Problem 61 - Introductory Chemistry Atoms First

Question

Two beakers each contain 1 L of water. Then \(0.1\) mole of HCl gas is dissolved in one beaker, and \(0.1\) mole of HF gas is dissolved in the other beaker. The HCl solution causes a light bulb to glow intensely, and the HF solution causes the same bulb to glow dimly. Explain these observations.

Step-by-Step Solution

Answer

The difference in the light bulb's glow intensity can be explained by the difference in the dissociation characteristics of strong and weak acids. HCl (a strong acid) completely dissociates in water, resulting in a higher concentration of H+ ions and a more conductive solution, causing the light bulb to glow intensely. In contrast, HF (a weak acid) only partially dissociates, producing fewer H+ ions and a less conductive solution, causing the light bulb to glow dimly.

Step 1: The problem statement involves two acids, HCl and HF, dissolved in water. HCl is a strong acid while HF is a weak acid. The difference between a strong and a weak acid lies in how they dissociate (break down) into ions when they dissolve in water. This will be important to understand the behavior of the two solutions and the subsequent impact on the light bulb.

#Step 2: Understand the concept of strong acids and their dissociation#

Step 2: Strong acids like HCl completely dissociate in water into their constituent ions. For example, when HCl is dissolved in water, it releases hydrogen ions (H+) and chloride ions (Cl-): \[ HCl_{(aq)} \rightarrow H^+_{(aq)} + Cl^-_{(aq)} \] The increase in the concentration of H+ ions causes the solution to be highly conductive, as the ions can move freely and carry an electric charge. This conductivity allows the solution to transmit electricity and causes the light bulb to glow intensely.

#Step 3: Understand the concept of weak acids and their dissociation#

Step 3: Weak acids, like HF, do not fully dissociate in water. Instead, they partially dissociate: \[ HF_{(aq)} \rightleftharpoons H^+_{(aq)} + F^-_{(aq)} \] In this equilibrium, there are fewer H+ ions produced than in the case of a strong acid like HCl. This means that the solution is less conductive as there are fewer ions to carry the electric charge. This results in the light bulb glowing dimly when brought into contact with the HF solution.

#Step 4: Relate the conductivity to the light bulb intensity#

Step 4: Both the HCl and HF solutions have the ability to conduct electricity due to the presence of ions, which is the reason why the light bulb glows in both cases. However, the difference in the glow's intensity can be attributed to the higher concentration of H+ ions in the HCl solution compared to the HF solution. The higher concentration of ions in the HCl solution results in higher conductivity and therefore a more intense glow from the light bulb. On the other hand, the lower concentration of ions in the HF solution leads to lower conductivity, causing the light bulb to glow dimly.

In conclusion, the difference in the intensity of the light bulb's glow in HCl and HF solutions can be explained by the difference in the dissociation characteristics of strong and weak acids. The higher conductivity of the HCl solution due to its greater H+ ion concentration leads to a more intense glow, while the lower conductivity of the HF solution causes the light bulb to glow dimly.

Key Concepts

Strong Acids

When we talk about strong acids, we refer to their powerful ability to dissociate in water. For instance, hydrochloric acid (HCl) is a perfect example. It completely breaks down into its individual ions, hydrogen ions (H⁺) and chloride ions (Cl⁻), when it dissolves in water.
This means that in a solution of HCl, you'll find a high concentration of H⁺ ions floating freely.

• These ions are crucial as they can move around easily and carry a charge, which is why such solutions conduct electricity very well.
• In a typical experiment, like dissolving HCl in water and connecting it with a circuit, the high conductivity will make a light bulb glow brightly.
• This phenomenon occurs because the freely moving ions facilitate the flow of electric current efficiently.

So, for strong acids like HCl, the complete dissociation is synonymous with high conductivity and intense electrical signals.

Weak Acids

Weak acids, in contrast, do not fully dissociate in water. Hydrofluoric acid (HF) is a classic example of a weak acid. Instead of breaking all its molecules into ions, only a fraction of HF molecules release hydrogen ions (H⁺) when dissolved in water.

• This partial dissociation creates an equilibrium between the non-dissociated HF molecules and the ions produced.
• Compared to strong acids, weak acids produce fewer H⁺ ions.
• As a result, the lower concentration of ions leads to a solution with reduced conductivity.

When a light bulb is connected to such a weakly conductive solution, the dim glow signifies the minimal movement of electric current. Thus, weak acids are characterized by their incomplete ionization and lesser ability to transmit electricity.

Conductivity

Conductivity in a solution essentially describes how well it can transmit electric signals. The presence and concentration of charged particles, like ions, dictate this behavior. Strong acids such as HCl lead to high conductivity due to their complete ionization that releases more ions.
On the other hand, weak acids like HF result in lower conductivity because fewer ions are released upon partial ionization.

• High concentration of ions means higher conductivity, leading to strong reactions observable through devices like glowing bulbs or meters.
• Conversely, lower ion concentration corresponds with weaker conductivity, showing milder reactions.

In practical experiments, such as the one involving strong and weak acids, the intensity of the glow from a connected light bulb is directly linked to the ion concentration and hence the conductivity of the solution. Understanding this relationship helps clarify how different acids interact with circuits and devices.