Chem 112, week 2-3 class notes
Chem 112, week 2-3 class notes CHEM 112
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This 5 page Class Notes was uploaded by Savannah Notetaker on Monday September 5, 2016. The Class Notes belongs to CHEM 112 at University of South Carolina taught by aaron vannucci in Fall 2016. Since its upload, it has received 66 views. For similar materials see General Chemistry II in Chemistry at University of South Carolina.
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Date Created: 09/05/16
Solution Terminology Summary Solute and Solvent When solids or gases are dissolved in liquids to form a liquid solution the liquid is called the solvent and the other component is called the solute. When two liquids are combined to form a solution, the one present in the largest amount is usually called thesolvent and the other component is called the solute. In the most general terms, the solute is said to dissolve in the solvent. Concentrated and Dilute A solution that has a relatively large amount of solute per unit amount of solvent is called concentrated. A solution that has a relatively small amount of solute per unit amount of solvent is called dilute. Solubility The maximum amount of a solute that will dissolve in a given amount of solvent at a given temperature is called the solubility of that solute in that solvent at that temperature. Saturated, Unsaturated and Supersaturated A solution whose concentration is at the solubility limit for a given temperature is said to be saturated. A solution whose concentration is lower than the solubility limit for a given temperature is said to be unsaturated. A solution whose concentration is greater than the solubility limit for a given temperature is said to be supersaturated. Such a solution can only be prepared under carefully controlled conditions and is not stable. Miscible and Immiscible These terms are customarily applied only to solutions of liquids in liquids. If two liquids dissolve in each other in all proportions, they are said to be miscible. If two liquids are insoluble in each other, they are said to be immiscible. Liquids that will dissolve in each other in appreciable, but limited, amounts are said to be partially miscible. The Relationship between K and K c p For the general chemical reaction: aA + bB cC + dD c d [C] [D] K c [A] [B] b If the reactants or products are gases, the equilibrium constant can also be expressed in terms of PRESSURES because (n/V) in the ideal gas law is just the concentration of the gas: P = (n/V) RT = [gas] RT For the general GAS PHASE reaction: aA(g) + bB(g) cC(g) + dD(g) c d (P C (P D K p (P A (P B b To determine the relationship between K and K supstitute c = [gas] RT, for each pressure in the equation above: c d ([C]RT) ([D]RT) K p ([A]RT) ([B]RT) b c d c+d ... = ([C] [D] )(RT) ([A] [B] )(RT) a+b ... = Kc(RT) (c+d)-(a+b) Or K p K (Rc) n(gas) Where: n(gas) = (c+d)-(a+b) = moles gaseous products - moles gaseous reactants Note that K acd K arp expressed as unitless quantities. You must use the ideal gas constant, R, in units of (liter atmospheres/mole Kelvin) and temperature in Kelvin, for this equation to hold. Le Chatelier's Principle: A change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner as to reduce or counteract the effect of the change. Factors that determine the equilibrium conditions of a system are: 1) Concentration, 2) Temperature (K changes), and 3) Volume (Pressure) for gaseous systems. Concentration When a system is at equilibrium, Q = K. Changing the concentration of reactants or products that are part of the equilibrium constant expression cause the reactant quotient, Q, to change. Q will no longer be equal to K and the system will seek to restore itself to a new equilibrium that offsets the change. For example: Adding reactant to an equilibrium system. 2 2 NO (2) N O (2) 4..... K = [c O ] /2[N4 ] 2 Adding NO (g)2will cause the reaction to run in the forward direction to consume the added reactant. Since NO is in 2he denominator, Q is less than K, and more product needs to be produced to reestablish equilibrium. Therefore the concentration of N O2(g4 will increase. 2 2 H 2(l) 2 H (g2 + O (g) 2..... K = [Hc] [O ]2 2 Adding H O(2) will have no effect on the equilibrium because the reactant is a liquid. Notice that the concentration of H O(2) is not a part of the equilibrium constant expression. Since the concentration of product gases do not change when H 2(l) is added, Q is still equal to K and there is no driving force for the system to change. As long as some liquid is present to establish the equilibrium, adding more will have no effect on the concentration of products. LeChatelier's Principle: A change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner as to reduce or counteract the effect of the change. Factors that determine the equilibrium conditions of a system are: 1. Concentration 2. Temperature (K changes) 3. Volume (Pressure) for gaseous systems. Temperature Raising the temperature on an equilibrium system causes the system to try to absorb the added energy. Reducing the temperature causes the system to try to restore the energy. You must, therefore, consider whether the reaction isendothermic or exothermic. Endothermic Reactions For an endothermic reaction, energy is absorbed when the reaction runs in the forward direction. You can think of energy as a reactant. Reactants + Energy Products If the temperature is raised, the system responds by converting reactants to products and absorbing the added energy in the process. Similar to when a reactant concentration increases, the system shifts toward products to consume the added energy. If the temperature is lowered, the opposite happens. The equilibrium constant increases when the temperature is raised on an endothermic reaction. Exothermic Reactions For an exothermic reaction, energy is absorbed when the reaction runs in the reverse direction. You can think of energy as a product. Reactants Products + Energy When the temperature is raised, the system responds by converting products to reactants and absorbing the added energy in the process. Similar to when a product concentration increases, the system shifts toward reactants to consume the added energy. If the temperature is lowered, the opposite happens. The equilibrium constant decreases when the temperature is raised on an exothermic reaction. Temperature is the only equilibrium variable that changes the value of the equilibrium constant.
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