Hydrogen Bonding in Water

What is hydrogen bonding?

A hydrogen (H) bond is a bond that forms between a partially positively charged hydrogen atom and an electronegative atom, typically fluorine (F), nitrogen (N), or oxygen (O).

How do hydrogen bonds form?

When atoms share valence electrons, a covalent bond is formed. Covalent bonds are either polar or non-polar depending on the electronegativity of the atoms (the ability of an atom to attract electrons when in a bond).

• Non-polar covalent bond: electrons are shared equally.

• Polar covalent bond: electrons are shared unequally.

Due to the unequal sharing of electrons, a polar molecule has a partially positive region on one side and a partially negative region on the other. Because of this polarity, a hydrogen atom with a polar covalent bond to an electronegative atom (for example, nitrogen, fluorine, and oxygen) is attracted to electronegative ions or negatively charged atoms of other molecules.

Hydrogen bonding in water: biology

Water consists of two hydrogen atoms attached via covalent bonds to one oxygen atom (H-O-H). Water is a polar molecule because its hydrogen and oxygen atoms share electrons unequally due to differences in electronegativity.

The oxygen atom has a higher electronegativity than the hydrogen atom, so electrons are attracted to oxygen and repelled by hydrogen. When the water molecule is formed, the ten electrons pair up into five orbitals distributed as follows:

• One pair is linked to the oxygen atom.

• Two pairs are linked to the oxygen atom as outer electrons.

• Two pairs form the two O-H covalent bonds.

When the water molecule is formed, two lone pairs are left. The two lone pairs associate themselves with the oxygen atom. As a result, oxygen atoms have a partial negative (δ-) charge, while hydrogen atoms have a partial positive (δ+) charge.

Because the hydrogen atoms in a water molecule are partially positively charged, they are attracted to partially negative oxygen atoms in nearby water molecules, allowing hydrogen bonds to form between nearby water molecules or other molecules with a negative charge. Hydrogen bonding occurs constantly between water molecules. While individual hydrogen bonds tend to be weak, they create a considerable impact when they form in large numbers, which is usually the case for water and organic polymers.

What is the number of hydrogen bonds that can form in water molecules?

Water molecules contain two lone pairs and two hydrogen atoms, all of which are connected to the strongly electronegative oxygen atom. This means that up to four bonds (two where it is the receiving end of the h-bond, and two where it is the giver in the h-bond) can be formed by each water molecule.

However, because hydrogen bonds are weaker than covalent bonds, they form, break, and reconstruct easily in liquid water. As a result, the precise number of hydrogen bonds created per molecule varies.

What are the effects and consequences of hydrogen bonding in water?

Hydrogen bonding in water imparts several properties that are important in sustaining life. In the following section, we will talk about some of these properties.

Solvent property

Water molecules are excellent solvents. Polar molecules are hydrophilic ("water-loving") substances.

This is because the negative ion of the solute will attract the positively charged region of the water molecule and vice versa, causing the ions to dissolve.

Moderation of temperature

The hydrogen bonds in water molecules react to changes in temperature, giving water its unique characteristics in its solid, liquid, and gas states.

• In its liquid state, water molecules constantly move past each other as the hydrogen bonds continuously break and recombine.

• In its gas state, water molecules have higher kinetic energy, causing hydrogen bonds to break.

• In its solid state, water molecules expand because the hydrogen bonds push the water molecules apart. At the same time, the hydrogen bonds hold the water molecules together, forming a crystalline structure. This gives ice (solid water) a lower density compared to liquid water.

Hydrogen bonding in water molecules gives it a high specific heat capacity.

The high specific heat capacity of water means it takes a lot of energy to cause changes in temperature. The high specific heat capacity of water allows it to maintain a stable temperature, vital in sustaining life on Earth.

Similarly, hydrogen bonding gives water high heat of vaporization,

In fact, it takes 586 cal of heat energy to change one gram of water to gas. This is because hydrogen bonds need to be broken for liquid water to enter its gas state. Once it reaches its boiling point (100° C or 212° F), the hydrogen bonds in water break, causing water to evaporate.

Cohesion

Hydrogen bonding causes water molecules to stay close to each other which makes water a highly cohesive substance.

Water clumps together to form "drops" because of its cohesive property. Cohesion results in another property of water: surface tension.

Surface tension

Surface tension is the property that allows a substance to resist tension and prevent rupture.

Both the cohesion of water to itself and the strong adhesion of water to the surface that it is touching cause water molecules close to the surface to move down and to the side.

On the other hand, the air pulling up exerts a little force on the water's surface. As a result, a net force of attraction is produced between water molecules at the surface, resulting in a highly flat, thin sheet of molecules. Water molecules on the surface adhere to one another, preventing items lying on the surface from sinking.

Adhesion

Water is highly adhesive; it adheres to a wide range of various things. Water attaches to other things for the same reason it sticks to itself — it is polar; thus, it is attracted to charged substances. Water attaches to various surfaces, including plants, utensils, and even your hair when it is wet after showering.

Capillarity

This tendency is due to the water molecules being more attracted to such surfaces than other water molecules.

What is the importance of hydrogen bonding in water in biology?

In the previous section, we discussed the properties of water. How are these enabling biochemical and physical processes that are essential in sustaining life on Earth? Let's discuss some specific examples.

Water being an excellent solvent means it can dissolve a wide range of compounds. Since most crucial biochemical processes occur in a watery environment inside cells, this property of water is critical in allowing these processes to occur. Water's high specific heat capacity enables large bodies of water to regulate temperature.

Similarly, water's high heat of vaporization means that in the process of changing from liquid to gas state, a lot of energy is consumed, causing the surrounding environment to cool down.

The cohesion, adhesion, and capillarity are important properties of water that enable the uptake of water in plants. Water can climb up the roots thanks to capillarity. It can also move through the xylem to bring water up to the branches and leaves.