Question

Simple soaps, like sodium laurate, \(\mathrm{NaC}_{12} \mathrm{H}_{22} \mathrm{O}_{2}\) are ionic compounds with small cations and large covalent anions. These soaps are somewhat soluble in water but form a soap scum if the water is "hard," which means it contains calcium ions \(\mathrm{Ca}^{2+}\) or magnesium ions, \(\mathrm{Mg}^{2+}\). Write a net ionic equation for the formation of soap scum. Also classify the reaction type or types.

Short answer

The net ionic equations for the formation of soap scum are: Calcium ions reaction: \(\mathrm{Ca}^{2+}\) + 2 \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{2}^{-}\) \(->\) \(\mathrm{Ca(C}_{12} \mathrm{H}_{22} \mathrm{O}_{2})_2\) Magnesium ions reaction: \(\mathrm{Mg}^{2+}\) + 2 \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{2}^{-}\) \(->\) \(\mathrm{Mg(C}_{12} \mathrm{H}_{22} \mathrm{O}_{2})_2\) These reactions are classified as precipitation reactions.

Step-by-step solution

Write down the given formulae and ions

Sodium laurate: \(\mathrm{NaC}_{12} \mathrm{H}_{22} \mathrm{O}_{2}\) Calcium ions: \(\mathrm{Ca}^{2+}\) Magnesium ions: \(\mathrm{Mg}^{2+}\)

Predict the products

When sodium laurate reacts with calcium or magnesium ions, the sodium ions will be replaced by calcium or magnesium ions, forming calcium or magnesium laurate, which is the soap scum. Calcium laurate: \(\mathrm{Ca(C}_{12} \mathrm{H}_{22} \mathrm{O}_{2})_2\) Magnesium laurate: \(\mathrm{Mg(C}_{12} \mathrm{H}_{22} \mathrm{O}_{2})_2\)

Write the balanced chemical equation

Let's consider the reaction between sodium laurate and calcium ions: 2 \(\mathrm{NaC}_{12} \mathrm{H}_{22} \mathrm{O}_{2}\) \(+\) \(\mathrm{Ca}^{2+}\) \(->\) \(\mathrm{Ca(C}_{12} \mathrm{H}_{22}\mathrm{O}_{2})_2\) \(+\) 2 \(\mathrm{Na}^+\) And the reaction between sodium laurate and magnesium ions: 2 \(\mathrm{NaC}_{12} \mathrm{H}_{22} \mathrm{O}_{2}\) \(+\) \(\mathrm{Mg}^{2+}\) \(->\) \(\mathrm{Mg(C}_{12} \mathrm{H}_{22} \mathrm{O}_{2})_2\) \(+\) 2 \(\mathrm{Na}^+\)

Determine the net ionic equation

Removing the spectator ions (sodium ions) from each reaction, we obtain the following net ionic equations: Calcium ions reaction: \(\mathrm{Ca}^{2+}\) + 2 \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{2}^{-}\) \(->\) \(\mathrm{Ca(C}_{12} \mathrm{H}_{22} \mathrm{O}_{2})_2\) Magnesium ions reaction: \(\mathrm{Mg}^{2+}\) + 2 \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{2}^{-}\) \(->\) \(\mathrm{Mg(C}_{12} \mathrm{H}_{22} \mathrm{O}_{2})_2\)

Classify the reaction type

These reactions are examples of precipitation reactions, where an insoluble solid (the soap scum) is formed from the reaction between soluble reactants (the soap and the ions in hard water).

Key concepts

Precipitation Reactions

Precipitation reactions involve the formation of an insoluble solid, known as a precipitate, when two aqueous solutions are mixed. These reactions occur when the products formed from the mixture have low solubility in water.
In our example of soap scum formation, when sodium laurate reacts with calcium or magnesium ions present in hard water, a solid precipitate (calcium or magnesium laurate) forms. This solid is what we typically refer to as soap scum.
These reactions are crucial in chemistry because they help us understand the interactions and solubility of various ions in solutions.

• Two soluble solutions combine.
• An insoluble product (precipitate) is formed.
• Commonly involves ionic compounds.

When observing such reactions in the laboratory or in everyday life, like the formation of soap scum, it helps to note that a visible change such as a solid forming or a change in solution clarity is often indicative of a precipitation reaction.

Soaps and Detergents

Soaps like sodium laurate are amphiphilic compounds, meaning they possess both hydrophilic (water-attracting) and hydrophobic (water-repellent) parts. This structure allows them to effectively clean by interacting with a wide range of substances.
The hydrophobic tails of soap molecules interact with oils and greases, while the hydrophilic heads interact with water, allowing them to emulsify fats and oils, suspending them in water so they can be rinsed away. In essence, soaps and detergents work by surrounding dirt or oil with their molecules, making it easier for these contaminants to be washed away.
However, the presence of calcium and magnesium ions in hard water can disrupt this cleaning process. These ions react with the soap to form an insoluble substance, which is the basis for our soap scum problem. This interaction highlights the important roles that chemical properties play in the effectiveness of cleaning agents.

Hard Water Chemistry

Hard water is characterized by high concentrations of calcium ( Ca^{2+} ) and magnesium ( Mg^{2+} ) ions. These ions naturally occur in water when it flows through deposits rich in limestone or chalk, which mainly consist of calcium carbonate and magnesium carbonate.
When hard water reacts with soap, it leads to the formation of soap scum. The calcium and magnesium ions replace the sodium ions in the soap, resulting in compounds such as calcium laurate, which are not soluble in water. This is what forms the cloudy, sometimes slimy residue on bathroom fixtures, and it also affects soap's ability to lather.
The challenges posed by hard water can be mitigated by using water softeners. These typically exchange calcium and magnesium ions with sodium or potassium ions to prevent scum formation and improve cleaning efficiency.

Solubility

Solubility refers to the ability of a substance to dissolve in a solvent, such as water. It is a fundamental concept in chemistry that affects many everyday processes, including the preparation and use of cleaning products.
In the context of soap and hard water, solubility plays a key role. The sodium salts present in soaps like sodium laurate are generally soluble in water, meaning they dissolve to release their ions. However, when these sodium soaps encounter calcium or magnesium ions in hard water, the resulting calcium or magnesium soaps are much less soluble.
Factors affecting solubility include temperature, the nature of the solute and solvent, and the presence of other solutes. In hard water, the solubility of the soap decreases due to the interaction with calcium and magnesium ions, causing precipitation as soap scum. Understanding these interactions can help in choosing the right type of soap or detergent for different water types, ensuring effective cleaning.