Question

When ammonium nitrate, \(\mathrm{NH}_{4} \mathrm{NO}_{3}\), dissolves in water, the solution feels cool. What can we say about the relative strengths of the forces between solute and solvent? Between the particles in the pure substances?

Short answer

The reaction of ammonium nitrate dissolving in water is endothermic, indicating that the forces of attraction between solute and solvent are stronger than those existing within the pure substances. This is because more energy is absorbed to break the bonds within the solute and solvent than is released during the formation of new bonds between solute and solvent.

Step-by-step solution

Understanding the heat absorption

When ammonium nitrate dissolves in water, the solution feels cool. This is an indication that the process of ammonium nitrate dissolving in water is endothermic in nature i.e. it absorbs heat from the surroundings. The heat is essentially absorbed in order to break the ionic bonds in ammonium nitrate per the equation \(\mathrm{NH}_{4} \mathrm{NO}_{3}(s) \rightarrow \mathrm{NH}_{4}^{+}(aq) + \mathrm{NO}_{3}^{-}(aq)\)

Analyze the forces between solute and solvent

The forces between solute and solvent (ammonium nitrate and water, respectively) engage in ion-dipole interactions, which are formed between the ions of the solute and the polar water molecules. This interaction is sufficiently strong to allow for the ions to be pulled apart and surrounded by water molecules, leading to dissolution.

Analyze the forces within the pure substances

In pure substances, the forces involved are those within the ionic compound ammonium nitrate and those within water molecules. In the case of ammonium nitrate, the forces are ionic bonds which are quite strong and require a large amount of energy to break. On the other side, within water, the forces are hydrogen bonds, which are one of the strongest types of intermolecular forces.

Key concepts

Ion-Dipole Interactions

When we talk about ion-dipole interactions, we're looking at the attractive force that occurs between a charged ion and a molecule that has a dipole, or difference in electrical charge, within it. This is a key concept when exploring why certain substances dissolve in water.

Let's consider our exercise example where ammonium nitrate dissolves in water. The water molecule, H₂O, has a distinct dipole moment due to its shape and the electronegativity difference between hydrogen and oxygen atoms. Each water molecule has a partially negative oxygen atom and two partially positive hydrogen atoms. When the ionic compound ammonium nitrate enters water, the positive ammonium ions (\(NH_4^+\)) are attracted to the partial negative charge on the oxygen atoms of water molecules. Similarly, the negative nitrate ions (\(NO_3^-\)) are attracted to the partial positive charges of the hydrogen atoms.

This interaction is crucial in the dissolution process of ionic compounds in polar solvents like water. The feeling of coolness detected when ammonium nitrate dissolves in water indicates that these ion-dipole interactions are strong enough to disrupt the ionic bonds within the solid ammonium nitrate, but the overall process requires the absorption of heat, showcasing the endothermic nature of the dissolution.

Ionic Bonds

The formation and disruption of ionic bonds are at the heart of many chemical processes, including the dissolution of salts like ammonium nitrate in water. An ionic bond is a type of chemical bond that forms between atoms with notably different electronegativities, resulting in the complete transfer of electrons from one atom to another.

These bonds create oppositely charged ions, which solidify into a lattice structure of alternating positive and negative charges. The strength of these bonds is significant, giving ionic compounds high melting and boiling points. When discussing the dissolution process of ammonium nitrate, it's crucial to understand that breaking these strong ionic bonds between the ammonium ions and nitrate ions requires energy input – part of the reason the dissolution feels cold to the touch is that energy (in the form of heat) is taken from the surroundings, confirming the endothermic dissolution.

Hydrogen Bonds

Hydrogen bonds are a strong type of attraction that plays a critical role in the structure and properties of many compounds, including water. These bonds occur when a hydrogen atom, covalently bound to a highly electronegative atom such as oxygen, nitrogen, or fluorine, experiences attraction to a lone pair of electrons on another electronegative atom.

In water, each molecule can form up to four hydrogen bonds—with two bonds using the lone pairs on oxygen and two more from the hydrogen atoms binding to the oxygen atoms of neighboring water molecules. This interaction is what gives water its unique properties, such as high boiling point, surface tension, and ability to dissolve many substances. When ammonium nitrate dissolves and the solution feels cool, the hydrogen bonds between water molecules must be disrupted to accommodate the incoming ions. However, the formation of new ion-dipole interactions compensates for this disruption, allowing for the ions to become hydrated and dispersed throughout the solution.