Problem 70 - Introductory Chemistry Atoms First

Question

Describe what happens when \(\mathrm{CaCl}_{2}(\mathrm{~s})\) and \(\mathrm{NaNO}_{3}(\mathrm{~s})\) are added to the same beaker of water. What, if anything, dissolves, and what, if anything, precipitates?

Step-by-Step Solution

Answer

When solid calcium chloride (CaCl2) and solid sodium nitrate (NaNO3) are added to water, both compounds dissolve, forming a mixture of calcium ions (Ca2+), chloride ions (Cl-), sodium ions (Na+), and nitrate ions (NO3-). No precipitation occurs as the resulting compounds, calcium nitrate (Ca(NO3)2) and sodium chloride (NaCl), are both soluble in water.

Step 1: Determine the solubility of initial compounds in water

To determine the solubility of CaCl2 and NaNO3 in water, we can refer to the following solubility rules for inorganic compounds: 1. All group 1 (alkali metal) and ammonium salts are soluble. 2. All nitrates, acetates, and perchlorates are soluble. 3. All chlorides, bromides, and iodides are soluble, except those of silver, lead, and mercury(I). 4. All sulfates are soluble, except those of barium, calcium, strontium, lead, and mercury(II). 5. All carbonates, phosphates, sulfites, and sulfides are insoluble, except those of group 1 and ammonium. 6. Most hydroxides and oxides are insoluble, except those of group 1, calcium, barium, and strontium. Using these rules, we can determine that: - Calcium chloride (CaCl2) is soluble in water because it is a chloride salt (rule 3). - Sodium nitrate (NaNO3) is soluble in water because it is a nitrate salt (rule 2).

Step 2: Analyze possible ion combinations after dissolution

After dissolving the soluble compounds in water, we will have the following ions: - Ca2+ (from CaCl2) - 2 Cl- (from CaCl2) - Na+ (from NaNO3) - NO3- (from NaNO3) Now, let's consider all possible combinations of these ions: 1. Ca2+ and NO3- forming Ca(NO3)2 2. Na+ and Cl- forming NaCl

Step 3: Check the solubility of new combinations

Using the solubility rules, let's determine if the new combinations will precipitate: 1. Calcium nitrate (Ca(NO3)2): It is soluble in water because all nitrates are soluble (rule 2). 2. Sodium chloride (NaCl): It is soluble in water because it is a group 1 metal salt (rule 1).

Step 4: Final conclusion

Both calcium chloride (CaCl2) and sodium nitrate (NaNO3) will dissolve in water, resulting in the formation of soluble calcium nitrate (Ca(NO3)2) and sodium chloride (NaCl). There will be no precipitation reaction, as all resulting compounds remain soluble in water.

Key Concepts

Solubility of Compounds

Understanding the solubility of compounds is vital when predicting the outcome of mixing chemicals in a solution. In inorganic chemistry, solubility refers to the ability of a substance to dissolve in a solvent, forming a homogeneous mixture at the molecular or ionic level. Most often, the solvent we consider is water due to its high polarity and ability to solvate ions and polar molecules.

In the context of the exercise, we recognize that both calcium chloride (\(\mathrm{CaCl}_{2}\)) and sodium nitrate (\(\mathrm{NaNO}_{3}\)) are ionic compounds. The guidelines provided by solubility rules are incredibly helpful. For example, we can deduce that sodium nitrate (\(\mathrm{NaNO}_{3}\)) will dissolve because nitrates are universally soluble in water. Similarly, calcium chloride, being a chloride (except for certain cations like silver, lead, and mercury(I)), is also soluble in water.

These rules are practical tools for students as they provide a methodical way to predict whether a substance will dissolve in water. Memorizing these can be a daunting task, but understanding the pattern behind them—such as most compounds of group 1 elements and nitrates being soluble—can make the learning process much easier and help students to solve exercises effectively.

Precipitation Reactions

A precipitation reaction occurs when two dissolved ionic compounds react to form an insoluble product, known as the precipitate. This is the crux of many classical experiments in inorganic chemistry and is a visually dramatic representation of chemical change.

For precipitation to occur, the product of the ion combination must not be soluble in water according to the solubility rules. In the given exercise, we might initially suspect that combining the ions from (\(\mathrm{CaCl}_{2}\)) and (\(\mathrm{NaNO}_{3}\)) could lead to precipitation. However, when referring to the solubility rules, we find that all possible new combinations—in this case, calcium nitrate (\(\mathrm{Ca(NO}_{3})_{2}\)) and sodium chloride (NaCl)—remain soluble. Therefore, no precipitation reaction will occur in this scenario.

Understanding these reactions is crucial because they are key to many processes, such as water purification, mineral extraction, and even in medical diagnostics where certain precipitation reactions are used to detect the presence of specific ions.

Ionic Compounds in Water

When discussing the ionic compounds in water, what we're really talking about is the process by which these compounds dissociate into their constituent ions. This dissociation occurs because water molecules are polar, with a partial positive charge near the hydrogen atoms and a partial negative charge near the oxygen atom. As ionic compounds are made up of positive and negative ions, these charges attract the ions, leading to their separation and uniform distribution throughout the water – a process we call dissolution.

In the solution from the exercise, calcium chloride separates into \(\mathrm{Ca}^{2+}\) and two \(\mathrm{Cl}^{-}\) ions. Similarly, sodium nitrate breaks down into \(\mathrm{Na}^{+}\) and \(\mathrm{NO}_{3}^{-}\) ions. The water molecules surround these ions, 'solvating' them and preventing them from recombining, which is crucial for maintaining a stable solution and allowing for reactions to happen in an aqueous environment—such as the functioning of biological systems or industrial chemical processes.