Question

Instructions on a container of antifreeze (ethylene glycol; \(\left.\mathrm{fp},-12.6^{\circ} \mathrm{C}, \mathrm{bp}, 197.3^{\circ} \mathrm{C}\right)\) give the following volumes of Prestone to be used in protecting a \(12 \mathrm{qt}\) cooling system against freeze-up at different temperatures (the remaining liquid is water): \(10^{\circ} \mathrm{F}, 3 \mathrm{qt}\) \(0^{\circ} \mathrm{F}, 4 \mathrm{qt} ;-15^{\circ} \mathrm{F}, 5 \mathrm{qt} ;-34^{\circ} \mathrm{F}, 6 \mathrm{qt} .\) since the freezing point of the coolant is successively lowered by using more antifreeze, why not use even more than 6 qt of antifreeze (and proportionately less water) to ensure the maximum protection against freezing?

Short answer

Using more than a certain proportion of antifreeze can lead to decreased cooling capabilities due to reduced water (which has a high heat capacity), a potential increase in the freezing point, and potentially cause mechanical problems due to increased solution viscosity.

Step-by-step solution

Understanding Freezing Point Depression

Freezing point depression is a colligative property of solutions. This means it depends on the amount of dissolved substances, not the type. When solute (like antifreeze) is added to a solvent (like water), it lowers the freezing point of the solvent. This property is why antifreeze is used in cooling systems. The more antifreeze, the lower the freezing point.

Importance of Balanced Proportions

Why not add more antifreeze for further protection? It's all about balance. The cooling capacity of the solution is not purely a role of antifreeze. Water has a notably high heat capacity, helping keep engines at an optimal operating temperature. If too much antifreeze is added, it could adversely affect the cooling capability of the system - the overall cooling capacity of the solution could decrease, causing the engine to overheat.

Understanding Concentration and Molality

Remember that the freezing point depression depends on the molality - the number of moles of solute (antifreeze) per kilogram of solvent (water). Replacing too much water with antifreeze would reduce this ratio, potentially causing a higher freezing point than desired.

Increasing Antifreeze beyond a Certain Point is Counter-Productive

The antifreeze usage beyond a certain point (given here as 6 quarts) would not only decrease the cooling efficiency but also potentially increase the freezing point of the solution. As higher concentrations also make the liquid more viscous, which can cause mechanical problems in the pumps and pipes of the cooling system.

Key concepts

Colligative Properties

Colligative properties are unique features of solutions that depend entirely on the number of solute particles available, irrespective of their nature. A common example is freezing point depression. When a solute like ethylene glycol, which is commonly used as antifreeze, is added to water, it lowers the solution's freezing point. The effect becomes more significant as the concentration of the solute increases.

In your car's engine cooling system, using antifreeze based on this principle ensures that the fluid doesn't freeze in cold temperatures. But the extent depends on how much antifreeze is mixed with water. More antifreeze means a lower freezing point and better protection in colder environments. However, it’s crucial to maintain balance, as these properties only function optimally within certain solute-to-solvent ratios.

Cooling Systems

Cooling systems in automotive engines are designed to keep the engine within a specific temperature range. This is essential for optimal performance and to prevent overheating. The system circulates a liquid coolant through engine components where it absorbs heat, which is then expelled through the radiator.

Antifreeze contributes to maintaining this balance by lowering the freezing point and elevating the boiling point of the coolant. But excessive antifreeze can disrupt this balance.

• Water has an excellent heat capacity, meaning it can absorb a lot of heat before getting hot. Therefore, water plays a crucial role in the cooling process.
• A perfect blend of water and antifreeze ensures that the coolant remains liquid in extreme temperatures while retaining thermal efficiency.

Too much antifreeze, however, can decrease the overall cooling efficiency, leading to engine failures due to inadequate heat dissipation.

Heat Capacity

Heat capacity refers to the amount of heat needed to increase the temperature of a given amount of substance by a specific temperature interval. Water, with its high heat capacity, is particularly adept at absorbing significant amounts of heat, making it an essential component in many cooling systems.

In the context of engine cooling, water's high heat capacity plays a pivotal role. It helps dissipate the heat generated by the engine efficiently. When antifreeze is added to the water, it also affects the total heat capacity of the mixture:

• A lower proportion of antifreeze ensures optimal heat absorption and transfer.
• Adding too much antifreeze can reduce the mixture’s ability to efficiently absorb and release heat, leading to potential overheating.

This is why balancing antifreeze and water is vital to maintain both the freezing point depression needed in winter and the heat capacity required for cooling efficiency.