Problem 16 - General Chemistry: Principles and Modern Applications

Question

Which has the (a) higher melting point, MgO or BaO; (b) greater solubility in water, \(\mathrm{MgF}_{2}\) or \(\mathrm{MgCl}_{2}\) ? Explain.

Step-by-Step Solution

Answer

(a) MgO has a higher melting point than BaO. (b) \(\mathrm{MgCl}_{2}\) is more soluble in water than \(\mathrm{MgF}_{2}\).

Step 1: Identify the relative atomic sizes

Identify that Magnesium (Mg) is higher up on the Periodic Table compared to Barium (Ba), which makes Mg smaller in atomic size than Ba.

Step 2: Predict Melting Points based on Trends

Recognize from the Periodic Table trend that as you go down the group (from Mg to Ba), atomic size increases and thus the forces between the ions become weaker. This leads to a decrease in melting points.

Step 3: Check Solubility Rules

Know that general solubility rules state that salts of fluorides are less soluble than salts of chlorides in water.

Step 4: Predict Solubility

Considering the aforementioned rules, conclude that \(\mathrm{MgF}_{2}\) is less soluble than \(\mathrm{MgCl}_{2}\) in water.

Key Concepts

Solubility Rules

Solubility rules help us understand which substances are likely to dissolve in water and other solvents. These rules are essential for predicting chemical behavior in solutions. General guidelines tell us:

• Chloride, bromide, and iodide salts are usually soluble except when paired with silver, lead, or mercury.
• Fluoride salts tend to be less soluble compared to chloride salts.
• Sulfates are generally soluble, but exceptions exist for barium, lead, and calcium.

In our exercise involving \(\mathrm{MgF_2}\) and \(\mathrm{MgCl_2}\), the solubility rules indicate that \(\mathrm{MgCl_2}\) is more likely to dissolve in water because fluorides are generally less soluble than chlorides. This understanding helps us predict the behavior of these compounds in aqueous environments.

Melting Points

Melting points refer to the temperature at which a solid becomes a liquid. For ionic compounds like MgO and BaO, melting points are influenced by the strength of the ionic bonds, which in turn are affected by the size of the ions and the charge on them.

• Smaller ions usually have stronger ionic bonds because they are able to get closer to each other, increasing the electrostatic attraction between ions.
• Higher charges on the ions also increase the melting point as they enhance the strength of the ionic bond.

In comparing MgO and BaO, we observe that Mg is smaller than Ba, leading to shorter distance and stronger bonds in MgO. Therefore, MgO has a higher melting point than BaO due to the combined effects of smaller ionic size and stronger bond strength.

Atomic Size

Atomic size can be understood as the distance from the nucleus of an atom to its outermost electrons. On the periodic table, atomic size tends to increase as you move down a group and decreases as you move across a period from left to right.

• As you move down a group, additional electron shells are added, increasing the size of the atom.
• Going across a period, electrons are added to the same outer shell and the increasing number of protons draws the electrons closer, reducing the atomic radius.

In the context of our exercise, we see that magnesium (\(\mathrm{Mg}\)) is smaller than barium (\(\mathrm{Ba}\)) because it is higher up in the same group. This concept helps to explain why MgO has stronger ionic bonds and therefore a higher melting point than BaO.