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  Teaching about the Chemistry of Oxygen Solubility
 

Goals
This activity introduces students to:

  1. Factors that control the solubility of gases in water
  2. Henry's Law and LeChatelier's Principle
  3. supersaturation of gases in water
  4. Dynamics of oxygen solubility in lake ecosystems

Introduction
This lesson introduces students to the principles that control the solubility of gases in
water, and includes applications to the dynamics of oxygen solubility in lake ecosystems. For chemistry (and physics) students this lesson provides explanation and animation of the physical principles that control the solubility of a non-polar gas in water, as well as a practical application that allows students to observe these attractive forces in action in a lake ecosystem. For biology students, this lesson demonstrates applicability of chemistry to changes in environmental conditions and illustrates the role organisms can play in altering the chemical condition of their own environment.

Outcomes
Students will be able to:

  1. Explain and diagram how nonpolar gases enter into solutions in water.
  2. Explain the roles that pressure and temperature play in altering the concentration of gases in water.
  3. Explain the physical basis for oxygen supersaturation and how it is related to
    photosynthesis.

Keywords
gas solubility, dipole, dipole-induced dipole, saturation, pressure, temperature, polar,
non-polar

Materials/Resources/Software
Excel, Shockwave

Time Required
1-2 hours

Curriculum Connections
Biology - abiotic factors in the environment, photosynthesis
Chemistry - chemical equilibrium, solubility, polar and nonpolar materials,
endothermic/exothermic reactions

WOW Curriculum Links
Thermal stratification; oxygen; aquatic respiration

Procedure
Work through the following discussions with the students. This could be offered as a demonstration and lecture using computerized projections, or with students in a computer lab while stopping periodically for discussions.

  1. Discuss how oxygen dissolves and review the animation, Shockwave plug-in required.

Notes: Water, as a polar molecule, induces an accumulation of electron density(dipole moment) at one end of nonpolar gas molecules such as oxygen (O2) and carbon dioxide (CO2). In animation, observe a polar water molecule approaching a nonpolar O2 molecule. The electron cloud of O2 is normally distributed symmetrically between the bonded O2 atoms. As the negative end of the H2O molecule approaches the oxygen molecule, the electron cloud of the O2 moves away to reduce the negative-to-negative repulsion. As a result, a dipole (a molecule with positive and negative charges separated by a distance) has been induced in the nonpolar O2 molecule, causing O2 and H2O to become weakly attracted to each other. This intermolecular attraction between the oppositely charged poles of nearby molecules is termed a dipole- induced dipole force. The creation of these forces then explains the mechanism by which gases dissolve in water.

  1. Discuss the effects of pressure on oxygen solubility and review the animation, Shockwave plug-in required.

Notes: Because dipole-induced dipole forces are very weak, the quantity of nonpolar gases (such as O2) that will dissolve in a given volume of water is strongly affected by temperature and pressure. Henry's Law describes the effect of pressure on the solubility of a gas in a liquid. The law states that the amount of gas that dissolves in a given volume of solvent at a specified
temperature (usually 25°C for water) is proportional to the partial pressure of the gas above the liquid. When gas under pressure contacts a liquid, the pressure tends to force gas molecules into solution. At a given pressure the number of gas molecules that will enter into solution rises until equilibrium is reached. By definition, at equilibrium, the number of gas molecules entering and leaving the solution is balanced and the concentration of the gas in solution remains constant. If the partial pressure of a gas increases, more gas enters into solution. If partial pressure drops, gas comes out of solution and reaches a new equilibrium. Illustrate this by opening a can or bottle of soda pop.

At sea level, total atmospheric pressure is 760 mm Hg. This means that the
gravity-induced weight of the atmosphere generates enough force to move a
sufficient volume of mercury (Hg) 760 mm up a tube. At sea level, approximately 20.8 percent of this air is oxygen gas (O2). The partial pressure of oxygen at sea level is, therefore, 158 mm Hg (760 mm Hg x 0.208 = 158.08 mm Hg). Oxygen has a Henry's Law constant of 1.7 x 10-6 molal/mm Hg when dissolved in water at 25°C. 

Molality of O2 = (1.7 x 10-6 molal/mm Hg)
(158.08 ) = 2.687 x 10-4 m

From the above value the number of milligrams per liter of oxygen that will
dissolve in 25°C water can be calculated.

2.687 x 10-4 moles/kg x 32g/mole x 1000 mg/g = 8.6 mg/liter

  1. Discuss the effects of temperature on solubility: Le Chatelier's Principle (review the computer animation) and refer to the table oxygen solubility).

Notes: Begin with a demonstration. Open two cans of soda pop, one warm and one
cold (students can also work in small groups). It is easily observed that more
gas is released if the can is warm than when it is cold. Before this lesson,
pour cold water into a glass. During the lesson students can observe the oxygen
bubbles that formed inside the glass of water that had been poured cold and
warmed up over time. Both of these demonstrations illustrate the fact that the
temperature of a solvent (recall that water is the "universal solvent") affects
the solubility of gases.

Gases that dissolve in solvents usually release heat in an exothermic process as they dissolve .

gas + liquid solvent ---> saturated solution + heat

This process continues until saturation is reached. At this point gas will still dissolve, but will be balanced by the gas that leaves solution. If heat is added to a solution, gas is released in this endothermic reaction:

saturated solution + heat ---> gas + liquid solvent

At equilibrium, as many molecules come out of solution as dissolve in a given time period.

gas + liquid <------> saturated solution + heat

Le Chatelier's principle states that a change in any of the factors determining
equilibrium will cause the system to adjust in order to reduce or counteract
the effect of the change. Le Chatelier's principle predicts that the solubility of a gas will increase as a system loses heat, and will decrease as it gains heat.

  1. Discuss how O2 supersaturation is possible.

Notes: Due to the effects of hydrostatic pressure on gases in solution, water can become supersaturated with oxygen and other gases (exceed 100% saturation). The attractive forces that hold excess oxygen in solution similar to the dipole-induced dipole forces discussed earlier, but a smaller number of water molecules are available to induce dipoles in oxygen molecules. This leads to weaker attraction of oxygen molecules when water is supersaturation with oxygen. Hypothesize what conditions in lakes could cause supersaturation. What clues could you look for in a lake that may indicate O2 supersaturation is occurring?

  1. Assign student pairs to complete the student Studying Lesson.
  2. Discuss student results.

Resources

  1. Horne, A.J. and C.R. Goldman. 1994. Limnology. McGraw-Hill
  2. Wetzel, R.G. Limnology, 1983. W.B. Saunders Publishing.
  3. Kotz, J. and K. Purcell. 1991. Chemistry and Chemical Reactivity.

Extensions
Monitor dissolved O2 levels in aquaria with and without plant life. Graph the O2 concentrations over time. Describe the patterns that emerge.


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date last updated: Wednesday March 03 2004