Problem 30 - Chemistry: The Central Science

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.

Step-by-Step Solution

Answer

When ammonia melts, the kinetic energy of the molecules increases as they can move more freely in the liquid state compared to the solid state. This increase in kinetic energy allows for some hydrogen bonding to be overcome, resulting in a decrease in hydrogen bonding energy (26% during melting). As ammonia transitions from a liquid to a gas, the kinetic energy of the molecules further increases, providing even more freedom to move and overcome a larger portion of hydrogen bonding. In this phase, 67% of the hydrogen bonding breaks down, causing most of the loss in hydrogen bonding to occur during the transition from liquid to vapor state.

Step 1: Understanding kinetic energy and hydrogen bonding

Kinetic energy refers to the energy possessed by an object due to its motion. In the case of ammonia, this refers to the energy of the molecules as they move. Hydrogen bonding is a type of intermolecular force that occurs between molecules containing hydrogen atoms bonded to highly electronegative atoms (such as nitrogen in NH₃). These forces cause the molecules to stick together, and they need to be overcome for a substance to change its state (e.g., from liquid to gas). Now, let's use this understanding to explain the observations in the exercise.

Step 2: Explaining the decrease of hydrogen-bonding energy on melting

When ammonia melts, meaning it transitions from a solid to a liquid state, the kinetic energy of the molecules increases. This is because in the liquid state, the molecules can move more freely compared to the solid state. The increase in kinetic energy allows the molecules to overcome some of the hydrogen bonding, causing a decrease in hydrogen bonding energy. Therefore, 26% of the hydrogen bonding breaks down during the melting process.

Step 3: Explaining the loss of hydrogen bonding in transition from liquid to vapor state

As ammonia transitions from a liquid state to a vapor (gas) state, the kinetic energy of the molecules further increases. In the gas phase, the molecules have even more freedom to move compared to the liquid state, allowing them to overcome a larger portion of the hydrogen bonding. This is why 67% of the hydrogen bonding breaks down during the transition from liquid to vapor state, making it the phase where most of the loss in hydrogen bonding occurs. Overall, the increase in kinetic energy as ammonia changes from different states allows the molecules to overcome hydrogen bonding to various extents, which explains the given observations about hydrogen bonding in ammonia.