1. Lecture 49 – Lecture 50 Reactions in Aqueous Solutions Ozgur Unal 1

2.  What changes take place at the atomic level, when salt is dissolved in water? 2

3.  An aqueous solution contains one or more substances called solutes dissolved in the water.  In this case water is the solvent – the most plentiful substance in the solution. Salt is the solute. 3  Molecular compounds can be dissolved in water.  Some of them exist as molecules in the solution  sugar solution.  Some molecular compounds form ions when dissolved in water  HCl HCl (aq)  H + (aq) + Cl - (aq)

4.  Ionic compounds (such as NaCl) can also be dissolved in water.  When ionic compounds dissolve, their ions separate. NaCl (aq)  Na + (aq) + Cl - (aq) 4

5.  When two aqueous solutions that contain ions as solutes are combined, the ions might react with one another.  These reactions are always double-replacement reactions.  There are 3 types of products that can form from double reactions in aqueous solutions: a precipitate, water or a gas. 5 Reactions that form precipitate:  Some reactions that occur in aqueous solutions produce precipitates.  Example: 2NaOH (aq) + CuCl2 (aq)  2NaCl (aq) + Cu(OH)2 (s)  Na + and Cl - ions after the reaction remain dissolved in the new solution.  Cu(OH)2 precipitates at the bottom of the solution

6.  To show the details of reactions that involve ions in aqueous solutions, chemists use ionic equations  different than chemical equations  In ionic equations, the ions in the solution are explicitly written: 2Na + (aq) + 2OH - (aq) + Cu +2 (aq) + 2Cl - (aq)  2Na + (aq) + 2Cl - (aq) + Cu(OH)2 (s) 6  An ionic equation that shows all of the particles in a solution as they exist is called a complete ionic equation.  Note that the sodium and chlorine ions do not participate in the reaction  spectator ions  Net ionic equations are ionic equations that include only the particles that participate in the reaction. 2OH - (aq) + Cu +2 (aq)  Cu(OH)2 (s)

7.  Consider the reaction between hydrobromic acid and sodium hydroxide: HBr (aq) + NaOH (aq)  ???  What are the products of this reaction?  Does this reaction produce a precipitate? 7  The reaction above produces NaBr (aq) and water.  The complete ionic equation for this reaction is: H + (aq) + Br - (aq) + Na + (aq) + OH - (aq)  H2O (l) + Na + (aq) + Br - (aq)  The net ionic equation is: H + (aq) + OH - (aq)  H2O (l)

8.  Example: Write the chemical, complete ionic and net ionic equations for the reaction between hydrochloric acid and aqueous lithium hydroxide. The reaction produces water and aqueous lithium choride. 8  Example: Write the chemical, complete ionic and net ionic equations for the reaction between the following substances. Mixing sulfuric acid and aqueous potassium hydroxide produces water and aqueous potassium sulfate.

9.  Some double replacement reactions produce gases, such as CO2, HCN and H2S  Example: 2HI (aq) + Li2S (aq)  H2S (g) + 2LiI (aq) 9  The complete ionic equation for this reaction is: 2H + (aq) + 2I - (aq) + 2Li + (aq) + S +2 (aq)  H2S (g) + 2Li + (aq) + 2I - (aq)  What are the spectator ions here?  The net ionic equation is: 2H + (aq) + S -2 (aq)  H2S (g)

10.  Write down the complete ionic, net ionic and chemical equations for the reaction between baking soda and vinegar. 10  Example: Write chemical, complete ionic, and net ionic equations for the reactions between perchloric acid (HClO4) and aqueous potassium carbonate forming CO2 and water.

11.  Consider the reaction between HCl and NaHCO3. HCl (aq) + NaHCO3 (aq)  H2CO3 (aq) + NaCl (aq)  When H2CO3 is formed, it decomposes immediately: H2CO3 (aq)  H2O (l) + CO2 (g)  Therefore, there are two reactions taking place: double replacement and decomposition reactions. 11  When these two reactions are combined, we get the overall reaction: H + (aq) + HCO3 - (aq)  H2O (l) + CO2 (g)  Follow the steps on page 307.