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

(a) On melting, ammonia molecules gain kinetic energy, causing them to move more freely and expand the intermolecular spaces. This weakens the hydrogen bonds between neighboring molecules, resulting in a 26% decrease in hydrogen bonding. This allows the molecules to move more freely in the liquid phase. (b) During the transition from liquid to vapor, the kinetic energy further increases, overpowering the remaining intermolecular forces, including hydrogen bonding. A significant 67% loss in hydrogen bonding occurs at the boiling point, allowing the molecules to move even more freely in the gas phase. This is due to the increased kinetic energy required to break the remaining hydrogen bonds and separate the molecules in the gas phase.

Step-by-step solution

(Hydrogen Bonding in Ammonia)

Ammonia (NH3) is a polar molecule where the nitrogen atom is bonded to three hydrogen atoms. It experiences hydrogen bonding between these nitrogen and hydrogen atoms. Hydrogen bonding is an intermolecular force that arises due to an attraction between the partial negative charge on the nitrogen atom and the partial positive charge on a hydrogen atom in a neighboring molecule.

(Kinetic Energy and Phase Transitions)

The kinetic energy of the molecules increases with an increase in temperature. This higher kinetic energy allows the molecules to overcome the intermolecular forces holding them together. During a phase transition, such as melting or evaporation, molecules gain enough kinetic energy to break the intermolecular forces present in the substance.

(a) Decrease of Hydrogen-Bonding Energy on Melting)

On melting, the ammonia molecules gain kinetic energy as the temperature increases. This added kinetic energy causes the molecules to move more freely, leading to an expansion of the substance's intermolecular spaces. Consequently, the hydrogen bonds between the neighboring molecules weaken, and 26% of the hydrogen bonding breaks down. As the system transitions from a solid to a liquid, the decrease in hydrogen bonding allows molecules to move more freely in the liquid phase.

(b) Loss of Hydrogen Bonding in the Transition from the Liquid to the Vapor State)

During the transition from the liquid to the vapor phase, the kinetic energy of the ammonia molecules further increases, providing enough energy to overpower the remaining intermolecular forces, including hydrogen bonding. At the boiling point, a large portion of the hydrogen bonding (67%) breaks down, allowing the molecules to overcome the attractive forces and move even more freely in the gas phase compared to the liquid phase. This significant loss of hydrogen bonding during the vaporization process is mainly due to the increased kinetic energy of ammonia molecules, which is required to break the remaining hydrogen bonds and allow the separation of molecules in the gas phase.

Key concepts

Kinetic Energy and Phase Transitions

Understanding how molecules behave during phase transitions requires a look at kinetic energy. This energy is related to the movement of particles; for example, the speed with which molecules move around. When you heat a substance like ammonia, the molecules inside it start moving faster. As their temperature increases, their kinetic energy does too.

Now, imagine those particles are at a school dance, initially moving slowly in solid state. As the music tempo (temperature) goes up, they begin to 'dance' more energetically. This is what happens with ammonia molecules as they transition from solid to liquid, and then to gas. At the melting point, they've gathered enough energy to loosen up, breaking some of the hydrogen bonds that hold them close together in a tightly choreographed pattern. This 'break in the dance' allows them to flow more fluidly—hence we get a liquid.

Then comes evaporation, which is like the end of the dance when everyone heads off home. As kinetic energy increases further, ammonia molecules have enough energy to break away from any intermolecular attractions. The transition into gas phase at the boiling point means they're moving with the full freedom of ‘dancing’ through the entire room—that is, the molecules are completely separated, turning into vapor.

Intermolecular Forces

Like invisible threads, intermolecular forces pull substances together. These forces, including hydrogen bonding, are the reasons why materials have different states of matter at room temperature—some are gases, others liquids, or solids. Ammonia, being a compound with a moderate strength hydrogen bond, exhibits fascinating behavior in this regard.

The hydrogen bonding in ammonia is a type of glue that holds its molecules of nitrogen and hydrogen together. Think of each molecule as having a tiny magnet where the nitrogen end is slightly negative and the hydrogen end slightly positive. Like magnets, the opposite charges attract, and that's what pulls different ammonia molecules towards each other. However, when heat is added, it's as if you're shaking a table full of magnetic pieces. The pieces jiggle and move apart, weakening the overall magnetic pull. This is similar to what happens to ammonia molecules as they undergo phase transitions when subjected to heat.

Ammonia Phase Changes

Phase changes of ammonia demonstrate the drama of chemistry in action. Ammonia has relatively strong hydrogen bonds due to its polar nature, but these bonds are not unbreakable. When we consider the phase changes, we're observing a three-act play where temperature and kinetic energy dictate the storyline.

In the solid phase, the ammonia molecules are like actors on stage in a tight formation. Heat is the director telling them to increase their movements, and as the curtain rises to the melting point, the formation breaks. We then witness a 26% break down of those special hydrogen bonds. However, it's during the final act, as we approach the boiling point and the molecules transition to the gas phase, that a grand 67% of these bonds snap. This is because the molecules now have enough kinetic energy to completely override the attractive forces, allowing them to transition into the gas phase and disperse throughout the atmosphere like actors leaving the stage.