Problem 42 - Chemistry The Central Science

Question

Determine whether each of the following is true or false: (a) All strong bases are salts of the hydroxide ion. (b) The addition of a strong base to water produces a solution of \(\mathrm{pH}>\)7.0 .(c) Because \(\mathrm{Mg}(\mathrm{OH})_{2}\) is not very soluble, it cannot be a strong base.

Step-by-Step Solution

Answer

Based on the given step-by-step solution, the short answer is: (a) True - All strong bases are salts of the hydroxide ion, as they dissociate completely in water to form a high concentration of hydroxide ions (OH⁻). (b) True - The addition of a strong base to water produces a solution of pH > 7.0, as it increases the basicity of the solution. (c) False - Despite its low solubility, Mg(OH)₂ can still be considered a strong base due to its complete dissociation in water.

Step 1: Statement (a) - All strong bases are salts of the hydroxide ion.

A strong base is a substance that can dissociate completely in water to form a high concentration of hydroxide ions (OH⁻). Salts are ionic compounds that result from the neutralization reaction between an acid and a base. In this context, a salt of the hydroxide ion means it has a metal cation combined with a hydroxide anion (OH⁻). For example, sodium hydroxide (NaOH) and potassium hydroxide (KOH) are strong bases and salts of the hydroxide ion. Thus, this statement is true.

Step 2: Statement (b) - The addition of a strong base to water produces a solution of pH > 7.0.

The pH scale is used to determine the acidity or basicity of a solution. It ranges from 0 to 14, with 7 being neutral. A solution with a pH less than 7 is acidic, while a solution with a pH greater than 7 is basic. Since a strong base dissociates completely in water to produce a high concentration of hydroxide ions (OH⁻), it increases the basicity of the solution. This results in a pH value greater than 7. Therefore, this statement is true.

Step 3: Statement (c) - Because Mg(OH)₂ is not very soluble, it cannot be a strong base.

Solubility refers to the ability of a substance to dissolve in a given solvent, such as water. In the case of Mg(OH)₂, it is considered a sparingly soluble substance, which means it does not dissolve well in water. However, the classification of a substance as a strong base or a weak base depends on the degree of dissociation of that substance in water. While Mg(OH)₂ is not very soluble, the small amount that does dissolve in water dissociates completely to form magnesium ions (Mg²⁺) and hydroxide ions (OH⁻). Therefore, Mg(OH)₂ can still be considered a strong base due to its complete dissociation in water, despite its low solubility. Thus, this statement is false.

Key Concepts

Hydroxide Ion

The hydroxide ion, represented as \(\text{OH}^{-}\), plays a crucial role in defining the nature of basic solutions. It is a negatively charged ion formed when a base dissociates in water. This dissociation process involves breaking apart a compound to release hydroxide ions into the solution.

These ions are essential in determining the basicity of a solution. When you have a strong base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH), the compound dissociates completely in water to produce a large number of \(\text{OH}^-\) ions. These ions then interact with other compounds in the solution, shifting the balance towards a more basic environment.

• Strong bases fully dissociate, releasing high concentrations of hydroxide ions.
• The presence of \(\text{OH}^{-}\) ions increases the pH, making the solution basic.

The behavior and concentration of hydroxide ions are central to understanding both the strength and the effects of bases in chemical reactions.

pH Scale

The pH scale is a measure of how acidic or basic a solution is. It ranges from 0 to 14, with a pH of 7 considered neutral. Any solution with a pH below 7 is acidic, and those with a pH above 7 are basic. This scale is logarithmic, meaning that each whole number change represents a tenfold change in acidity or basicity.

When a strong base is added to water, it increases the concentration of hydroxide ions, which results in a higher pH value. By increasing the number of \(\text{OH}^-\) ions, the solution becomes more basic, thus raising the pH above 7.0. This is why the addition of a strong base to water will always result in a pH that's more than 7.

• pH reflects the concentration of hydrogen ions (less in bases) and hydroxide ions (more in bases).
• The scale ranges from acidic (<7) to basic (>7).

Understanding the pH scale helps in predicting the behavior and reactivity of different solutions in various chemical contexts.

Solubility

Solubility refers to the ability of a substance to dissolve in a solvent, such as water. It's an important property that influences the availability of ions in a solution, impacting the solution's overall behavior.

When discussing bases like magnesium hydroxide \(\text{Mg(OH)}_2\), we encounter situations where a compound is not very soluble, meaning it doesn’t dissolve well in water. However, even tiny amounts that dissolve can dissociate completely. So, although \(\text{Mg(OH)}_2\) is sparingly soluble, the portion that does dissolve releases hydroxide ions fully, defining it as a strong base.

• Solubility influences how ions are released into a solution.
• Even sparingly soluble compounds can be strong bases if they dissociate completely once in solution.

Grasping the concept of solubility is essential to understanding the behavior of different substances in reactions, especially in aqueous solutions.

Dissociation

Dissociation is the process by which molecules split into smaller particles, such as ions, typically when dissolved in water. For bases, this usually means breaking down to release hydroxide ions \(\text{OH}^-\).

Strong bases, like NaOH and KOH, dissociate completely, liberating a high concentration of hydroxide ions into the solution. This complete dissociation is characteristic of strong bases and determines their ability to affect pH significantly.

• Dissociation is vital in understanding how substances interact and change the nature of the solution.
• Complete dissociation in strong bases results in substantial changes in the solution's properties, notably an increase in pH.

Understanding dissociation helps in predicting how different substances will behave in aqueous environments and underlines the differences between strong and weak acids and bases.

Acid-Base Reactions

Acid-base reactions involve the transfer of hydrogen ions (protons) from an acid to a base. These reactions are foundational in chemistry and happen often in everyday chemical interactions. They are typified by acid-base neutralization, which produces water and a salt.

In these reactions, bases contribute hydroxide ions \(\text{OH}^-\), which can react with the \(\text{H}^+\) ions from acids to form water. For instance, when hydrochloric acid \(\text{HCl}\) reacts with sodium hydroxide \(\text{NaOH}\), the products are water and NaCl, common table salt.

• Reactions often result in the formation of water and a salt.
• The principles governing these reactions are crucial for understanding many biological and environmental processes.

Through mastering acid-base reactions, one can predict the outcomes of mixing different acids and bases, leading to a deeper understanding of chemical dynamics in various settings.