CH145L_09_06 - CH 145- Experiment VI- Fall 2009 Laboratory...

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CH 145- Experiment VI- Fall 2009 1 Laboratory VI: Chemical Equilibria 1 Pre-laboratory Assignment: Textbook, Chapter 15 (all sections) Ocean Optics handouts This laboratory handout There will be a pre-laboratory quiz!! Introduction Chemical Equilibria No chemical reaction goes to completion: there will always be some amount of reactants left with the products. For example, although we write 2 CO 2 (g) 2 CO (g) + O 2 (g) (1) as though it goes entirely to products, at 2000K only 2% of the CO 2 decomposes. A chemical reaction reaches equilibrium when the concentrations of the reactants and products no longer change over time. The position of the equilibrium describes the relative amounts of reactants and products that remain at the end of a chemical reaction. The position of the equilibrium for reaction (1) is said to lie with the reactants, or to the left, because at equilibrium very little of the carbon dioxide has reacted. On the other hand, in the reaction 2 H 2 (g) + O 2 (g) 2 H 2 O (g) (2) the equilibrium position lies very far to the right since only very small amounts of H 2 and O 2 remain after the reaction reaches equilibrium. Since chemists often wish to maximize the yield from a reaction, it is vital to determine how to control the position of the equilibrium. The equilibrium position of a reaction may shift if an external stress is applied. The stress may be in the form of a change in temperature, pressure, or the concentration of one of the reactants or products. For example, consider a flask with an equilibrium mixture of CO 2 , CO, and O 2 , as in reaction (1). If a small amount of CO is then injected into the flask, we will find that the concentration of CO 2 increases. Here the external stress is the increase in concentration of CO. The system responds by reacting some of the added CO with O 2 to yield an increased amount of CO 2 . That is, the position of equilibrium shifts to the left, yielding more reactant and less CO. Reaction (1) will also shift with changes in pressure. Starting with reaction (1) at equilibrium, an increase in pressure will cause the position of equilibrium to shift to the side of the reaction with the smaller number of moles of gas. That is, by shifting the equilibrium position to the left, the reaction will decrease the number of moles of gas, thereby decreasing the pressure in the flask. In so doing, some of the applied stress is relieved. On the other hand, an 1 This experiment was modified from Laboratory Inquiry in Chemistry by Bauer, Birk, and Sawyer.
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CH 145- Experiment VI- Fall 2009 2 increase in pressure for reaction (2) will result in a shift of the equilibrium to the right to decrease the number of moles of gas. The Iron-Thiocyanate Equilibrium
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CH145L_09_06 - CH 145- Experiment VI- Fall 2009 Laboratory...

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