Question

Describe a hydrogen bond. What conditions must exist for a hydrogen bond to form? (Chapter 12\()\)

Short answer

A hydrogen bond is an attractive interaction between a hydrogen atom bonded to a highly electronegative atom (e.g., nitrogen, oxygen, or fluorine) and another electronegative atom with a lone pair of electrons. These bonds are weaker than covalent and ionic bonds but significantly influence the properties of substances, such as water and DNA base pairs. For a hydrogen bond to form, there must be a hydrogen atom bonded to a highly electronegative atom, another molecule or atom with an electronegative atom having a lone pair of electrons, and a significant difference in electronegativity between the hydrogen-bonded atom and the atom to which the hydrogen is covalently bonded. These conditions lead to the formation of partial charges, creating an attractive force and resulting in a hydrogen bond.

Step-by-step solution

Define a hydrogen bond

A hydrogen bond is a type of attractive interaction between a hydrogen atom that is bound to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) and another electronegative atom with a lone pair of electrons. Hydrogen bonds are generally weaker than covalent bonds and ionic bonds, but they are still significant in influencing the physical and chemical properties of various substances.

Provide examples of hydrogen bonds

Some common examples of hydrogen bonds include the bonds within water molecules (H2O) and between DNA base pairs (adenine with thymine, and guanine with cytosine). In water, the hydrogen bond occurs between the hydrogen atom of one water molecule and the oxygen atom of another. These bonds give water its unique properties, such as high boiling point, surface tension, and ability to act as a solvent.

Describe the conditions necessary for the formation of a hydrogen bond

For a hydrogen bond to form, several conditions must be met: 1. The presence of a hydrogen atom bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine). 2. The presence of another molecule or atom containing an electronegative atom with a lone pair of electrons. 3. A significant difference in electronegativity between the hydrogen-bonded atom and the atom to which the hydrogen is covalently bonded. When these conditions are met, the hydrogen atom becomes partially positively charged due to the electronegativity difference, while the electronegative atom with a lone pair becomes partially negatively charged. This creates an attractive force between the opposite charges, which results in the formation of a hydrogen bond.

Key concepts

Chemical Bonding

Chemical bonds form the backbone of all substances, holding atoms and molecules together. They can be divided into three main types: ionic, covalent, and metallic bonds. Ionic bonds occur when electrons are transferred from one atom to another, creating oppositely charged ions that attract each other. Covalent bonds, on the other hand, involve the sharing of electron pairs between atoms. Lastly, metallic bonds occur between a sea of delocalized electrons and positively charged metal ions.

Each type significantly affects a substance's properties. For instance, ionic solids often have high melting points due to the strong attractions between ions, while covalent compounds can be gases, liquids, or solids, showing versatile characteristics based on the nature of the covalent bonding. In understanding hydrogen bonds, it's vital to acknowledge that they fall under a special subcategory within intermolecular forces, which we'll explore next.

Intermolecular Forces

Intermolecular forces are the attractive forces that occur between molecules. They are responsible for the physical states and some properties of substances. The strongest of these is the hydrogen bond, discussed in the exercise. Meanwhile, dipole-dipole interactions occur when molecules with permanent dipoles attract each other, and the London dispersion forces are present in all molecules, resulting from momentary dipole changes caused by electron motion.

Understanding the differences in these forces is critical. While hydrogen bonds are strong enough to affect boiling points and solubility, the London dispersion forces are generally weak and are dominant only in nonpolar compounds. The balance and competition among these forces determine characteristics like state of matter at room temperature, viscosity, and melting points.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond. The more electronegative an atom, the more strongly it will pull shared electrons towards itself. This concept is crucial when discussing hydrogen bonds. Fluorine, oxygen, and nitrogen are highly electronegative and tend to create the partial charges necessary for hydrogen bonding.

In our exercise, the difference in electronegativity between the hydrogen atom and the electronegative atom it is bound to creates a dipole. This happens because the electronegative atom, having a stronger pull, gains a slight negative charge by drawing electrons closer to itself, while the hydrogen atom becomes slightly positive. Notably, hydrogen bonds are only as strong as the difference in electronegativity between the involved atoms.

Physical and Chemical Properties

The physical and chemical properties of a substance are intrinsically linked to the types of chemical bonding and intermolecular forces present. As highlighted in the exercise, hydrogen bonds greatly influence properties such as a compound’s boiling point, melting point, viscosity, and solubility. For instance, water’s unusually high boiling point for a molecule of its size can be attributed to hydrogen bonding between water molecules, which requires more energy to overcome.

Chemical properties like reactivity and stability can also be affected by hydrogen bonds as they can influence the three-dimensional structure of molecules like DNA. The bonds between the base pairs in DNA, for example, maintain the double helix structure which is essential for its function in heredity. Therefore, understanding how hydrogen bonds shape physical and chemical properties is essential for grasping why different substances behave in specific ways.