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 the physical properties of hydrogen peroxide (H2O2) and water (H2O), specifically their boiling points and vapor pressures, can be attributed to the difference in their molecular structures and the strength of their intermolecular forces. H2O2 has a dihedral molecular structure, allowing it to form up to four hydrogen bonds, while water has a bent structure and can form up to two hydrogen bonds. This leads to stronger hydrogen bonding in H2O2, resulting in a higher boiling point of \(152.2^{\circ} \mathrm{C}\) as compared to water's \(100^{\circ} \mathrm{C}\), and a lower vapor pressure due to its more stable liquid state and lower tendency to evaporate.

Step-by-step solution

Compare molecular structures of H2O and H2O2

First, we should look at the molecular structures of both hydrogen peroxide (H2O2) and water (H2O) to understand their physical properties. Water has a bent molecular shape with an angle of 104.5 degrees between the two hydrogen atoms and the oxygen atom. On the other hand, hydrogen peroxide has a dihedral molecular shape, with two groups of hydrogen-oxygen pairs connected by a single bond. This difference in molecular structures defines the possible intermolecular forces between the molecules in each compound.

Identify the intermolecular forces present in both substances

In both compounds, the primary intermolecular force is hydrogen bonding, which occurs between the hydrogen atom of one molecule and the oxygen atom of another molecule. However, there are some additional forces present, such as dipole-dipole forces and London dispersion forces. In water, due to its bent structure, the molecule is polar, and as a result, a stronger force arises, called dipole-dipole interaction. Hydrogen peroxide is also polar but to a lesser extent than water due to its dihedral structure. The London dispersion forces are relatively weak in both compounds but slightly stronger in H2O2 because it has more electrons.

Compare the boiling points of H2O and H2O2

The normal boiling point of water is \(100^{\circ} \mathrm{C}\) while that of hydrogen peroxide is \(152.2^{\circ} \mathrm{C}\). This difference can be attributed to the strength of the hydrogen bonding in both compounds. The hydrogen bonding in H2O2 is stronger due to the presence of more hydrogen bond donors and acceptors, as each molecule can form up to four hydrogen bonds. Water, on the other hand, can form up to two hydrogen bonds. The stronger the intermolecular forces, the higher the boiling point since more energy is required to separate the molecules.

Compare the vapor pressures of H2O and H2O2

Water has a higher vapor pressure than hydrogen peroxide because the stronger hydrogen bonding in H2O2 results in more stable liquid state, making it more resistant to becoming a vapor. Lower vapor pressure indicates that hydrogen peroxide has a lower tendency to evaporate under normal conditions as compared to water. In conclusion, the differences in the physical properties of hydrogen peroxide and water, specifically their boiling points and vapor pressures, can be attributed to the difference in their molecular structures and the strength of their intermolecular forces. Hydrogen peroxide has a higher boiling point and lower vapor pressure than water due to the stronger hydrogen bonding arising from its dihedral structure and the presence of more hydrogen bond donors and acceptors.

Key concepts

Molecular Structure

The molecular structure plays a critical role in determining the physical and chemical properties of a substance. In the case of water \(\left(\text{H}_2\text{O}\right)\) and hydrogen peroxide \(\left(\text{H}_2\text{O}_2\right)\), their differences start at the molecular level. Water has a bent molecular shape with an angle of 104.5 degrees between the hydrogen atoms and the oxygen atom.
This specific angle results in a polar molecule, where the distribution of electrical charges leads to a stronger dipole moment.

• Water (H₂O): Bent shape, 104.5° bond angle, polar molecule.
• Hydrogen Peroxide (H₂O₂): Dihedral shape, linkage of two hydrogen-oxygen pairs, still polar but less so due to a different geometric structure.

The dihedral conformation in hydrogen peroxide implies that the molecule has a greater degree of rotational freedom and can form more complex bonding interactions. This structural difference is a foundation for the uniqueness in physical properties like boiling point and vapor pressure.

Hydrogen Bonding

Hydrogen bonding is a key intermolecular force influencing the properties of substances like water and hydrogen peroxide. Both molecules possess hydrogen bonds, but the extent and impact of these bonds vary greatly between them. In water, hydrogen bonds occur due to the attraction between the slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another. These bonds are significantly strong because of the highly polar nature of water.
Hydrogen peroxide, with its dihedral molecular shape, allows each molecule to form multiple hydrogen bonds. In fact, each molecule can both donate and accept more hydrogen bonds than a water molecule can.

• Strength: Water's bonds are strong due to significant polarity which enhances dipole-dipole interactions.
  Hydrogen peroxide has additional bonding capabilities, which makes its hydrogen bonds particularly robust.
• Impact: The additional hydrogen-bonding capability helps explain the higher boiling point and lower vapor pressure of hydrogen peroxide compared to water.

Through this stronger hydrogen bonding network, hydrogen peroxide achieves a more stable liquid state and a different thermal profile.

Boiling Point

The boiling point of a substance is determined largely by its intermolecular forces. Water has a boiling point of \(100°\text{C}\), while hydrogen peroxide boils at a notably higher \(152.2°\text{C}\).
This difference can be attributed to the number and strength of hydrogen bonds.A stronger network of intermolecular forces in hydrogen peroxide, due to its ability to form more hydrogen bonds per molecule, means that more energy is required to break these interactions and convert the liquid into a gaseous state.

• Water (H₂O): Boiling point \(100°\text{C }\), with each molecule forming two hydrogen bonds.
• Hydrogen Peroxide (H₂O₂): Boiling point \(152.2°\text{C}\), forming up to four hydrogen bonds per molecule.

Thus, hydrogen peroxide's high boiling point underscores the critical influence of its specific molecular arrangements and intermolecular bonding capabilities.

Vapor Pressure

Vapor pressure is an essential property reflecting how readily a liquid substance evaporates. Lower vapor pressure indicates a compound that doesn't easily become gas, while higher vapor pressure corresponds to a volatile substance. Water has a relatively high vapor pressure compared to hydrogen peroxide, which means it evaporates more easily at room temperature. This is because water, despite its strong hydrogen bonds, has fewer of them per molecule compared to hydrogen peroxide.

• Water (H₂O): Higher vapor pressure due to fewer hydrogen bonds which allows ease of transition from liquid to gas.
• Hydrogen Peroxide (H₂O₂): Lower vapor pressure, as the stronger and more abundant hydrogen bonds create a more stable liquid state, hindering evaporation.

Ultimately, the strong hydrogen bonding in hydrogen peroxide stabilizes its molecules significantly more in the liquid state, reducing its vapor pressure and decreasing its likelihood of evaporation compared to water. This difference significantly impacts its usage and behavior in various chemical and practical applications.