Problem 58 - Introductory Chemistry Atoms First

Question

What is the name of the attractive force between dissolved \(\mathrm{Na}^{+}\) ions and water molecules? Diagram this force, showing how a water molecule would approach an \(\mathrm{Na}^{+}\) ion. Do the same for a \(\mathrm{Cl}^{-}\) ion.

Step-by-Step Solution

Answer

The attractive force between dissolved Na+ ions and water molecules is called ion-dipole interaction. The interaction between the Na+ ion and water molecule can be represented as: \(Na^+(aq) + H_2O -> [Na(H_2O)]^+\) Similarly, for the attractive force between the Cl- ion and a water molecule, the interaction can be represented as: \(Cl^-(aq) + H_2O -> [Cl(H_2O)]^-\)

Step 1: Identify the Attractive Force

The attractive force between dissolved Na+ ions and water molecules is called ion-dipole interaction.

Step 2: Diagram the Interaction Between Na+ Ion and Water Molecule

To draw a diagram showing the attractive force between the Na+ ion and a water molecule, follow these steps: 1. Draw an Na+ ion (a small circle labeled Na+) representing the dissolved sodium ion. 2. Draw a water molecule (two small circles labeled H connected to a big circle labeled O) representing a water molecule. 3. Show the partial positive charges on the hydrogen atoms (labeled δ+) and the partial negative charge on the oxygen atom (labeled δ-). 4. Draw an arrow from the oxygen atom (with the δ- sign) toward the Na+ ion, which represents the attractive force between the Na+ ion and the water molecule's oxygen atom. The interaction between the Na+ ion and water molecule can be represented as: Na+(aq) + H₂O -> [Na(H₂O)]^+

Step 3: Diagram the Interaction Between Cl- Ion and Water Molecule

Similarly, to draw a diagram showing the attractive force between the Cl- ion and a water molecule, follow these steps: 1. Draw a Cl- ion (a small circle labeled Cl-) representing the dissolved chloride ion. 2. Draw a water molecule (two small circles labeled H connected to a big circle labeled O) representing a water molecule. 3. Show the partial positive charges on the hydrogen atoms (labeled δ+) and the partial negative charge on the oxygen atom (labeled δ-). 4. Draw an arrow from one of the hydrogen atoms (with the δ+ sign) toward the Cl- ion, which represents the attractive force between the Cl- ion and the water molecule's hydrogen atom. The interaction between the Cl- ion and water molecule can be represented as: Cl-(aq) + H₂O -> [Cl(H₂O)]^-

Key Concepts

Dissolved Ions in Water

When ionic compounds like table salt (sodium chloride) are mixed with water, they dissolve by dissociating into their constituent ions. This dissociation occurs because of the polar nature of water molecules, which can attract and stabilize the charged particles. For instance:

• The positively charged sodium ions (a^+)
• The negatively charged chloride ions (cl^-)

as they interact with water molecules, they become surrounded by a layer of water in a process known as hydration. This phenomenon is crucial for many biological and chemical processes, as it allows ions to be transported in solution and participate in reactions.

Na+ and Water Interaction

Sodium ions (a^+) carry a positive charge. In an aqueous solution, these ions form hydration shells, where water molecules orient themselves around the ion. The negatively charged ends, which are the oxygen atoms of the water molecules, face the sodium ion due to the ion-dipole interaction. This interaction is a type of chemical bonding that helps keep the sodium ion in solution, and it's driven by the electrostatic attraction between the opposite charges.

Drawing a diagram to illustrate this, one would sketch a central sodium ion surrounded by several water molecules, all pointing with their oxygen atoms, marked with a partial negative charge (δ-), towards the a^+ ion.

Cl- and Water Interaction

Conversely, chloride ions (cl^-), which have a negative charge, are also stabilized in water by ion-dipole interactions. This time, however, it is the partial positive charges on the hydrogen atoms of the water molecules that are attracted to the chloride ion. As a result, water molecules arrange with their hydrogen atoms, marked as δ+, pointing toward the cl^- ion, forming a hydration shell.

When you draw this interaction, you'd depict the chloride ion at the center with several water molecules around it, oriented in such a way that their hydrogen atoms are directed toward the central cl^- ion, symbolizing the nature of the ion-dipole bond.

Chemical Bonding

The term 'chemical bonding' refers to the force that holds atoms or ions together in a compound. The ion-dipole interaction discussed earlier is just one type of such bonding. In an aqueous solution, this ion-dipole bonding ensures the ions remain dispersed and are prevented from coming back together to form a precipitate. It's the varying types of such chemical bonds that dictate the structure, properties, and reactions of chemical compounds.

Apart from ion-dipole bonds, other types include:

• Covalent bonding where two atoms share electrons
• Hydrogen bonding where a hydrogen atom is attracted to an electronegative atom, like oxygen or nitrogen
• Van der Waals forces that include dipole-dipole interactions and London dispersion forces

The strength and nature of these bonds play critical roles in the physical and chemical behavior of substances.