Problem 66 - Chemistry: Principles and Reactions

Question

A plot of the solubility of a certain compound \(\left(\mathrm{g} / 100 \mathrm{~g} \mathrm{H}_{2} \mathrm{O}\right)\) against temperature \(\left({ }^{\circ} \mathrm{C}\right)\) is a straight line with a positive slope. Is dissolving that compound an exothermic process?

Step-by-Step Solution

Answer

Explain your reasoning. Answer: No, the solubility plot suggests that dissolving the compound is not an exothermic process because the solubility increases with increasing temperature, as indicated by the positive slope. In an exothermic process, the solubility would decrease with increasing temperature due to the release of heat.

Step 1: Understanding Positive Slope Relationship

A straight line with a positive slope signifies that as the temperature increases, so does the solubility of the compound in water. This means that the compound becomes more soluble.

Step 2: Analyzing the Relationship with Exothermic Processes

In an exothermic process, heat is released when the compound dissolves, which should result in a decrease in solubility as the temperature increases. However, the given plot shows that the solubility of the compound increases with increasing temperature, which suggests that this process is not exothermic.

Step 3: Conclusion

Since the solubility of the given compound increases with the increasing temperature, as shown by the positive slope of the solubility plot, dissolving the compound is not an exothermic process.

Key Concepts

Exothermic process

An exothermic process is a chemical reaction or physical change that releases heat to the surroundings.
Imagine a cozy campfire; as it burns, it releases heat, warming everything around it.
In terms of dissolving substances, if dissolving a compound is exothermic, the solution releases heat, leading to the immediate warming of the solvent. An important characteristic of exothermic processes is the inherent decrease in solubility with rising temperature.
Because heat is being released, an increase in temperature typically disrupts the dissolving process, causing less solute to dissolve.
For example:

• If you dissolve sodium acetate in water and it releases heat, it is exothermic.
• Such solutions often crystallize as temperature rises, since heat addition slows down the dissolve reaction.

The exercise suggests an alternative observation where solubility increases with temperature, which is not typical for most exothermic processes.

Solubility

Solubility is the measure of how much solute can dissolve in a solvent at a given temperature.
It's like determining how much sugar can be stirred into your tea before it stops dissolving and starts settling at the bottom. Several factors influence solubility:

• Nature of solute and solvent: Like attracts like. Polar substances usually dissolve better in polar solvents.
• Temperature: Most solids become more soluble in liquids as temperature increases.
• Pressure: This primarily affects gases, with higher pressure often increasing solubility.

When plotting solubility against temperature, the graph can tell us a lot: - A positive slope indicates increasing solubility with temperature.
- A negative slope would show decreasing solubility, hinting at potential exothermic processes.
Understanding solubility helps explain why certain compounds dissolve better under specific conditions.

Temperature dependence of solubility

The temperature dependence of solubility is crucial in understanding how substances interact in solutions.
Just like how an ice cube melts faster on a hot day, solubility can increase as temperature rises. For many solid solutes, an increase in temperature means an increase in solubility.
This is because higher temperatures generally provide more energy, promoting the solute to dissolve more readily.
When solubility increases with temperature:

• The solvent molecules move faster, interact more, and break apart solute particles more efficiently.
• The energy supplied can overcome attractive forces between solute particles.

In the given exercise, the compound's solubility increases with temperature, opposing exothermic characteristics.
This generally suggests an endothermic process, where heat absorption aids solubility.
Recognizing these patterns assists in predicting how substances behave under varied thermal conditions.