Colligative Properties

7 Key excerpts on "Colligative Properties"

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  eBook - ePub

  Chemistry II For Dummies

  • John T. Moore(Author)
  • 2012(Publication Date)
  • For Dummies

    (Publisher)

  By law, the maximum contamination level of lead in drinking water is 0.05 ppm. This number corresponds to 0.05 milligrams of lead per liter of water. That’s pretty dilute. But mercury is regulated at the 0.002 ppm level. Sometimes, even this unit isn’t sensitive enough, so environmentalists have resorted to the parts per billion (ppb) or parts per trillion (ppt) concentration units. Some neurotoxins are deadly at the parts per billion level.

  Contemplating Colligative Properties

  Some properties of solutions depend on the specific nature of the solute. For example, sugar solutions taste sweet, whereas salt solutions taste salty. Salt solutions conduct electricity (they’re electrolytes), while sugar solutions don’t (they’re nonelectrolytes). Solutions containing the copper cation are commonly blue, while those containing the nickel cation are green.

  However, some properties of solution don’t depend on the specific type of solute — just the number of solute particles. Properties that simply depend on the relative number of solute particles are called Colligative Properties . The effect the solute has on the properties of the solution simply depends on the number of solute particles present. The following sections discuss in greater detail these Colligative Properties — these effects — including

  Vapor-pressure lowering

  Boiling-point elevation

  Freezing-point depression

  Osmotic pressure

  Vapor-pressure lowering

  A liquid that is contained in a closed container will eventually evaporate, and the gaseous molecules contribute to the pressure of the gas above the liquid. The pressure due to the gaseous molecules of the evaporated liquid is called the liquid’s vapor pressure.

  But if you make that same liquid the solvent in a solution, the vapor pressure due to the solvent evaporation is lower, because the solute particles in the liquid take up space at the surface and the solvent can’t evaporate as easily. Also many times the solute and solvent may have an attractive force that also makes it more difficult for the solvent to evaporate. That lowered vapor pressure is independent of what kind of solute you use. Instead, it just depends on the number of solute particles that are present in the solution.

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  eBook - ePub

  The Britannica Guide to Matter

  • Britannica Educational Publishing, Erik Gregersen(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  2 . The equation is:

  The essence of this technique follows from the observation that, in a dilute solution of a nonvolatile solute, the rise in boiling point is proportional to the number of solute molecules, regardless of their size and mass.

  DECREASE IN FREEZING POINT

  Another colligative property of solutions is the decrease in the freezing temperature of a solvent that is observed when a small amount of solute is dissolved in that solvent. By reasoning similar to that leading to equation (5), the freezing-point depression,

  ΔTf

  , the freezing temperature of pure solvent, T

  f 1

  , the heat of fusion (also called the heat of melting) of pure solvent per unit weight, l 1 fusion , and the weights of solute and solvent in the solution, w 2 and w 1 , respectively, are so related as to equal the molecular weight of solute, M 2 , in the equation

  A well-known practical application of freezing-point depression is provided by adding antifreeze to the cooling water in an automobile’s radiator. Water alone freezes at 0 °C (32 °F), but the freezing temperature decreases appreciably when ethylene glycol is mixed with water.

  OSMOTIC PRESSURE

  A third colligative property, osmotic pressure, helped to establish the fundamentals of modern physical chemistry and played a particularly important role in the early days of solution theory. Osmosis is especially important in medicine and biology, but in recent years it has also been applied industrially to problems such as the concentration of fruit juices, the desalting of seawater, and the purification of municipal sewage. Osmosis occurs whenever a liquid solution is in contact with a semipermeable membrane—i.e., a thin, porous wall whose porosity is such that some, but not all, of the components in the liquid mixture can pass through the wall. A semipermeable membrane is a selective barrier, and many such barriers are found in plants and animals. Osmosis gives rise to what is known as osmotic pressure in which a container at uniform temperature is divided into two parts by a semipermeable membrane that allows only molecules of component A to pass from the left to the right side; the selective membrane does not allow molecules of component B to pass. Example compounds for A and B might be water and sodium chloride (table salt), respectively.

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  eBook - ePub

  Physicochemical and Environmental Plant Physiology

  • Park S. Nobel(Author)
  • 2020(Publication Date)
  • Academic Press

    (Publisher)

  cell sap can be calculated using the freezing point depression of 1.86°C for a 1-molal solution together with the Van't Hoff relation.

  Figure 2-8  Schematic diagram indicating the principle underlying an osmometer in which a semipermeable membrane (permeable to water, but not to solutes) separates pure water (region A) from water containing solutes (region B). Water tends to diffuse toward regions where it has a lower mole fraction, in this case into region B (region of higher osmotic pressure). This causes the solution to rise in the open central column until, at equilibrium, the hydrostatic pressure (P ) at the horizontal dashed line is equal to the osmotic pressure (Π) of the solution. Alternatively, we can apply a hydrostatic pressure P to the right-hand column to prevent a net diffusion of water into region B, this P again being equal to Π.

  2.2F. Van't Hoff Relation

  For many purposes in biology, osmotic pressures are related to the concentration of solutes instead of expressing Π in terms of the more abstract water activity, a w , as is done in Equation 2.7 . In general, the greater is the concentration of solutes, the lower a w becomes and hence the more negative is ln a w , and the larger is the osmotic pressure. Therefore, some way of expressing a w in terms of the properties of the solutes is needed. The ensuing derivation not only will show how a w

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  eBook - ePub

  Concise Physical Chemistry

  • Donald W. Rogers(Author)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  free solvent concentration may be reduced by solvation.

  If NaCl is the nonvolatile solute in water for example, there will be an effective molality approximately two times the anticipated value because NaCl exists as Na+ ions and Cl− ions in aqueous solution. The number of particles in solution is the van’t Hoff i factor, 2 for NaCl solutions, 1 for sucrose, which does not ionize, 3 for ZnCl2 , and so on. Van’t Hoff i factors are, however, integers only at infinite dilution.

  In real solutions, van’t Hoff i factors show a systematic deviation from integral values due to strong solvation (hydration) of the molecules or ions. When there is a strong association between a solute molecule and solvent molecules, the solvent molecules are effectively “taken away” from the solution. The amount of free solvent is reduced and the relative amount of solute is greater than we conventionally calculate it to be. The measured change in Colligative Properties is augmented.

  The freezing points of aqueous solutions of NH3 , which is not ionized to any appreciable extent, are shown in Fig. 12.7 . The freezing point of water decreases with ammonia concentration according to the van’t Hoff equation Δ

  Tf

  = − 1.86 m to about m ≅ 4.0 but then the freezing point becomes more negative than theory predicts, as though the solution were more concentrated than it actually is. The effective molality (Zavitsas, 2001) is reduced to m 1 − hm 2 , where h is a parameter called the solvation number, which gives the number of solvent molecules held so tightly by solute as to be ineffective. In water solution, h is called the hydration number . The solvation number h is not an integer because it is an average over many solute molecules or ions. It is not difficult to determine h ; it is just an empirical parameter chosen to cause real colligative behavior to approach the van’t Hoff equation. In the case of ammonia dissolved in water, the choice h = 1.8 leads to the function shown by open circles in Fig. 12.7 . These experimental freezing points differ from those shown by solid circles only in that the molalities have been recalculated as moles of solute per kilogram of free

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  eBook - ePub

  Molecular Driving Forces

  Statistical Thermodynamics in Biology, Chemistry, Physics, and Nanoscience

  • Ken Dill, Sarina Bromberg(Authors)
  • 2010(Publication Date)
  • Garland Science

    (Publisher)

  16 Solvation & the Transfer of Molecules Between Phases The Chemical Potential Describes the Tendency of Molecules to Exchange and Partition

  We now consider the solvation, partitioning, and transfer of molecules from one medium to another. For example, toxins dissolve in one medium, such as water, and partition or transfer into another medium, such as the oil components of fish. A central type of action in biology—the binding of a drug or metabolite to a protein—requires first desolvation, i.e., the stripping away of water molecules from the ligand and the receptor. Drugs that act on the brain must be designed not only for their ability to bind their biological targets but also for their ability to transport across an interface called the blood–brain barrier. Related to partitioning and solvation are Colligative Properties: vapor pressure depression by solutes, boiling temperature elevation, freezing temperature depression, and osmotic pressure. For example, salt dumped on icy roads will melt the ice. Salt added to a boiling pot of water will reduce the boiling. Reverse osmosis is a way to purify salt water to make it drinkable. In this chapter, we use the liquid mixture lattice model to illustrate, in a simple approximate way, the molecular basis for solvation and colligative processes.

  What drives transfer and partitioning processes? On one hand, atoms and molecules move from regions of high concentration to regions of low concentrations, to increase the entropy of the system. On the other hand, molecules also move into regions for which they have high chemical affinity. In this chapter, we explore the chemical potential, an expression of these driving forces. While the free energy describes the tendency of a whole system toward equilibrium, the chemical potential describes the tendency toward equilibrium for each individual chemical component. In a mixture, the low-concentration component is called the solute and the high-concentration component is the solvent

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  eBook - ePub

  Let's Review Regents: Chemistry--Physical Setting Revised Edition

  • Albert S. Tarendash(Author)
  • 2021(Publication Date)
  • Barrons Educational Services

    (Publisher)

  Chapter Twelve

  Solutions and their properties

  Key Ideas

  This chapter focuses on the properties of the homogeneous mixtures we call solutions. The concepts of solubility, concentration, and Colligative Properties are explored. The chapter concludes with a discus sion of suspensions and colloidal dispersions.

  Key Objectives

  At the conclusion of this chapter you will be able to:

  • Define the terms solution, solute, and solvent.
  • Provide examples of various types of solutions.
  • Define the terms miscible, saturated, unsaturated, solubility, and supersaturation.
  • Describe the factors that affect the solubility of a substance.
  • Use the “solubility rules” to predict the solubility of an ionic compound in water.
  • Interpret a solubility curve, and solve problems involving solu- bility curves.
  • Describe how the concentration of a solution can be expressed with respect to the following terms: percent, parts per million, mole fraction, molarity, and molality, and solve problems involving these measurements of concentration.
  • Describe how the solute affects the boiling point and the freezing point of a solution.
  • Solve problems involving freezing point depression and boiling point elevation (Colligative Properties).
  • Solve problems involving solutions and chemical equations.
  • Define the term electrolyte, and indicate why solutions of electrolytes exhibit abnormal behavior.
  • Distinguish among suspensions and types of colloidal dispersions.

  Passage contains an image

  section I—basic (regents-level) material

  NYS REGENTS CONCEPTS AND SKILLS

  Note:

  By the time you have finished Section I, you should have mastered the concepts and skills listed below. The Regents chemistry examination will test your knowledge of these items and your ability to apply them. Concepts are the basic ideas that form the body of the Regents chemistry course (what you need to know!). Skills are the activities

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  eBook - ePub

  Chemistry

  With Inorganic Qualitative Analysis

  • Therald Moeller(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  14

  SOLUTIONS AND COLLOIDS

  Publisher Summary

  This chapter describes the types of solutions, such as gases in gases, gases in liquids, solids in solids, solids in liquids, and liquids in liquids. It further describes the properties of ideal vs. non-ideal, and electrolyte vs. non-electrolyte solutions. The chapter discusses two ways of expressing solution concentrations-mole fraction and molality. It illustrates dilution problems and the types of standard solutions. The chapter explains the vapor-pressure related properties of liquid solutions quantitatively. It discusses the properties and types of colloids. In a saturated solution, the mass of the crystals remains the same as long as the temperature is unchanged. Molecules or ions join and leave the crystal surface in a dynamic equilibrium. During careful cooling of a saturated solution, no solid crystallizes out, and the solution becomes supersaturated. The chapter explains Henry’s law and vapor pressure of a solution that follows Raoult’s law. It describes fractional distillation method. The change in any colligative property of a solution, with changing concentration, is directly proportional to the amount of solute dissolved in a definite amount of solvent.

  In this chapter we first give a qualitative description of the types of solutions—gases in gases, gases in liquids, and so on—and of the properties of ideal vs. nonideal, and electrolyte vs. nonelectrolyte solutions. Next we introduce two more ways of expressing solution concentrations (mole fraction and molality), illustrate dilution problems, and summarize the types of standard solutions. The next sections treat the vapor-pressure related properties of liquid solutions quantitatively. Finally, we discuss the properties and types of colloids.

  E

  nclosed within your skin is a complex biological–physical–chemical system. Molecules are organized into nerves, muscles, bones, cartilage, and connective tissue. Maintaining all of these structures are the body fluids–blood, lymph, tissue fluids, intestinal fluids, urine–each a complex mixture of molecules and ions carried in an aqueous medium

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