Question

What does it mean to say that two solutions have a common ion? Give an example.

Short answer

Two solutions have a common ion when they both contain the same ion in their respective ionic compounds. This is typically observed in the context of the common ion effect, which influences the solubility of other ions in the solution. For example, sodium chloride (NaCl) and sodium sulfate (Na2SO4) share a common ion, the sodium ion (\(Na^+\)), as both compounds dissociate in water to release this ion along with their respective anions.

Step-by-step solution

Definition of a common ion

A common ion is an ion that is present in two different ionic compounds that are dissolved in the same solution. When both of these compounds are dissolved, they will both release the common ion among other ions. This is often associated with the common ion effect, which refers to how the common ion can affect the solubility of the other ions.

Choosing an example

Now, let's choose two appropriate compounds as examples. We will choose sodium chloride (NaCl) and sodium sulfate (Na2SO4) as our compounds.

Dissolving the compounds

When sodium chloride (NaCl) and sodium sulfate (Na2SO4) are dissolved in water, they dissociate into their respective ions. The dissociation can be represented as follows: For NaCl: \[ NaCl \rightarrow Na^+ + Cl^- \] For Na2SO4: \[ Na_2SO_4 \rightarrow 2Na^+ + SO_4^{2-} \]

Identifying the common ion

In the dissociation equations above, we can see that both solutions contain \(Na^+\) ions. The sodium ion (\(Na^+\)) is the common ion shared by both sodium chloride and sodium sulfate.

Key concepts

Ionic Compounds Dissolution

Understanding how ionic compounds dissolve in water is essential for grasping the common ion effect. Ionic compounds are made up of positively charged cations and negatively charged anions. When these compounds are introduced into water, they dissociate into their individual ions.

This process of dissolution can be demonstrated using table salt, sodium chloride (NaCl), as an example. Upon dissolving, NaCl separates into sodium ions (\(Na^+\)) and chloride ions (\(Cl^-\)):\[\begin{equation}NaCl \rightarrow Na^+ + Cl^-\[\begin{equation}Dissolving ionic compounds leads to a mixture where ions are free to move, which is critical to the conductivity of the solution and its various chemical reactions. The degree to which a compound dissolves in water is known as its solubility. The common ion effect directly influences solubility by the presence of a shared ion in solution, which we see with sodium ions in the case of sodium chloride and sodium sulfate.

Solubility Equilibria

The concept of solubility equilibria is central to understanding how the presence of a common ion affects the behavior of ionic compounds in solution. Solubility equilibrium refers to the dynamic balance between the solid and dissolved forms of an ionic compound in a saturated solution.At equilibrium, the rate at which the solid dissolves is equal to the rate at which the dissolved ions recombine to form the solid. This balance can be expressed as a constant, known as the solubility product constant (\(K_{sp}\)).Imagine a saturated solution of calcium fluoride (\(CaF_2\)) which has the equilibrium expression:\[\begin{equation}CaF_2(s) \rightleftharpoons Ca^{2+}(aq) + 2F^-(aq)\[\begin{equation}The solubility product constant for this reaction would include the concentrations of the ions raised to the power of their coefficients in the balanced equation. In the context of the common ion effect, if fluoride ions are added from another source, such as sodium fluoride (\(NaF\)), the equilibrium shifts to decrease the solubility of calcium fluoride because of the increased fluoride ion concentration.

Chemical Equilibrium

Chemical equilibrium is a state in which the concentrations of reactants and products remain constant over time because the forward and reverse reactions occur at the same rate. In the context of dissolution and solubility, it plays a crucial role as it allows us to predict how the common ion effect will influence the solubility of ionic compounds.For instance, when a solvent contains a common ion, the equilibrium is shifted according to Le Chatelier's principle. This principle states that if a system at equilibrium experiences a change in concentration, temperature, or pressure, the system will adjust to counteract the imposed change and a new equilibrium will be established.Applying this to our earlier example with sodium chloride and sodium sulfate, if sodium ions are added to a solution in equilibrium, the presence of the common ion (\(Na^+\)) will push the equilibrium of dissolution to favor the formation of undissolved salt, effectively reducing the solubility of the other compounds containing sodium ions. This is a practical illustration of chemical equilibrium in action and highlights the delicate balance within solubility equilibria.