Question

At room temperature, water is a liquid with a molar volume of 18 \(\mathrm{mL}\) . At \(105^{\circ} \mathrm{C}\) and 1 atm pressure, water is a gas and has a molar volume of over 30 \(\mathrm{L}\) . Explain the large difference in molar volumes.

Short answer

The large difference in molar volumes of water at room temperature (liquid state) and at 105°C and 1 atm pressure (gaseous state) can be explained by the changes in intermolecular forces during the phase transition. In its liquid state, water molecules are held closely together by hydrogen bonding, resulting in a smaller molar volume (18 mL). When the temperature is raised and water becomes a gas, hydrogen bonding is no longer effective, causing the water molecules to be much more dispersed and leading to a significantly larger molar volume (over 30 L). The increase in kinetic energy and the breaking of intermolecular forces result in a much larger separation between molecules in the gaseous state compared to the liquid state.

Step-by-step solution

At room temperature, water is normally found in the liquid state. This is because the kinetic energy of water molecules is not enough to overcome the intermolecular forces holding them together, which in the case of water are mainly hydrogen bonds. In this state, the molecules are relatively close together, which results in a smaller molar volume. At 105 °C and 1 atm pressure, water exists as a gas. In this state, the kinetic energy of the water molecules is high enough to overcome the intermolecular forces, and the molecules are now much further apart from each other. This results in a significantly larger molar volume. #Step 2: Intermolecular forces in water#

In water, the primary intermolecular force is hydrogen bonding. Hydrogen bonding is a type of dipole-dipole interaction that occurs between molecules with a hydrogen atom bonded to a highly electronegative atom such as oxygen. The difference in electronegativity between hydrogen and oxygen creates a polar bond, leading to a partial positive charge on the hydrogen atom and a partial negative charge on the oxygen atom. This results in attraction between the positively charged hydrogen atoms and the negatively charged oxygen atoms in neighboring molecules. #Step 3: Phase transitions#

When we raise the temperature of water from room temperature to 105 °C, we are increasing the kinetic energy of the water molecules. At a certain point, the kinetic energy becomes high enough to overcome the intermolecular forces holding the molecules together, resulting in a phase transition from liquid to gas. This phase transition is known as boiling, and it occurs at 100 °C and 1 atm pressure for pure water. However, since we are asked about water at 105 °C and 1 atm pressure, it is safe to assume the transition has already occurred and the water is in its gaseous state. #Step 4: Explaining the molar volume difference#

The reason for the large difference in molar volumes between the liquid and gaseous states of water is due to the changes in intermolecular forces during the phase transition. When water is in its liquid state, the hydrogen bonding between molecules keeps them relatively close together, resulting in a smaller molar volume (18 mL). When the water undergoes a phase transition to become a gas, the hydrogen bonding is no longer effective in holding the molecules together, and they become much more dispersed, leading to a significantly larger molar volume (over 30 L). The increase in kinetic energy and the breaking of intermolecular forces result in a much larger separation between molecules in the gaseous state compared to the liquid state.

Key concepts

Molar Volume

Molar volume is an essential concept in chemistry that refers to the volume occupied by one mole of a substance. It's useful for understanding how substances behave under different conditions. For instance, water at room temperature, around 25°C, has a molar volume of about 18 mL when in liquid form. This relatively small volume is due to the molecules being tightly packed because of strong intermolecular forces.

However, when water is heated to 105°C and still at 1 atm pressure, it transforms into a gas. In this gaseous state, the molar volume expands dramatically to over 30 L. This expansion occurs because the water molecules are much further apart, as the kinetic energy has increased sufficiently to overcome the intermolecular forces. The molar volume difference highlights how state changes can significantly affect a substance's density and occupying space.

The concept of molar volume is especially important when examining phase changes, as it gives insight into how substances react to changes in temperature and pressure, thus affecting their physical states.

Phase Transition

Phase transitions are transformations between different states of matter, such as solid, liquid, and gas. These changes occur when environmental conditions, primarily temperature or pressure, shift sufficiently to alter the physical state. For example, water experiencing a phase transition from liquid to gas around 100°C—known as boiling—is influenced by increased kinetic energy overcoming the bonds holding molecules together.

During such transitions, the arrangement of molecules changes significantly. In the liquid state, water molecules are close due to hydrogen bonding, but when transitioning to a gaseous state at 105°C, these molecules move far apart. This spatial change underlies the substance's expansion and altered density. It explains why boiling water takes up much more space as gas than it does as a liquid—illustrating phase transition's impact on molar volume.

Understanding phase transitions is critical for appreciating how materials behave under different temperatures and pressures. These transitions are the foundation of many natural and industrial processes where temperature regulation is crucial.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole attraction that plays a crucial role in the properties of water. This force occurs when a hydrogen atom covalently bonded to a highly electronegative atom, like oxygen, interacts with an electronegative atom in a nearby molecule. Such bonds give water many of its unique characteristics, such as its relatively high boiling point.

In water, each hydrogen atom develops a partial positive charge, while the oxygen atom holds a partial negative charge. This polarity leads to hydrogen bonds forming between the hydrogen atoms of one molecule and the oxygen atoms of another, keeping the molecules close in the liquid form. This bonding also leads to water’s smaller molar volume in its liquid state because the molecules don't move as freely as they do in the gaseous state.

When water is heated to a temperature beyond its boiling point, like 105°C, these hydrogen bonds break, allowing the molecules to disperse into a gaseous state. Despite their strength in liquid forms, these bonds aren't sufficient to prevent transition into a gas under such conditions, which results in a drastic increase in molar volume. Understanding hydrogen bonding provides valuable insight into why substances like water behave the way they do under different conditions.