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Freezing Point Depression

Freezing Point Depression. the change that occurs in the temperature at which a liquid freezes (or a solid melts) when a solute is dissolved in it . Pure water freezes at 0°C (32°F )

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Freezing Point Depression

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  1. Freezing Point Depression the change that occurs in the temperature at which a liquid freezes (or a solid melts) when a solute is dissolved in it.

  2. Pure water freezes at 0°C (32°F) • If 10 grams (0.35 ounces) of sodium chloride (table salt) is dissolved in 100 grams (3.53 ounces) of water, the freezing point goes down to −5.9°C (21.4°F). • Why does the freezing point of water drop when sodium chloride is added to it?

  3. When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change. The ice and water are said to be in dynamic equilibrium with each other.

  4. Here's the same container with the water at 0°C, only this time the water contains salt molecules. Adding salt disrupts the equilibrium. The salt molecules dissolve, but do not attach easily to the solid ice. There are fewer water molecules; some of the water has been replaced by salt. The number of water molecules able to be captured by the ice decreases, so the rate of freezing slows.

  5. The rate of melting of the ice is unchanged by the salt so melting is now occurring faster than freezing. So the ice eventually melts. The temperature drops so that the rate of freezing can increase to reach equilibrium. In this example, the new freezing/melting point is reached at -4°C. The higher the concentration of salt, the lower the freezing point drops.

  6. If sucrose (table sugar) is used instead of sodium chloride, will the freezing point of the solution be lower or higher than that of the salt solution? • 10 grams (0.35 ounces) of sucrose C12H 22O11in 100 grams (3.53 ounces) of water gives a solution with a freezing point of −0.56°C (31°F). • Why is the freezing point of the salt solution lower than that of the sugar solution?

  7. The reason that the salt solution has a lower freezing point than the sugar solution is that there are more particles in 10 grams of sodium chloride than in 10 grams of sucrose. • Freezing point depression is a colligative property which is a property of solutions that depends on the number of dissolved particles in solution, not on the identities of the solutes.

  8. Each sodium chloride unit comes apart into two ions (a sodium cation and a chloride anion ) when dissolved in water.

  9. Calculate the change in freezing point (ΔT f ) relative to the pure solvent using the equation: • Δ T f = i K f m • where K f is the freezing point depression constant for the solvent (1.86°C·kg/mol for water) • m is the number of moles of solute in solution per kilogram of solvent • and iis the number of ions present per formula unit (e.g., i= 2 for NaCl).

  10. Calculating Freezing Point Depression 1.60 g of naphthalene (C10H8) is dissolved in 20.0 g of benzene. The freezing point of pure benzene is 5.5 oC, and the freezing point of the mixture is 2.8 oC. What is the molal freezing point depression constant, Kf of benzene?

  11. Step 1: Calculate the freezing point depression of benzene.

Tf = (Freezing point of pure solvent) - (Freezing point of solution)
(5.5 oC) - (2.8 oC) = 2.7 oC • Step 2 : Calculate the molal concentration of the solution.
molality = moles of solute / kg of solvent
moles of naphthalene = (1.60 g) (1 mol / 128 g) = 0.0125 mol naphthalene
molality of solution = (0.0125 mol) / (0.0200 kg) = 0.625 m

  12. Step 3: Calculate Kf of the solution.Tf = (Kf) (m)(2.7 oC) = (Kf) (0.625 m)Kf= 4.3 oC/m Everyday uses of Freezing Point Depression • What ideas do you have? • Salt melting ice • Car antifreeze • Making ice cream • This is a lab activity we will be doing in class!

  13. References • Read more: Colligative Properties - Chemistry Encyclopedia - water, uses, examples, gas, number, equation, salt, property, Freezing Point Depression, Boiling Point Elevation, Vapor Pressure Lowering, Osmotic Pressure http://www.chemistryexplained.com/Ce-Co/Colligative-Properties.html#ixzz1ESZg9En9 • http://www.worsleyschool.net/science/files/saltandfreezing/ofwater.html • http://chemed.chem.purdue.edu/genchem/probsolv/colligative/kf1.3.html • Image: http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/chemistry/chemistr.htm

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