Question

When dry ammonia gas \(\left(\mathrm{NH}_{3}\right)\) is bubbled into a 125 -mL sample of water, the volume of the sample (initially, at least) decreases slightly. Suggest a reason for this.

Short answer

The slight decrease in volume of the water sample when dry ammonia gas is bubbled into it is due to the formation of hydrogen bonds between ammonia and water molecules, leading to a more dense arrangement of molecules in the solution. Ammonia molecules essentially "fill in the gaps" between water molecules, causing a tighter packing and ultimately, a decrease in volume.

Step-by-step solution

Understanding Ammonia Gas and Water Interaction

Ammonia is a polar molecule, which means it has a positive and a negative end (charges are not evenly distributed). In ammonia's case, nitrogen has a slightly negative charge and the hydrogen atoms have slightly positive charges. When ammonia is mixed with water, it can easily dissolve due to hydrogen bonding between ammonia and water molecules.

Hydrogen Bond Formation

When ammonia gas comes into contact with water molecules, the positively charged hydrogen atoms in ammonia (H) get attracted to the negatively charged oxygen atoms in water (O). This leads to the formation of hydrogen bonds between the ammonia molecules and the water molecules. As a result, NH3 molecules are surrounded by water molecules, forming hydration shells or clusters.

Decrease in Volume

When ammonia gas is bubbled into the water, the hydrogen bonding between the ammonia and water molecules causes the formation of clusters. This arrangement has a higher density as compared to individual water molecules. The attraction between ammonia and water molecules causes a tighter packing, which results in a slight decrease in the volume of the water sample. In other words, ammonia molecules are occupying some of the empty spaces between water molecules, essentially "filling in the gaps" and leading to a more compact arrangement of molecules. Overall, the reason behind the observed decrease in the volume of the water sample when ammonia gas is bubbled into it can be attributed to the formation of hydrogen bonds, resulting in a more dense arrangement of molecules in the solution.

Key concepts

Hydrogen Bonding

One of the most fascinating aspects of chemistry is the way molecules interact with one another. A prime example of this is hydrogen bonding, a specific type of interaction that occurs between polar molecules. Hydrogen bonds are relatively weak compared to covalent or ionic bonds, but they are strong enough to have a significant impact on the properties of substances.

For instance, when ammonia (\textbf{NH}\(_3\)), a molecule with a lone pair of electrons on the nitrogen atom, comes into contact with water (\textbf{H}\(_2\)O), a fascinating phenomenon occurs. The slightly positive hydrogen atoms of the ammonia are attracted to the slightly negative oxygen atom of water molecules. This attraction forms a hydrogen bond, which is the key to understanding not only the behavior of many biological molecules but also the reason why your sample of water decreases in volume when ammonia is introduced. It's a kind of intermolecular embrace that brings the molecules closer together, leading to a more compact and dense structure.

Hydrogen bonding is important because it explains a multitude of phenomena, such as the boiling and freezing points of water, the secondary structure of proteins, the property of water as a universal solvent, and even the delicate dance of DNA strands.

Polar Molecules

Polar molecules are the social butterflies of the molecular world; they like to interact, and they do this through their partial charges. To comprehend this concept, picture a molecule as a tiny magnet with a positive end and a negative end. This polarity is due to the unequal sharing of electrons within the molecular bonds, leading to a distribution of electric charge that is not even.

Ammonia is an exemplary polar molecule. The nitrogen atom hoards more of the electrons, bequeathing it with a slight negative charge, while the hydrogen atoms are left with a slight positive charge. This uneven charge distribution permits ammonia to mingle with other polar substances, like water, because opposite charges attract. The polar nature of water itself, with its oxygen atom holding a partial negative charge and hydrogen atoms a partial positive charge, allows it to dissolve many substances and earn the title of the 'universal solvent'.

Understanding the concept of polar molecules is fundamental in grasping the detailed interactions involved in solution chemistry and the behaviors of compounds in various environments–from the human body to the world's oceans.

Solution Chemistry

Solution chemistry is the section of chemistry that studies the combination of substances that form a single homogeneous phase – a solution. When we mix substances like ammonia gas and water, we're inviting them to a molecular level party where they mingle and form a uniform mixture. This involves solutes (like ammonia) dissolving in solvents (like water), which is a process governed by the nature of the molecules involved and their interactions through hydrogen bonding and polarity.

In the case of the previously mentioned gaseous ammonia and water, when the two are combined, the ammonia doesn't just 'disappear' into the water. Rather, it forms a tight community with water molecules through hydrogen bonding, where they hold on to each other, leading to a decrease in the overall volume of the solution. This is because these bonds allow the molecules to pack more efficiently, occupying less space than they would individually.

Furthermore, the solubility of a gas in a liquid is an intriguing aspect of solution chemistry. As more ammonia gas is bubbled into the water, more molecules find their place within the liquid, up to a point where the solution reaches saturation. Beyond this point, no more ammonia can dissolve. This concept plays a vital role in various industries and natural processes, including environmental control, chemical manufacturing, and even in carbonated beverages. By understanding solution chemistry, we can better comprehend and predict the outcomes of mixing various chemicals—knowledge that is invaluable in both laboratory and real-world applications.