Question

Discuss the interpretation of the observation that magnesium oxide is more soluble in aqueous magnesium chloride than in pure water.

Short answer

Magnesium oxide is more soluble in aqueous MgCl₂ due to common ion effects and complex formation with additional Mg²⁺ ions.

Step-by-step solution

Understand Solubility Concepts

Solubility refers to the ability of a substance to dissolve in a solvent. Magnesium oxide (MgO) is a compound that shows limited solubility in water, which is a common solvent.

Consider the Common Ion Effect

The common ion effect occurs when the presence of an ion common to a dissolved substance decreases its solubility. In water, MgO dissociates to form Mg²⁺ and O²⁻ ions. In aqueous magnesium chloride (MgCl₂), extra Mg²⁺ ions are already present.

Analyze Chemical Equilibrium

In pure water, the dissolution of MgO typically reaches an equilibrium: \[ \text{MgO (s)} \rightleftharpoons \text{Mg}^{2+} (aq) + \text{O}^{2-} (aq) \]. However, in a MgCl₂ solution, the increased concentration of Mg²⁺ shifts the equilibrium according to Le Chatelier's principle, potentially affecting solubility.

Evaluate the Solvation Dynamics

MgO dissolving in MgCl₂ could be partially explained by aggregation of O²⁻ ions with Mg²⁺, forming complexes that are less likely to precipitate than MgO alone, increasing the apparent solubility.

Formulate Conclusion

Increased solubility of MgO in MgCl₂ is likely due to more Mg²⁺ influencing equilibrium dynamics and complex formation leading to more MgO dissolution than in pure water.

Key concepts

Common Ion Effect

The common ion effect is a fundamental concept in chemistry that can alter the solubility of substances. When a solution containing a common ion is mixed with a solute, the solubility of the solute can decrease. This happens because the dissolved ions of the solute and the common ions from the solution together create an environment that feels saturated quickly.
In the case of magnesium oxide dissolving in water or magnesium chloride (MgCl₂) solution, the common ion effect plays a major role. Magnesium oxide dissociates into magnesium ions (Mg²⁺) and oxide ions (O²⁻), and when MgCl₂ is added, it already contains Mg²⁺ ions. The presence of these additional Mg²⁺ ions causes the solubility of MgO to change.

• The added magnesium ions heighten the Mg²⁺ concentration in the solution.
• This increased concentration discourages additional MgO dissolution due to the common ion effect.

Thus, although the common ion effect often decreases solubility, the scenario here is different because of dynamic chemical and solvation interactions.

Chemical Equilibrium

Chemical equilibrium involves a balance of competing reactions; it occurs when the rate of dissolution equals the rate of precipitation. In our scenario of MgO in water, equilibrium is established via the reaction: \[ \text{MgO (s)} \rightleftharpoons \text{Mg}^{2+} (aq) + \text{O}^{2-} (aq) \].
This equilibrium can be disrupted or adjusted when external factors such as concentration, pressure, or temperature change.

• When extra Mg²⁺ ions are introduced from MgCl₂, it disrupts this balance.
• The equilibrium might seem to shift towards forming more solids, yet increased dissolution is observed here due to other principles at play.

Such adjustments in chemical equilibria can actually sometimes enhance solubility when it involves ionic solutions and more complex reactions.

Le Chatelier’s Principle

Le Chatelier's Principle provides insight into how systems at equilibrium respond to external changes. According to this principle, if a stress (such as concentration change) is applied to a system at equilibrium, the system will respond by adjusting the equilibrium balance to counteract the stress.
In the presence of increased Mg²⁺ ions due to MgCl₂, traditionally you would expect a reduction in solubility. However, in our scenario:

• The principle suggests the system might adapt to lessen the concentration of the additional ions.
• This could cause more MgO to dissolve as the system seeks a new state of balance.

The real-world interactions of these ions, and potential complex formations, mean that Le Chatelier's principle guides but doesn't fully dictate the results here.

Solvation Dynamics

Solvation dynamics explores how molecules or ions become surrounded and stabilized by solvent molecules. This offers a deeper explanation for the MgO solubility in MgCl₂ solution. As MgO dissolves, Mg²⁺ and O²⁻ ions interact with surrounding water molecules. But more importantly, they can form complex pairs themselves:

• These ion pairs or complexes can have different properties from the unattached ions.
• By forming these stable complexes, they can be less likely to precipitate than their individual counterparts.

Such interactions in the aqueous MgCl₂ environment can facilitate increased solubility of MgO compared to pure water.
Thus, understanding solvation dynamics enhances comprehension of how complex ion interactions and solvent properties lead to shifts in apparent solubility and reaction outcomes.