chapter 14: ions in aqueous solutions

chapter 14: ions in aqueous solutions

Dissociation When a compound that is made of ions dissolves in water, the ions separate from one another. This is called dissociation. NaCl(s) Na + (aq) + Cl - (aq) For ionic compounds, formula units ( molecules for ionic compounds ) tell how many ions will be formed during dissociation. Dissociation of NaCl

Dissociation (cont d) Similarly, CaCl2 undergoes dissociation as follows: CaCl2(s) Ca 2+ (aq) + 2Cl - (aq) Assuming 100% dissociation, a solution containing 1 mol of NaCl will produce 1 mol of Na + ions and 1 mol of Cl - ions. Assuming 100% dissociation, a solution containing 1 mol of CaCl2 will produce 1 mol of Ca 2+ ions and 2 mol of Cl - ions.

Dissociation Example 1 Write the equation for the dissolution of aluminum sulfate, Al2(SO4)3, in water. How many moles of aluminum ions and sulfate ions will be produced by dissolving 1 mol of aluminum sulfate? What is the total number of moles of ions produced by dissolving 1 mol of aluminum sulfate?

Dissociation Example 2 Write the equation for the dissolution of 1 mol of ammonium chloride, NH4Cl. Determine the number of moles of each ion produced as well as the total number of ions produced.

Dissociation Example 3 Write the equation for the dissolution of 0.5 mol of barium nitrate, Ba(NO3)2. Determine the number of moles of each ion produced as well as the total number of moles of ions produced.

Dissociation Example 4 Write the equation for the dissolution of NH4NO3 in water. If 3.5 mol of NH4NO3 are dissolved, how many moles of each type of ion are produced? How many total moles of ions are produced?

Precipitation Reactions Sometimes ionic compounds are combined together and solids form as a result. These solids are called precipitates. Prior to combining these ionic compounds, it may be necessary to predict whether a precipitate will form or not. Solubility rules will be useful in the prediction of precipitates. It is difficult to write solubility rules that cover all possible conditions. YOUR SOLUBILITY RULES CHART WILL BE YOUR BEST FRIEND.

General Solubility Rules Sodium, potassium, and ammonium compounds are soluble in water. Nitrates, acetates, and chlorates are soluble. Most chlorides are soluble, except those of silver, mercury (I), and lead. Lead(II) chloride is soluble in hot water. Most sulfates are soluble, except those of barium, strontium, lead, calcium, and mercury. Most carbonates, phosphates, and silicates are insoluble, except those of sodium, potassium, and ammonium. Most sulfides are insoluble, except those of calcium, strontium, sodium, potassium, and ammonium.

Precipitation Reactions Using your solubility rules, determine whether or not each chemical compound will be a precipitate. If not, write an equation that shows its complete dissociation. Example 1 A. Ca 3 (PO 4 ) 2 B. (NH 4 ) 2 S C. Cd(NO 3 ) 2 D. PbSO 4 E. Al 2 (CO 3 ) 3

Precipitation Reactions (cont d) Sometimes, we use the solubility rules to confirm the prediction of precipitates in a doublereplacement reaction. If we were to react ammonium sulfide and cadmium nitrate, the two products that form are ammonium nitrate and cadmium sulfide. But the question must be asked, are there any precipitates formed? (NH4)2S (aq) + Cd(NO3)2 (aq) 2NH4NO3 (?) + CdS (?)

Precipitation Reactions Example 2 Identify the precipitate that forms when aqueous solutions of zinc nitrate, Zn(NO3)2, and ammonium sulfide, (NH4)2S, are combined. Write the equation for the possible double-replacement reaction including all physical states of products.

Precipitation Reactions Example 3 Will a precipitate form if solutions of potassium sulfate, K2SO4, and barium nitrate, Ba(NO3)2, are combined? If so, write the equation of the possible double-replacement reaction indicating the physical states of all products.

Precipitation Reactions Example 4 Will a precipitate form if solutions of potassium nitrate, KNO3, and magnesium sulfate, MgSO4, are combined? Write the equation of the double-replacement reaction indicating the physical states of all products formed.

Net Ionic Equations Reactions of ions in aqueous solution are typically shown using net ionic equations rather than traditional formula equations. In net ionic equations, we only use those compounds and ions that undergo a chemical change in a reaction in an aqueous solution. There is a systematic approach to arriving at a correct net ionic equation.

How to Write a Net Ionic Equation 1. Write out the formula equation with all physical states of reactants and products. 2. Write out the overall ionic equation by breaking down all reactants and products that are not insoluble into their respective ions. All precipitates are shown using (s). 3. Remove all spectator ions. These are ions that appear on both sides of the overall ionic equation. These do not contribute to the formation of the precipitate. 4. The leftovers = net ionic equation!

Net Ionic Equations Example 1 In a reaction between strontium chloride, SrCl2, and sodium sulfate, Na2SO4, a white precipitate is observed. Write the net ionic equation for the precipitate observed in the reaction above.

Net Ionic Equations For the following pairs, identify the precipitate formed, if any, and write the net ionic equation for the reaction. Example 2 A. KCl and AgNO3 B. Na2CO3 and CaCl2 C. Na2S and Fe(NO3)2 D. K2SO4 and Ba(NO3)2 E. Cu(CH3COO)2 and K2CO3