carbon atom is bonded to one hydrogen atom and triple-bonded to the other carbon atom, which corresponds to a total of two sigma bonds for each carbon. According to hybridization theory, two sigma bonds correspond to sp hybridization.

Step 2: Relate hybridization to molecular geometry

The sp hybridization implies that the molecule has a linear geometry. In a linear geometry, the orbitals are as far apart as possible, which guides us to predict the bond angles.

Step 3: Predict the bond angles

Knowing that the molecule has a linear geometry, we can conclude that the H-C-C bond angle in ethyne (acetylene) is expected to be 180 degrees since that is the characteristic bond angle in molecules with linear geometry.

Key Concepts

Lewis Structure

The Lewis structure is a visual representation of the arrangement of electrons around atoms in a molecule. It's fundamental for understanding chemical bonding and molecule formation. In the case of acetylene, the Lewis structure is denoted as H-C ≡ C-H, with the triple bond (represented by three parallel lines, ≡) between the two carbon atoms indicating they share three pairs of electrons. The hydrogen atoms are each single-bonded (denoted by a single line, -) to the carbon atoms, implying they share one pair of electrons with carbon. This structure is crucial for predicting the molecule's shape and bond angles by revealing the number of bonding and nonbonding electron pairs around the central atom.

Hybridization Theory

Hybridization theory is integral to understanding how atoms in molecules bond and form specific shapes. It provides insight into the combination of atomic orbitals to form new hybrid orbitals that can better accommodate bonding with other atoms within a molecule. In acetylene's context, each carbon atom undergoes sp hybridization because it forms two sigma bonds: one with a hydrogen atom and one as part of the carbon-carbon triple bond. The two sp hybrid orbitals form a linear arrangement, leading us to anticipate the molecule's geometric structure. Hybridization is the key to connecting a molecule's electron arrangement with its overall geometry.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It dictates the molecule's overall shape and can affect its physical and chemical properties. For acetylene, the theory predicts a linear geometry due to the sp hybridized orbitals of the carbon atoms. In a linear configuration, atoms are arranged in a straight line. This prediction is crucial as it impacts how molecules interact with each other and with light, influencing the molecule's reactivity and other characteristics.

sp Hybridization

In sp hybridization, one s orbital and one p orbital from the same atom mix to form two new equivalent hybrid orbitals. This process occurs in acetylene's carbon atoms, resulting in two linearly arranged sp hybrid orbitals. These orbitals are responsible for the formation of strong sigma bonds, with one used to bond with hydrogen and the other for the carbon-carbon triple bond. The linear arrangement of these sp orbitals is what gives acetylene its characteristic straight-line shape with an angle of 180 degrees between the bonds.

Sigma Bonds

Sigma bonds (σ) are the strongest type of covalent chemical bond and are formed by the direct overlap of atomic orbitals. In acetylene, each carbon atom forms two sigma bonds, one with hydrogen and one with the other carbon atom as part of the carbon-carbon triple bond. These bonds define the basic structural framework of the molecule and directly influence its bond angles and geometry. Understanding sigma bonds is essential for predicting molecular behavior and reactivity, making them a cornerstone of chemical bonding theory.