FINAL_Studio_4_Identifying Salts_S2006_1

FINAL_Studio_4_Identifying Salts_S2006_1 - Chem 25 Studio#4...

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Chem. 25: Studio #4 Identifying Salts Using Precipitation Reactions NAME:___________________________ STUDIO:_______ GROUP: _______ Identifying Salts Using Precipitation Reactions How do you know if a chemical reaction occurs after mixing two reactants? You can often answer this common question from direct observations. A chemical reaction can be detected upon mixing separate solutions of two potential reactants if any of the following things happen: o the color changes o a gas is evolved, o a solid precipitates, o the temperature changes, or o the pH changes. In this Studio, you examine whether a number of potential chemical reactions occur upon mixing two colorless salt solutions. The occurrence of a chemical reaction is signaled by deposition of a white precipitate. Consider the chemical reaction that occurs upon mixing equimolar aqueous solutions of the two salts barium chloride (BaCl 2 ) and sodium sulfate (Na 2 SO 4 ). The immediate formation of a thick, gelatinous white precipitate of barium sulfate indicates that the following chemical reaction has occurred: BaCl 2 (aq) + Na 2 SO 4 (aq) 2 NaCl (aq) + BaSO 4 (s) (1) Equation (1) is the balanced “ molecular ” equation indicating that only BaSO 4 of the four compounds involved deposits as a precipitate since it is insoluble. The subscript (aq) identifies a material that is soluble in water; the subscript (s) indicates a material that is insoluble in water and therefore present in the system as a solid. These reactions are generically referred to as double displacement or metathesis reactions and have the general form: AX + BZ AZ + BX A more detailed representation of the reaction that produces barium sulfate is given by the ionic equation for the process (2): Ba 2+ (aq) + 2 Cl¯ (aq) + 2 Na + (aq) + SO 4 (aq) 2 Cl¯(aq) + 2 Na + (aq) + BaSO 4 (s) (2) In water, the soluble salts BaCl 2 and Na 2 SO 4 dissociate and exist as their respective cations and anions. Of the possible ionic products, NaCl remains in solution (but dissociated into its ions) whereas BaSO 4 , an insoluble salt, drops out of solution. The full or complete ionic equation shows all the species present in the system. One more equation is frequently used to describe the formation of insoluble barium sulfate in water. The net ionic equation for this reaction is written as equation (3): 1
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Ba 2+ (aq) + (SO 4 ) 2- (aq) BaSO 4(s) (3) The net ionic equation shows only those ions that are involved in the reaction. Note that in equation (2), sodium ions show up on both sides of the equation. They are not involved in the chemistry – they start out in solution and remain in solution. They, and the chloride ions, are known as spectator ions , and as spectator ions they are not included in the net ionic equation. Let us re-examine equations (1) and (2) and see how we can predict if a chemical reaction will occur when two solutions are mixed. Dissolution of the reactants BaCl 2 and Na 2 SO 4 gives two cations, Ba 2+ (aq) and Na + (aq), and two anions, Cl¯ (aq), SO 4 (aq). The possible chemical reaction amounts to reshuffling the cation-anion pairs to form potential products NaCl and BaSO 4 . A chemical reaction takes place only if one (or both) of these two new salts is insoluble.
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