Question

Hydrogen peroxide \(\left(\mathrm{H}_{2} \mathrm{O}_{2}\right)\) is a syrupy liquid with a relatively low vapor pressure and a normal boiling point of \(152.2^{\circ} \mathrm{C}\) . Rationalize the differences of these physical properties from those of water.

Short answer

The differences in physical properties, such as boiling point and vapor pressure, between hydrogen peroxide (H2O2) and water (H2O) are a result of their different molecular structures and the presence or absence of strong intermolecular forces like hydrogen bonding. Water has stronger hydrogen bonds than hydrogen peroxide due to its overall dipole moment and more electronegative oxygen atoms, leading to a higher boiling point (\(100^{\circ} \mathrm{C}\)) and a lower vapor pressure. In contrast, the reduced dipole moment and weaker hydrogen bonding in hydrogen peroxide yield a lower boiling point (\(152.2^{\circ} \mathrm{C}\)) and higher vapor pressure.

Step-by-step solution

Understanding the chemical structures of H2O and H2O2

First, we need to understand the chemical structures of both compounds. Water, or H2O, is a simple molecule consisting of a single oxygen atom bonded to two hydrogen atoms. Hydrogen peroxide, or H2O2, is a larger molecule consisting of two oxygen atoms and two hydrogen atoms with a single bond connecting the two oxygen atoms. Although these molecules share similar elements, the arrangements and the types of bonds connecting the atoms are different.

Analyzing intermolecular forces in H2O

In water, the oxygen atom is more electronegative than the hydrogen atoms, leading to a polar covalent bond between oxygen and hydrogen atoms. This difference in electronegativity creates a dipole moment in the molecule, making water a polar molecule. Consequently,water molecules are attracted to each other through hydrogen bonding, which is a strong type of intermolecular force specifically between hydrogen atoms and highly electronegative atoms like oxygen.

Analyzing intermolecular forces in H2O2

Hydrogen peroxide also possesses polar covalent bonds between the oxygen and hydrogen atoms. However, the oxygen-oxygen single bond reduces the overall dipole moment of the H2O2 molecule. Consequently, hydrogen peroxide does not exhibit a strong dipole moment like water, and its capability to form hydrogen bonds is less than that of water.

Comparing boiling points

The boiling point of a substance is related to the strength of its intermolecular forces. Since hydrogen bonding is significantly stronger in water than in hydrogen peroxide, water molecules require more energy to break away from each other than the H2O2 molecules. This leads to water having a higher boiling point (\(100^{\circ} \mathrm{C}\)) than hydrogen peroxide (\(152.2^{\circ} \mathrm{C}\)).

Comparing vapor pressures

Vapor pressure is a measure of the tendency of a substance to evaporate. Substances with stronger intermolecular forces have lower vapor pressures because more energy is required to break the intermolecular forces holding the molecules together. Since water has stronger hydrogen bonds than hydrogen peroxide, water has a lower vapor pressure compared to H2O2.

Conclusion:

The differences in physical properties such as boiling point and vapor pressure between hydrogen peroxide and water are a result of the different molecular structures and the presence or absence of strong intermolecular forces like hydrogen bonding. Since hydrogen bonds are stronger in water than in hydrogen peroxide, water exhibits a higher boiling point and a lower vapor pressure than hydrogen peroxide.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that significantly influences the physical properties of molecules. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, is attracted to another electronegative atom nearby. This creates a strong dipole-dipole attraction.

In the case of water (\(\text{H}_2\text{O}\)), the oxygen atom is more electronegative than the hydrogen atoms. This results in a polar covalent bond, with a partial negative charge on the oxygen and a partial positive charge on the hydrogens. The hydrogen atoms in one water molecule are then attracted to the oxygen atoms of other water molecules, forming a hydrogen bond. These bonds are relatively strong for an intermolecular force, granting water its unusually high boiling point and low vapor pressure.

Hydrogen peroxide (\(\text{H}_2\text{O}_2\)) also involves hydrogen bonding, but the presence of an oxygen-oxygen single bond reduces its effectiveness. The geometry of the molecule minimizes the net dipole effect, making it weaker compared to water. This accounts for its lower boiling point relative to water, alongside a higher vapor pressure.

Polar Covalent Bonds

Polar covalent bonds are fundamental in understanding molecules like water and hydrogen peroxide. They occur when two atoms share electrons unevenly due to a difference in electronegativity. This causes one end of the bond to have a partial negative charge and the other a partial positive charge.

In water, the bond between hydrogen (less electronegative) and oxygen (more electronegative) is polar. Oxygen pulls the shared electrons closer, creating a dipole moment in the molecule. This polarity is why water molecules attract each other strongly through hydrogen bonds.

Hydrogen peroxide, on the other hand, consists of similar polar covalent bonds with its oxygen-hydrogen pairs. However, the presence of an oxygen-oxygen bond reduces the overall polarity of the molecule. The balance of charges results in a weaker dipole than in water, which lessens its hydrogen-bonding capacity. Understanding these bond differences helps explain the varied physical properties like boiling point and vapor pressure between water and hydrogen peroxide.

Boiling Point

The boiling point of a substance is a critical physical property indicative of the energy required to break intermolecular forces so that molecules can transition from liquid to gas.

In terms of hydrogen bonding, water is particularly unique. Due to strong hydrogen bonds, water requires a significant amount of energy to separate the molecules and turn them into vapor. As a result, water has a boiling point of\(100^{\circ} \text{C}\).

For hydrogen peroxide, though hydrogen bonds exist, their strength is mitigated by the molecular structure and polarity as discussed earlier. Consequently, less energy is needed to vaporize hydrogen peroxide, which reflects in its lower boiling point of\(152.2^{\circ} \text{C}\)compared to water. Understanding these points is vital when considering why substances behave differently under heat.

Vapor Pressure

Vapor pressure is a measure of a liquid's evaporation tendency and is closely related to the strength of intermolecular forces.

In general, the stronger the intermolecular forces, the lower the vapor pressure, as more energy is required to overcome these forces for evaporation to occur.

Water, with its extensive hydrogen bonding, exhibits a relatively low vapor pressure. Each water molecule is fairly strongly bound to its neighbors, resulting in fewer molecules escaping to form vapor. This accounts for water's smaller tendency to evaporate at a given temperature.

Hydrogen peroxide, with its weaker hydrogen bonding, exhibits a higher vapor pressure. This means it more readily turns into vapor than water under similar conditions. These distinctions highlight how varying intermolecular forces affect the physical behavior of substances such as water and hydrogen peroxide.