Bonding - Hydrogen Bonding in Water (A-Level Chemistry)
Hydrogen Bonding in Water
Hydrogen Bonding in Water
As we said in the previous chapter, hydrogen bonds are a special type of permanent dipole-dipole forces that form when hydrogen forms a covalent bond with a very electronegative element: either nitrogen, oxygen or fluorine. They are the strongest type of intermolecular force.
Water molecules are highly polarized molecules that are able to form an extensive network of hydrogen bonds. The lone pairs of electrons on the oxygen atom are attracted to the δ+ of the hydrogen in nearby water molecules. Each water molecule can form a total of 4 hydrogen bonds.
The extensive hydrogen bonding that occurs in water accounts for its anomalous physical properties.
Boiling Point
If we compare the boiling points of Group 6 hydrides, we can see that those for water are significantly higher than would have been expected.
As we go down the group and number of electrons increases, so does the strength of London forces between molecules, so that boiling point increases. However, the strong hydrogen bonds that form between water molecules, overrides the effects of their weaker London forces. The same principle applies to melting points.
Surface Tension
Surface tension refers to how strongly molecules are held to the surface of a liquid. As a result of hydrogen bonds which exert a strong downwards force on water molecules at the surface, water has a higher surface tension than most other liquids.
Viscosity
Viscosity is a measure of how resistant to flow a liquid is as a result of friction between its molecules. Due to the hydrogen bonds between them, water molecules are less capable to slide over each other so that water has a high viscosity.
Density
In general, solids are denser than their liquids but for water, the density of solid ice is lower than the density of liquid water.
When water freezes into ice, water molecules become arranged into a 3D network where each water molecule is connected to four other water molecules by hydrogen bonds. As a result of this regular lattice structure, the water molecules in solid ice are further apart than the molecules in liquid water. The structure is more open in ice which makes it less dense than water.
Water as a Solvent
A solvent is the liquid medium for a reaction.
Solvents with a permanent dipole moment, like water, are said to be polar and those without it are said to be non-polar.
When a compound dissolves in a solvent:
1) Intermolecular forces within the compound are broken down
2) New bonds with the solvent molecules are formed
Therefore, for a solvent to be able to dissolve a compound, both have to be able to form similar intermolecular forces.
- Water can dissolve ionic compounds. The partial charges on water molecules are attracted to the charged ions in the ionic lattice forming ion-dipole interactions. The overall strength of the large number of ion-dipole interactions that form is enough to overcome the electrostatic forces between oppositely charged ions in the lattice and break it apart. Ions dissociate and become surrounded by water molecules by a process known as hydration so that they are able to move freely in solution.
- Water can dissolve simple alcohols. Alcohols are compounds with a hydroxyl group and as a result can form hydrogen bonds with water. However, long chain alcohols are less soluble in water because more bonds have to be broken down than those that get formed. As a result, energy released from bond formation does not compensate for the energy lost during bond breakage so that the alcohol dissolves poorly in water.
- Not all polar molecules will dissolve well in water. Halogenoalkanes for example, don’t dissolve in water. This is because their dipole moment is too weak to form hydrogen bonds with water molecules. The hydrogen bonds between water molecules are stronger than the dipole interactions that can form between water molecules and the halogenoalkane so the compound does not dissolve.
- Non-polar compounds do not dissolve in water. Non-polar compounds like iodine are completely insoluble in water. Non-polar solvents like diethyl ether, which can form London forces with nonpolar compounds, should be used instead.
FAQs
Hydrogen bonding in water refers to the strong attractive forces between the positively charged hydrogen atoms of one water molecule and the negatively charged oxygen atoms of another water molecule. These bonds form due to the difference in electronegativity between hydrogen and oxygen atoms.
Hydrogen bonding in water occurs due to the polarity of the water molecule, which arises from the electronegativity difference between the oxygen and hydrogen atoms. The oxygen atom in a water molecule is more electronegative than the hydrogen atoms, meaning it has a stronger pull on the shared electrons in the covalent bond. As a result, the electrons spend more time around the oxygen atom, giving it a partial negative charge (denoted δ-) and the hydrogen atoms a partial positive charge (denoted δ+).
In a bulk sample of water, the δ+ hydrogen atoms of one water molecule are attracted to the δ- oxygen atoms of neighboring water molecules, forming a hydrogen bond. The hydrogen bond is an electrostatic attraction between the partially positive hydrogen atom and the partially negative oxygen atom of two adjacent water molecules.
Each water molecule can form up to four hydrogen bonds with neighboring water molecules, resulting in a complex network of hydrogen bonds in liquid water. This network of hydrogen bonds gives water many of its unique properties, such as its high boiling point, surface tension, and ability to dissolve a wide range of substances.
Hydrogen bonding occurs in water due to the difference in electronegativity between hydrogen and oxygen atoms. The hydrogen atoms of one water molecule are positively charged, while the oxygen atoms are negatively charged. When two water molecules come into close proximity, the positive hydrogen atoms are attracted to the negative oxygen atoms of another water molecule, forming a hydrogen bond.
Hydrogen bonds in water are constantly forming, breaking, and reforming due to the motion of water molecules. The hydrogen bond is an electrostatic attraction between the partially positive hydrogen atom of one water molecule and the partially negative oxygen atom of another water molecule.
In liquid water, the water molecules are in constant motion, and the hydrogen bonds are constantly breaking and reforming as the molecules collide and interact with each other. As a result, the hydrogen bonds in liquid water are not static, but rather dynamic and constantly changing.
The strength of hydrogen bonds in water is influenced by several factors, such as temperature, pressure, and the presence of other molecules. For example, at higher temperatures, the kinetic energy of water molecules is greater, which can break more hydrogen bonds and make the water less structured. Similarly, changes in pressure can affect the strength and stability of hydrogen bonds in water.
Hydrogen bonding affects many of the properties of water, including its high boiling and melting points, high surface tension, and high heat of vaporization. These properties make water a unique substance with important implications for life and the environment.
Hydrogen bonding in water is important in A-Level Chemistry because it helps students understand the properties and behavior of water, which is essential for many chemical reactions and processes. Understanding hydrogen bonding is also critical for understanding the behavior of other substances that contain hydrogen bonds, such as proteins, enzymes, and DNA.
Hydrogen bonding in water plays a critical role in many real-world applications, including water treatment and purification, food and beverage processing, and the pharmaceutical industry. It is also important in many biological processes, including protein and enzyme function, DNA structure and stability, and the regulation of water balance in cells. Understanding hydrogen bonding in water is essential for making informed decisions in many fields and industries.
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