Question

Energy is required to remove two electrons from Ca to form \(\mathrm{Ca}^{2+},\) and energy is required to add two electrons to \(\mathrm{O}\) to form \(\mathrm{O}^{2-} .\) Yet \(\mathrm{CaO}\) is stable relative to the free elements. Which statement is the best explanation? (a) The lattice energy of CaO is large enough to overcome these processes. (b) CaO is a covalent compound, and these processes are irrelevant. (c) CaO has a higher molar mass than either Ca or O. (d) The enthalpy of formation of CaO is small. (e) CaO is stable to atmospheric conditions.

Short answer

The best explanation for the stability of CaO relative to its constituent elements is (a) The lattice energy of CaO is large enough to overcome these processes. This is because a large lattice energy indicates strong attractive forces between the ions in the solid-state, contributing to the overall stability of the compound and making it favorable despite the energy required for electron transfer processes.

Step-by-step solution

Statement Analysis

The first step is to analyze each statement separately and consider its relevance to the stability of CaO. (a) The lattice energy of CaO is large enough to overcome these processes. (b) CaO is a covalent compound, and these processes are irrelevant. (c) CaO has a higher molar mass than either Ca or O. (d) The enthalpy of formation of CaO is small. (e) CaO is stable to atmospheric conditions.

Comparing Statements and Understanding Concepts

Now let's discuss the statements one by one. (a) The lattice energy is the energy required to separate one mole of a solid ionic compound into its gaseous ions. This energy contributes to the overall stability of ionic compounds. In the case of CaO, it's an ionic compound with Ca having a +2 charge and O having a -2 charge. This creates a strong attraction between the ions, leading to a large lattice energy. The larger the lattice energy, the more stable the compound. Therefore, this statement is a possible valid explanation. (b) CaO is an ionic compound, not a covalent one. This statement is not relevant to the discussion of CaO's stability and can be ruled out. (c) The molar mass of a compound has no direct influence on its stability. Therefore, this statement is not a valid explanation. (d) The enthalpy of formation measures the energy change when a compound is formed from its constituent elements. A smaller enthalpy of formation indicates that less energy is required to form the compound, but it doesn't explain CaO's stability compared to its elements. This statement is not the best explanation. (e) The stability of CaO to atmospheric conditions is irrelevant to the discussion of the compound's formation and energy processes. This statement is not a valid explanation.

Choosing the Best Explanation

Comparing the given statements, the best explanation for CaO's stability relative to its constituent elements is: (a) The lattice energy of CaO is large enough to overcome these processes. This is because a large lattice energy indicates strong attractive forces between the ions in the solid-state. This energy contributes to the overall stability of the compound, making it favorable despite the energy required for electron transfer processes.

Key concepts

Ionic Compounds

Ionic compounds are chemical compounds composed of oppositely charged ions. These ions form when atoms transfer electrons, resulting in one atom giving up electrons to become a positively charged cation and another gaining electrons to become a negatively charged anion. For example, in the formation of calcium oxide (CaO), calcium (\( \text{Ca} \)) loses two electrons to form \( \text{Ca}^{2+} \), and oxygen (\( \text{O} \)) gains those electrons to become \( \text{O}^{2-} \). This electron transfer leads to the formation of a strong ionic bond between \( \text{Ca}^{2+} \) and \( \text{O}^{2-} \). Ionic compounds like CaO are usually solid at room temperature and have high melting and boiling points. This is because the ionic bonds are strong, requiring significant energy to break.

• Key Feature: Formed by electron transfer.
• Strong attractions due to opposite charges.
• Usually solid with high melting points.

Understanding these characteristics helps explain why ionic compounds like CaO are stable in their solid form.

Enthalpy of Formation

Enthalpy of formation refers to the heat change that occurs when one mole of a compound is produced from its elemental constituents under standard conditions. This measurement is important because it provides insight into the energy required or produced when forming compounds from elements. For CaO, the enthalpy of formation involves the energy required for Ca to lose electrons and for O to gain electrons. When compounds such as CaO are formed, the lattice energy also plays a role, especially in ionic compounds. While the enthalpy of formation involves multiple energy changes, it typically considers the energy gained from forming the ionic lattice structure. However, in the case of the exercise, relying on enthalpy of formation alone does not fully explain CaO's stability relative to free elements.

• Measures energy change during compound formation.
• Involves multiple energy transformations.
• Lattice energy significantly contributes to stability.

This means that although it gives an idea of how energetically favorable the forming reaction is, other considerations like lattice energy must be factored in.

Electron Transfer

Electron transfer is a fundamental process in forming ionic compounds. It involves moving electrons from one atom to another, turning those atoms into ions. For instance, when calcium (\( \text{Ca} \)) transfers electrons to oxygen (\( \text{O} \)), calcium becomes a \( \text{Ca}^{2+} \) ion, and oxygen becomes an \( \text{O}^{2-} \) ion. This transfer forms the basis of an ionic bond, where the electrostatic attraction between the positive and negative ions holds the compound together.Electron transfer requires energy; for calcium, energy is required to remove electrons, and for oxygen, energy is absorbed when gaining electrons. Despite these energy requirements, the resulting compound, CaO, possesses significant lattice energy, contributing to its stability.

• Crucial for ionic bond formation.
• Involves energy expenditure and absorption.
• Allows compounds to achieve lower energy states.

This electron exchange leads to favorable energy states, crucial for the stability of ionic compounds like CaO.

Compound Stability

Compound stability refers to how resistant a compound is to changing or decomposing. In the case of ionic compounds such as CaO, one primary factor contributing to stability is the lattice energy. Large lattice energies indicate strong ionic bonds, which stabilize the compound. The strength of these bonds results from the transfer of electrons and the ionic structure, which causes a robust attraction between ions. For calcium oxide, the stability compared to its free elements (calcium and oxygen) is explained by its substantial lattice energy. This energy compensates for the energy needed during electron transfer, making the compound stable even though the electron transfer process requires energy.

• Depends heavily on lattice energy.
• Key to resistance against external change.
• Stability results from strong ionic attractions.

Overall, understanding compound stability helps explain why ionic compounds remain intact and difficult to decompose, highlighting why CaO is stable despite initial energy investments in forming ions.