Problem 16 - Chemistry and Chemical Reactivity

Question

Which compound in each of the following pairs should have the higher melting point? Explain briefly. (a) NaCl or RbCl (b) BaO or \(\mathrm{MgO}\) (c) NaCl or MgS

Step-by-Step Solution

Answer

(a) NaCl, (b) MgO, (c) MgS

Step 1: Understanding Melting Points

The melting point of a compound is determined by the strength of the ionic bonds holding the crystal lattice together. Greater forces result in higher melting points. Ionic compounds' melting points depend on the charges of ions and the distance between them, which is influenced by the ionic sizes.

Step 2: Comparing NaCl and RbCl

Both NaCl and RbCl are ionic compounds. NaCl consists of Na⁺ and Cl⁻ ions, while RbCl consists of Rb⁺ and Cl⁻ ions. Na⁺ is smaller than Rb⁺, leading to stronger ionic bonds in NaCl than in RbCl because the ions are closer together. Thus, NaCl should have a higher melting point than RbCl.

Step 3: Comparing BaO and MgO

BaO and MgO are ionic compounds with Ba²⁺, Mg²⁺, and O²⁻ ions. The Mg²⁺ ion is smaller than the Ba²⁺ ion, and due to the higher density of charge compared to the size of Mg²⁺, MgO has stronger ionic bonds. Consequently, MgO should have a higher melting point than BaO.

Step 4: Comparing NaCl and MgS

NaCl consists of Na⁺ and Cl⁻ ions, while MgS consists of Mg²⁺ and S²⁻ ions. MgS has ions with higher charges compared to NaCl, resulting in much stronger ionic bonds. Thus, MgS is expected to have a higher melting point than NaCl.

Key Concepts

Ionic Bonds Strength

The strength of ionic bonds is crucial in determining the melting point of ionic compounds. These bonds are formed when positively charged ions (cations) and negatively charged ions (anions) are attracted to each other. The attraction is due to electrostatic forces. The greater the electrostatic forces, the stronger the bond and, consequently, the higher the melting point.
The charge on the ions plays a significant role here:

• Higher charges on ions mean stronger attractions.
• Larger differences in charge densities increase the bond strength.

This is why compounds with multiply charged ions often have higher melting points compared to those with singly charged ions.

Ion Sizes and Charges

Ion sizes and their charges are two key factors that impact the strength of ionic bonds.
Smaller ions can get closer together, resulting in stronger electrostatic attractions.
Similarly, ions with higher charges generate stronger attractions.

• Small ions = Stronger bonds
• Higher charges = Stronger bonds

The combination of these factors can make a substantial difference in the physical properties of ionic compounds, especially in their melting points.

Comparing NaCl and RbCl

When comparing NaCl and RbCl, both consist of monovalent ions with charges of +1 and -1. The difference in melting points is due to the size of the cations:

• Na⁺ ion: smaller, allowing closer packing with Cl⁻ ions.
• Rb⁺ ion: larger, resulting in weaker ionic interactions.

Therefore, NaCl has a stronger ionic bond, leading to a higher melting point than RbCl. Smaller ions create proximity that intensifies the bond strength.

Comparing BaO and MgO

In comparing BaO and MgO, we notice a shift in the charges of the ions. Both BaO and MgO involve divalent cations (Ba²⁺ and Mg²⁺), but MgO is distinct because the Mg²⁺ ion is smaller:

• Mg²⁺ ion: smaller and has a higher charge density than Ba²⁺.
• Ba²⁺ ion: larger and offers lower charge density interaction.

Due to the smaller size of Mg²⁺, MgO exhibits stronger ionic bonds and therefore, a higher melting point compared to BaO.

Comparing NaCl and MgS

Although both NaCl and MgS are ionic compounds, they vary in the charges and sizes of their respective ions.
NaCl involves singly charged ions (Na⁺ and Cl⁻), while MgS has doubly charged ions (Mg²⁺ and S²⁻):

• Ions in MgS have higher charges, resulting in much stronger electrostatic attractions than those in NaCl.
• The increased charges and potentially closer packing lead to MgS having a significantly higher melting point.

Double charges amplify the forces holding the ions together, making MgS notably more heat-resistant.