Question

The following quote about ammonia \(\left(\mathrm{NH}_{3}\right)\) is from a textbook of inorganic chemistry: "It is estimated that \(26 \%\) of the hydrogen bonding in \(\mathrm{NH}_{3}\) breaks down on melting, \(7 \%\) on warming from the melting to the boiling point, and the final \(67 \%\) on transfer to the gas phase at the boiling point." From the standpoint of the kinetic energy of the molecules, explain (a) why there is a decrease of hydrogen-bonding energy on melting and (b) why most of the loss in hydrogen bonding occurs in the transition from the liquid to the vapor state.

Short answer

In short, the decrease in hydrogen bonding during phase transitions in ammonia is due to the increase in kinetic energy of the molecules. (a) During melting, increased molecular motion reduces the effectiveness of hydrogen bonding, leading to a 26% breakdown. (b) During the liquid-to-gas transition, the drastic increase in kinetic energy causes molecules to move more rapidly and spread out, overcoming the attractive hydrogen bonding interactions, resulting in a 67% loss of hydrogen bonding.

Step-by-step solution

Understanding hydrogen bonding in NH3

Ammonia (NH3) has strong hydrogen bonding due to the electronegativity of the nitrogen atom, which creates a partial positive charge on the hydrogen atoms and a partial negative charge on the nitrogen atom. These charges attract each other, leading to the formation of hydrogen bonds between adjacent NH3 molecules.

Change in hydrogen bonding during melting

(a) As the temperature increases and NH3 reaches its melting point, the kinetic energy of the molecules also increases. This increased molecular motion reduces the effectiveness of the hydrogen bonding forces between the molecules because they have less time to "stick" together due to hydrogen bonding. As a result, 26% of hydrogen bonding in ammonia breaks down during melting.

Change in hydrogen bonding during boiling

(b) When NH3 is heated further from melting to boiling point, the kinetic energy of the molecules continues to increase. This slight increase in heat and molecular motion only cause a 7% reduction in hydrogen bonding. However, when it reaches the boiling point and transitions from liquid to gas, the majority (67%) of the hydrogen bonding is lost.

Role of kinetic energy

The reason this dramatic loss of hydrogen bonding occurs during the liquid to vapor transition is because of the drastic increase in kinetic energy. The high kinetic energy causes the molecules to move rapidly and spread out, overcoming the attractive hydrogen bonding interactions. In the gas phase, molecules have enough kinetic energy to break free from the hydrogen bonding forces that held them together in the liquid phase. In conclusion, the decrease in hydrogen bonding during phase transitions in ammonia can be explained by the increase in the kinetic energy of the molecules. As their kinetic energy increases, the molecules move more rapidly, making hydrogen bonding less effective and eventually leading to the majority of the hydrogen bonds breaking during the liquid-to-gas phase transition.

Key concepts

Kinetic Energy

Kinetic energy refers to the energy that molecules possess due to their motion. In the case of ammonia (NH3), this concept is key to understanding changes in hydrogen bonding during phase transitions.
As the temperature increases, so does the kinetic energy of the NH3 molecules. This increase in motion means molecules spend less time interacting, or "sticking," with each other. Therefore, the increased motion reduces the effectiveness of intermolecular hydrogen bonding forces.

• At higher temperatures, kinetic energy is greater, making hydrogen bonds weaker or breaking them entirely.
• With more kinetic energy, molecules can move freely and widely, leading to phase transitions.
• Kinetic energy is necessary to achieve changes in physical states like melting and boiling.

Hence, kinetic energy is vital for understanding how molecular behaviors change with phase transitions.

Phase Transitions

Phase transitions are changes from one physical state to another, such as from solid to liquid (melting) or liquid to gas (boiling). These transformations occur due to changes in temperature, leading to corresponding changes in kinetic energy.
For ammonia, phase transitions play a crucial role in hydrogen bonding dynamics.

• During melting, NH3 transitions from a solid to a liquid. Around 26% of its hydrogen bonds are broken because the increased movement of molecules reduces the opportunity for bonds to form.
• From melting to boiling, only a small further reduction of bonding (7%) occurs, highlighting that even more energy is required to change from liquid to vapor.
• When NH3 reaches boiling point, 67% of hydrogen bonds break as the molecules gain enough kinetic energy to enter the gas phase.

This transition from liquid to vapor represents a significant change in molecular interaction due to increased kinetic energy.

Boiling Point

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. For ammonia, this is where the most drastic change in hydrogen bonding occurs. At boiling point, the kinetic energy of NH3 molecules is high enough for them to overcome hydrogen bonding.
This energy level enables molecules to move fast and separate, resulting in a gas state where intermolecular forces like hydrogen bonds are substantially weakened or entirely overcome.

• Ammonia's boiling point is where a large portion of hydrogen bonds break, particularly due to increased molecular energy and motion.
• This phase transition showcases a significant change where molecules are no longer bound by the strong interactions that are present in the liquid state.
• Achieving boiling point energy levels allows molecules to transition to the gas phase, a key characteristic of boiling.

The boiling point is crucial for understanding how kinetic energy facilitates the breakdown of molecular interactions.

Melting Point

The melting point is the temperature at which a solid becomes a liquid. When ammonia reaches its melting point, its molecules gain enough kinetic energy to break a portion of their hydrogen bonds.
Due to the increased motion, about 26% of hydrogen bonds in NH3 are broken at this stage. This is because the molecules vibrate or move more vigorously, not allowing enough time or stability to maintain hydrogen bonding.

• The melting point is where molecules begin to experiment increased energy that starts affecting hydrogen bonding stability.
• With enhanced kinetic energy at the melting point, NH3 transitions from a tightly packed solid to a more fluid liquid state.
• The melting point marks the beginning of kinetic energy's impact on molecular interaction, setting the stage for subsequent phase transitions.

Understanding the melting point helps illustrate how initial changes in kinetic energy can lead to phase transitions.