Exit Handout From DLM 03 FNTs 1.1.4-3 Perhaps you recall that when table salt, N

Question

Exit Handout From DLM 03 FNTs 1.1.4-3 Perhaps you recall that when table salt, NaCI, is added to water, the freezing point of water is lowered. Consider a system composed of a mixture of 2.5 kg of ice and 50 g of liquid water and a small, separate container of finely powdered salt. This physical system is contained in a fully insulated container that prevents all thermal interactions with the environment. Both the salt and the ice-water mixture are initially at the freezing point of water, 0 °C. The salt is then added to the ice. water mixture, and the system of ice-water and salt is allowed to come to thermal equilibrium. The final equilibrium temperature is less than 0 C. Use the energy-interaction model to predict if there will be a greater or lesser amount of ice in the final equilibrium state than in the initial state before the salt was added. Your explanation should include a complete energy-system diagram. [The simplest way to model this physical system is with one thermal energy system for everything and one bond energy system, i.e., in terms of the model, it is not useful to distinguish between the various chemical components in order to answer this particular question.]

Explanation / Answer

Initial state:

2.5 kg of ice + 50 g of liquid

there will be ice, since the specific heat of ice is much larger than that of liquid sensible heat requirement.

After addition of NaCl

Salt will dissociate in Na+ and Cl-, will lowe the freezing point of liquid water. Note that the salt is also soluble in ice, so it may lower the ice's freezing point as well.

In equilibrium (at the end)

Even though we ar elowering the freezing point, we are not converting all the ice to liquid, also, the container is insulated so expect no heat going in or out

The liquid will lower its freezig point and remain mostly liquid

Also, some fraction of ice must convet to liquid water, since it is being lowered. but there is still ice present in equilbirium (in a larger portion)

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