Physical Chemistry 2.pdf - OER@AVU - African Virtual University

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Effect of temperature on solubility of gases African Virtual University The solubility of most gases decrease with increasing temperature. For example oxygen and nitrogen, two major components of air are known to decrease as the temperature increases. It is for this reason that many types of fish can only survive in cold water as opposed to warm where there are lower amounts of dissolved oxygen. Comparative data is given in Table 4 showing solubility of selected gases at different temperatures. Solubility is in grams per 100 g of water when gaseous space over liquid is saturated with gas and the total pressure is 1 atm. Table 4 Comparative data depicting the solubility of the gases N 2 , O 2 and CO 2 . Gas Temperature (°C) 0 20 50 100 N 2 0.0029 0.0019 0.0012 0 O 2 0.0069 0.0043 0.0027 0 CO 2 0.335 0.169 0.076 0 The opposite phenomenon is often observed for solubility of gases in organic solvents. In this case, solubility increases with increase in temperature. For example, the solubility of hydrogen, nitrogen, carbon monoxide, helium and neon rise with increasing temperature in organic solvents such as carbon tetrachloride, toluene and acetone. Gases become more soluble at higher pressure A common phenomenon when one opens a can of many beverages is the fizzing sound as the gas escapes. Although the gases in air are not very soluble at room temperature, they become increasingly soluble at high pressures. The solubility of a gas is affected a lot more by pressure than temperature. To understand the effect of pressure, consider the following equilibrium for dissolution of a gas. Gas + solvent D solution An increase in pressure on the system reduces the volume available to the gas. According Le Châtelier’s principle the system must change to counter act the stress created. Consequently, the system reacts so as to reduce the pressure by decreasing the amount of gas. Gas molecules are absorbed from the gaseous phase into solution. The equilibrium in the above reaction equation is thus shifted to the right i.e. increased solubility. If the pressure on the solution is reduced then the reverse effect in equilibrium is observed, some of the gas molecules leave the solution into the gaseous phase. Equilibrium shifts to the left of the reaction equation. Fizzing of the cola drink or any other carbonated drink is a response to the lowering of pressure as the can (or
African Virtual University bottle is opened). The English chemist William Henry established that the solubility of a gas increased with increased pressure according to the mathematical relationship known as Henry’s law. C = kP gas (1.1) The solubility of a gas in a particular solvent is represented by C, P gas is the partial pressure of the gas and k is the proportionality constant. Henry’s constant is unique to each gas. Rationalization of Henry’s law In a saturated solution, the number of molecules leaving and entering the solution is equal. The rate at which molecules leave the gaseous phase into the solution and that at which the molecules leave the solution both depend on the number of molecules per unit volume. When the number per unit volume of the molecules is increased in the gaseous state, the number of molecules per unit volume in the solution also increases. When pressure is increased the number of molecules per unit volume in the solution must also increase. It is important to note, however, that Henry’s law fails for gases at high pressures and if there is a chemical reaction, or when the solventsolute interaction is significantly different from the solute-solute or solvent-solvent interaction. (1) (2) (3) Figure 1.2 Shows the effect of pressure on the solubility of the gas. The depth of the colour is indicative of concentration of the gas F ig ure 1.2 Shows the effect of pres sure on the solubilit y of the gas. The d epth of the co lour is indicative of conc entrat ion of the gas
Page 1 and 2: Physical Chemistry 2 Prepared by On
Page 3 and 4: Table of ConTenTs African Virtual U
Page 5 and 6: IV. Material African Virtual Univer
Page 7 and 8: Unit IV Electrochemistry (20 hours)
Page 9 and 10: Unit Learning objective(s) African
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Page 17 and 18: List of required readings • Solut
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Page 27 and 28: Simulation: Solution of salt 1.4 Th
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Practice problem 1 What is X in the
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Example 1 African Virtual Universit
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Practice problem 2 African Virtual
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5.7.2 Binding energy and nuclear st
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(ii) Photoelectric absorption Afric
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African Virtual University Colloid
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African Virtual University Standard
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Reading #3 COLLOIDS African Virtual
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Reading # 9 INTRODUCTION TO RADIOAC
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African Virtual University Rational
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Useful Link #2 Title : Chem1 Virtua
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Useful Link # 5 Title : Chemistry U
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Useful Link # 8 Title : Chemistry R
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Useful Link # 11 Title : RAdiation
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XVI. summative evaluation African V
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XVII. References African Virtual Un
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