Problem 48 - Chemistry The Molecular Science

Question

Solid magnesium oxide melts at \(2800^{\circ} \mathrm{C}\). This property, combined with the fact that magnesium oxide is not an electrical conductor, makes it an ideal heat insulator for electric wires in cooking ovens and toasters. In contrast, solid \(\mathrm{NaCl}\) melts at the relatively low temperature of \(801{ }^{\circ} \mathrm{C}\). What is the formula of magnesium oxide? Suggest a reason that it has a melting temperature so much higher than that of \(\mathrm{NaCl}\).

Step-by-Step Solution

Answer

The formula of magnesium oxide is MgO. Its higher melting point is due to stronger ionic bonds from Mg²⁺ and O²⁻ compared to NaCl.

Step 1: Identify the Elements

Magnesium oxide is composed of the elements magnesium (Mg) and oxygen (O).

Step 2: Determine the Charges

Magnesium ion has a charge of +2, represented as Mg²⁺, and oxide ion has a charge of -2, represented as O²⁻.

Step 3: Write the Formula

To form a neutral compound, one magnesium ion combines with one oxide ion, resulting in the formula MgO.

Step 4: Compare Ionic Structures

Both MgO and NaCl are ionic compounds, but MgO has ions with higher charges than NaCl, where sodium ion (Na⁺) has +1 and chloride ion (Cl⁻) has -1. This results in stronger ionic bonds in MgO.

Step 5: Explain the Melting Point

The higher ionic charges in MgO lead to stronger electrostatic forces between the ions, requiring more energy to break the bonds, resulting in a higher melting point compared to NaCl.

Key Concepts

Melting Point

The melting point of a substance is the temperature at which it transforms from solid to liquid. In the context of ionic compounds like magnesium oxide (MgO), a high melting point, such as 2800°C, indicates very strong interactions between the ions. This high energy requirement for melting exists because a lot of energy is needed to overcome these strong forces.

Keeping this in mind, when we compare MgO to sodium chloride (NaCl), which melts at 801°C, it is clear that MgO has much stronger forces binding its ions together. The high melting point of MgO makes it a suitable material for use in environments with extreme temperatures, such as in cooking appliances, because it remains stable without melting.

• High melting points indicate strong ionic bonds
• MgO can function in high temperatures without compromising its structure

Ionic Compounds

Ionic compounds are formed by the complete transfer of electrons from one atom to another, resulting in oppositely charged ions. These ions are then held together by strong electrostatic attractions, forming a lattice structure. In magnesium oxide, magnesium atoms lose two electrons to become Mg²⁺, while oxygen atoms gain those electrons to become O²⁻, forming the compound MgO.

The strength of an ionic compound largely depends on the charges of the ions. Compounds with higher charged ions, like MgO, tend to have stronger ionic bonds, compared to those formed by lower charges, such as NaCl. This is why MgO exhibits such impressive properties, including a high melting point. Ionic compounds also have notable electrical properties when molten or dissolved:

• Composed of positive and negative ions
• Strong ionic bonds result in high melting points
• Molten or dissolved ionics can conduct electricity

Electrostatic Forces

Electrostatic forces are the forces of attraction or repulsion between charged particles. In ionic compounds, these forces are responsible for holding the ions together in a crystal lattice structure. The magnitude of these forces is crucial for determining the properties of the compound.

In the case of magnesium oxide, the electrostatic forces are particularly strong due to the high charges on the Mg²⁺ and O²⁻ ions. These strong forces make it difficult to separate the ions, hence the requirement of significant energy to melt MgO. This is the key reason for the higher melting point of MgO compared to other ionic compounds such as NaCl, which has weaker electrostatic forces between the Na⁺ and Cl⁻ ions.

• Forces between charged particles
• Stronger forces in MgO due to +2 and -2 charges
• High energy needed to overcome these forces