Problem 98 - Chemistry Matter and Change

Question

Draw each molecule and show the locations of hydrogen bonds between the molecules. (Chapter 8\()\) a. two water molecules b. two ammonia molecules c. one water molecule and one ammonia molecule

Step-by-Step Solution

Answer

In order to show the locations of hydrogen bonds between the molecules, follow these steps: a. For two water molecules: 1. Draw the H2O molecular structure, with oxygen in the center and the hydrogen atoms at an angle of about 104.5 degrees. 2. Place a second H2O molecule nearby with the oxygen atoms close. 3. Show a hydrogen bond as a dotted line connecting one hydrogen atom from the first molecule to the oxygen atom of the second molecule. b. For two ammonia molecules: 1. Draw the NH3 molecular structure, with nitrogen in the center and hydrogen atoms spaced about 107.8 degrees apart. 2. Place a second NH3 molecule nearby with the nitrogen atoms close. 3. Show a hydrogen bond as a dotted line connecting one hydrogen atom from the first molecule to the nitrogen atom of the second molecule. c. For one water and one ammonia molecule: 1. Draw the H2O and NH3 molecular structures as described above. 2. Place the nitrogen atom of NH3 close to the oxygen atom of H2O. 3. Show a hydrogen bond as a dotted line connecting one hydrogen atom of the water molecule to the nitrogen atom of the ammonia molecule.

Step 1: a. Two water molecules

To draw two water molecules and show the locations of hydrogen bonds between them, follow these steps: 1. Draw the molecular structure of a water molecule (H2O), which consists of one oxygen atom bonded to two hydrogen atoms. The oxygen atom is at the center, with the hydrogen atoms at an angle of approximately 104.5 degrees to it. 2. Draw a second water molecule next to the first, with the oxygen atoms of both molecules close to each other. 3. Identify the locations of hydrogen bonds between the two water molecules. In this case, the hydrogen bond will form between one of the hydrogen atoms from the first water molecule and the oxygen atom in the second water molecule. Represent the hydrogen bond as a dotted line connecting the two interacting atoms.

Step 2: b. Two ammonia molecules

To draw two ammonia molecules and show the locations of hydrogen bonds between them, follow these steps: 1. Draw the molecular structure of an ammonia molecule (NH3), which consists of one nitrogen atom bonded to three hydrogen atoms. The nitrogen atom is at the center, with the hydrogen atoms spaced approximately 107.8 degrees apart around it. 2. Draw a second ammonia molecule next to the first, with the nitrogen atoms of both molecules close to each other. 3. Identify the locations of hydrogen bonds between the two ammonia molecules. In this case, the hydrogen bond will form between one of the hydrogen atoms from the first ammonia molecule and the nitrogen atom in the second ammonia molecule. Represent the hydrogen bond as a dotted line connecting the two interacting atoms.

Step 3: c. One water molecule and one ammonia molecule

To draw one water molecule and one ammonia molecule and show the location of hydrogen bonds between them, follow these steps: 1. Draw the molecular structure of a water molecule (H2O) as described in part a. 2. Draw the molecular structure of an ammonia molecule (NH3) as described in part b, with the nitrogen atom of the ammonia molecule close to the oxygen atom of the water molecule. 3. Identify the location of hydrogen bonds between the water and ammonia molecules. In this case, the hydrogen bond will form between a hydrogen atom from the water molecule and the nitrogen atom in the ammonia molecule. Represent the hydrogen bond as a dotted line connecting the two interacting atoms.

Key Concepts

Molecular Structures

Understanding the molecular structures of compounds is fundamental to visualizing how these tiny entities interact with one another. Molecules like water (H₂O) and ammonia (NH₃) have specific shapes due to the arrangements of their atoms. In water, the molecule adopts a V-shaped structure because of the two hydrogen atoms that are bonded to an oxygen atom at an angle, leading to its bent shape. Ammonia, on the other hand, features a trigonal pyramidal shape, with three hydrogen atoms bonded to a central nitrogen atom. The way these atoms are spatially arranged in a molecule is key to predicting how the molecule will behave and interact with other molecules.

These structural details are important when we consider properties such as boiling and melting points, solubility, and reaction kinetics. Keep in mind, the exact angles, like the 104.5 degrees in a water molecule or the 107.8 degrees in ammonia, are derived from the electron configurations and repulsions between electron pairs around the central atom as explained by VSEPR (Valence Shell Electron Pair Repulsion) theory.

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including forces of attraction or repulsion which act between neighboring particles. These are distinct from intramolecular forces, which are the forces within a molecule that hold it together. A key type of intermolecular force is hydrogen bonding, which is a strong type of dipole-dipole interaction. It occurs when a hydrogen atom covalently bonded to a highly electronegative atom, like nitrogen, oxygen, or fluorine, comes under the influence of another electronegative atom with a lone pair of electrons.

Hydrogen bonds greatly affect the physical properties of compounds. For instance, they are largely responsible for the high boiling point of water compared to other hydrides in the same group. Ammonia also has these bonds, but with nitrogen as the electronegative atom, conferring high solubility and a relatively high boiling point compared to other similar-sized molecules.

Chemical Bonding

The concept of chemical bonding is pivotal to understanding the stability and reactivity of molecules. Bonds within molecules, known as intramolecular bonds, can be either ionic, covalent, or metallic. In the context of water and ammonia, we are interested in covalent bonding, where atoms share pairs of electrons to achieve a stable electron configuration.

In H₂O, each hydrogen atom shares an electron with the oxygen atom, leading to two covalent bonds. In NH₃, the nitrogen atom shares an electron with each of the three hydrogen atoms. The distribution of these shared electrons around the central atom determines the shape of the molecule and contributes to the overall polarity, which in turn influences the substance's capacity to form hydrogen bonds with other molecules.

Drawing Molecules

When drawing molecules to represent their structures, the goal is to convey spatial relationships and bonding interactions accurately. For educational purposes, these drawings often simplify the molecules into ball-and-stick or space-filling models. In the case of water and ammonia, clear visualization requires correctly placing atoms at their respective angles and representing intermolecular interactions like hydrogen bonds with dotted lines.

This representation facilitates the comprehension of different concepts, such as molecular geometry and intermolecular forces. To improve the understanding, it's helpful to start with a straightforward sketch of each individual molecule, add in the hydrogen bonds, and then proceed to more complex interactions. With practice, students can visualize these structures and comprehend the physical implications of molecular geometry and bonding characteristics on a molecular level.