Chapter 13: Problem 69
Both \(\mathrm{NaCl}\) and \(\mathrm{CaCl}_{2}\) are used to melt ice on roads and sidewalks in winter. What advantages do these substances have over sucrose or urea in lowering the freezing point of water?
Short Answer
Expert verified
NaCl and CaCl_2 dissociate into more particles, providing a greater freezing point depression compared to sucrose and urea.
Step by step solution
01
Understanding Freezing Point Depression
Freezing point depression is a colligative property, which means its effect depends on the number of solute particles in a solution, not the identity of the solute. When a solute is added to a solvent, it disrupts the crystallization process, lowering the freezing point.
02
Determine Ionic vs. Molecular Compounds
Sodium chloride (NaCl) and calcium chloride (CaCl_2) are ionic compounds, which dissociate into multiple ions in solution. NaCl dissociates into two ions (Na⁺ and Cl⁻), whereas CaCl_2 dissociates into three ions (Ca²⁺ and two Cl⁻). In contrast, sucrose and urea are molecular compounds that do not dissociate into ions.
03
Calculate Van't Hoff Factor
The Van't Hoff factor (i) represents the number of particles the solute dissociates into. For NaCl, i = 2, while for CaCl_2, i = 3. Sucrose and urea have i = 1 as they don't dissociate into ions.
04
Impact on Freezing Point Depression
The effectiveness of a solute in lowering freezing point is proportional to its Van't Hoff factor. Since NaCl and CaCl_2 dissociate into more ions, they have a greater impact on lowering the freezing point than sucrose or urea, which remain as single particles.
05
Identify Practical Advantages
NaCl and CaCl_2 are more effective in melting ice due to their higher ability to lower the freezing point. Additionally, they are generally more cost-effective and easier to spread and handle compared to organic compounds like sucrose.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Colligative Properties
Colligative properties are fascinating characteristics of solutions that depend entirely on the number of solute particles present, regardless of what those particles are. One key aspect of these properties is that they result in changes to the physical behavior of the solvent.
- Freezing point depression
- Boiling point elevation
- Vapour pressure lowering
- Osmotic pressure
Van't Hoff Factor
The Van't Hoff factor \( (i) \) is an important concept in chemistry, especially when discussing colligative properties such as freezing point depression. It indicates the number of particles a solute splits into when dissolved.
- For example, \(\text{NaCl}\) splits into two particles: \( \text{Na}^+ \) and \( \text{Cl}^- \).
- \(\text{CaCl}_{2}\) splits into three particles: \( \text{Ca}^{2+} \) and two \( \text{Cl}^- \) ions.
Ionic vs. Molecular Compounds
The distinction between ionic and molecular compounds is crucial in understanding why some substances are more effective in freezing point depression.
- Ionic Compounds: These consist of metals combined with non-metals and dissociate into ions in a solution. For instance, \(\text{NaCl}\) breaks into \( \text{Na}^+ \) and \( \text{Cl}^- \) ions, while \(\text{CaCl}_{2}\) dissociates into \( \text{Ca}^{2+} \) and two \( \text{Cl}^- \) ions.
- Molecular Compounds: Composed of covalently bonded non-metal molecules, these do not break into ions in solution. A good example is sucrose, which stays intact as a whole molecule when dissolved in water.