Exp 14 - Double Displacement Reactions

Exp 14 - Double Displacement Reactions - UMass Lowell...

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Experiment 14 Double Displacement Reactions Overview In this experiment you ultimately want to try to identify two "unknown" solutions of ionic substances, by comparing the reactions of the "unknown" substances with the reactions of a set of "known" substances. When a chemical reaction takes place, there is usually some sort of observable evidence for the reaction: a gas may be produced (causing bubbling); the color of the sample may change; an odor may be noticed; heat may be evolved. Perhaps the most common bit of evidence that a reaction has taken place, however, is the formation of a solid precipitate when the reactants are mixed. For example, if clear, colorless solutions of barium nitrate [Ba(NO 3 ) 2 ] and sodium sulfate (Na 2 SO 4 ) are mixed, a precipitate of barium sulfate, BaSO 4 immediately forms: This reaction is an example of a general class of reactions called double displacement (or metathesis ) reactions. Such reactions have the general format Ba(NO 3 ) 2 ( aq ) + Na 2 SO 4 ( aq ) 2NaNO 3 ( aq ) + BaSO 4 ( s ) A + B - ( aq ) + X + Y - ( aq ) A + Y - ( aq ) + X + B - (s) Page 1 of 4 UMass Lowell Freshman Chemistry Lab 4/15/2008 http://faculty.uml.edu/james_hall/84123/14.htm
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In a sense, each positive ion in the reactants has "displaced" the other positive ion. One of the new combinations of ions is insoluble in water and forms a precipitate. The precipitate could be filtered, and the water could be evaporated from the ions remaining in solution, to give two new substances. In this experiment you will use the appearance (or non appearance) of a precipitate when two reagents are mixed as a means of identifying your two unknown solutions. The most important part of this experiment is learning to relate the Table of Solubilities on Page 106 with your experimental observations. For example, suppose you mixed together CaCl 2 solution with AgNO 3 solution: a precipitate forms. But what is the identity of the precipitate?? The two possible products of the reaction are AgCl and Ca(NO 3 ) 2 . Find the intersection of Ca 2+ and NO 3 - in the Table of Solubilities: there is an " S " there, which means that Ca(NO 3 ) 2 is soluble and must not be the substance that precipitated. However, if you look at the intersection of Ag
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Exp 14 - Double Displacement Reactions - UMass Lowell...

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