Problem 27 - Chemistry: The Central Science in SI Units, Global Edition

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 \(\mathrm{CaO}\) is large enough to overcome these processes. (b) \(\mathrm{CaO}\) 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 \(\mathrm{CaO}\) is small. (e) \(\mathrm{CaO}\) is stable to atmospheric conditions.

Step-by-Step Solution

Answer

The best explanation for the stability of CaO relative to the free elements is (a) The lattice energy of CaO is large enough to overcome these processes. The high lattice energy compensates for the energy required to remove electrons from Ca and add electrons to O, resulting in a stable ionic compound.

Step 1: Understanding the bonding in CaO

CaO (calcium oxide) is an ionic compound formed by the transfer of electrons from a metal atom (calcium) to a non-metal atom (oxygen). In this process, two electrons are removed from calcium (Ca) to form Ca²⁺ ion, and two electrons are added to oxygen (O) to form O²⁻ ion.

Step 2: Investigate the given statements

Let us go through the given statements one by one and analyze their relevance to the stability of CaO. (a) The lattice energy of CaO is large enough to overcome these processes. Lattice energy is the energy required to break a solid ionic compound into gaseous ions. A more significant lattice energy value means a more stable ionic compound. Thus, this statement could be a reason for the stability of CaO. (b) CaO is a covalent compound, and these processes are irrelevant. This statement is incorrect because CaO is an ionic compound, not a covalent one. (c) CaO has a higher molar mass than either Ca or O. The molar mass of a compound is not responsible for the stability of the compound, so this statement can be ignored. (d) The enthalpy of formation of CaO is small. A negative enthalpy of formation indicates an exothermic reaction and might contribute to the stability of the compound. However, it is not the only factor determining the stability of CaO. (e) CaO is stable to atmospheric conditions. Although this is a correct statement, it doesn't explain the inherent stability of CaO relative to the free elements.

Step 3: Select the best explanation

Based on our analysis, the best explanation for the stability of CaO relative to the free elements is (a) The lattice energy of CaO is large enough to overcome these processes. The high lattice energy compensates for the energy required to remove electrons from Ca and add electrons to O, resulting in a stable ionic compound.

Key Concepts

Lattice Energy

Lattice energy is a crucial factor in understanding the stability of ionic compounds like calcium oxide (CaO). It is the energy released when gaseous ions bond to form a solid ionic lattice. This process is equivalent to the reverse of breaking an ionic compound into its gaseous ions, which requires energy.
In essence, a higher lattice energy indicates a stronger ionic bond, contributing to the stability of the compound. For CaO, the lattice energy is significantly large, meaning it takes a considerable amount of energy to separate the Ca²⁺ and O^2- ions.

• This energy surpasses the energy required to remove electrons from Ca and add electrons to O.
• The large lattice energy is a key reason for CaO's stability compared to its free constituent elements.
• It acts as a driving force that makes the formation of CaO energetically favorable.

Understanding lattice energy provides insights into why certain ionic compounds form and remain stable. Since it involves electrostatic interactions, factors like ionic charge and size strongly influence lattice energy. High charges and smaller ionic sizes increase it, making compounds like CaO highly stable.

Enthalpy of Formation

The enthalpy of formation of a compound is the change in enthalpy when one mole of the compound forms from its constituent elements in their standard states. When it comes to calcium oxide (CaO), this value is crucial in explaining its formation and stability.
CaO has a negative enthalpy of formation, which means that energy is released when Ca and O₂ combine to form this compound.

• This releases energy due to the formation of strong ionic bonds between Ca²⁺ and O^2- ions.
• A negative enthalpy of formation typically indicates an exothermic reaction, which means the product compound, CaO, is more stable than the reactants.
• This released energy contributes indirectly to the compound's stability by making the reaction spontaneous under standard conditions.

While the enthalpy of formation isn't the sole reason for a compound's stability, it plays a vital role in understanding the energetics of compound creation. It's a piece of the puzzle that, along with lattice energy, gives a complete picture of why CaO forms and remains stable over time.

Electron Transfer

Electron transfer is a fundamental process in the formation of ionic compounds, including calcium oxide (CaO). At the core, electron transfer involves the movement of electrons from one atom to another, leading to the formation of ions with opposite charges which attract each other to form a stable compound.
In the case of CaO:

• Calcium ( Ca) donates two electrons to form a Ca²⁺ ion.
• Oxygen ( O) accepts these two electrons, forming an O^2- ion.
• This transfer results in a full valence shell for both ions, leading to a stable octet configuration.
• The strong electrostatic attraction between the positively charged Ca²⁺ and the negatively charged O^2- ions forms the ionic bond.

This process of electron transfer and resulting ionic bond formation is what drives the creation of compounds like CaO. The energy dynamics during electron transfer reflect the overall favorability of compound formation. While it does require some initial energy to transfer electrons, the payoff comes later with the release of large lattice energy as these freshly minted ions form a crystalline structure.