Water: removing dissolved solutes Chapter 12

Precipitation Reactions When two solutions are mixed, an insoluble compound sometimes forms. When two solutions are mixed, an insoluble compound sometimes forms. That is a solid forms and separates from the solution. That is a solid forms and separates from the solution. This solid is called a precipitate. This solid is called a precipitate.

Precipitation Reactions For example, when aqueous Na 2 SO 4 is added to aqueous BaCl 2, a precipitate of BaSO 4 forms. For example, when aqueous Na 2 SO 4 is added to aqueous BaCl 2, a precipitate of BaSO 4 forms. BaSO 4 is used in hospitals to contrast the intestinal tract for diagnostic X-rays. BaSO 4 is used in hospitals to contrast the intestinal tract for diagnostic X-rays. Barium chloride is a soluble compound. What is it made up of? Barium chloride is a soluble compound. What is it made up of? Sodium sulfate is also soluble, what is this made up of? Sodium sulfate is also soluble, what is this made up of?

Precipitations Reactions What happens when BaCl 2 and Na 2 SO 4 are added together? What happens when BaCl 2 and Na 2 SO 4 are added together? First the ionic compounds break up into their cations and anions. First the ionic compounds break up into their cations and anions. BaCl 2 becomes Ba 2+ and Cl - BaCl 2 becomes Ba 2+ and Cl - Na 2 SO 4 becomes Na + and SO 4 2- Na 2 SO 4 becomes Na + and SO 4 2-

What happens next? Next the products are formed by swapping the positive and negative ions of the reactants. Next the products are formed by swapping the positive and negative ions of the reactants. Ba 2+ from BaCl 2 combines with SO 4 2- from Na 2 SO 4 to form BaSO 4. Ba 2+ from BaCl 2 combines with SO 4 2- from Na 2 SO 4 to form BaSO 4. Na + and Cl - combine to form NaCl. Na + and Cl - combine to form NaCl. A direct swap has occurred between the positive and negative ions. A direct swap has occurred between the positive and negative ions.

Precipitation Reactions This reaction can be represented by the equation: This reaction can be represented by the equation: BaCl 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (s) + NaCl(aq) How did I know the it was the barium sulfate that was the precipitate?? How did I know the it was the barium sulfate that was the precipitate??

Steps for writing a precipitation reaction Step 1: Identify the reactants Step 2: Split the each reactant into its positive and negative ions. Step 3: Identify the possible products of the reactants by swapping the positive and negative ions of the reactants. Step 4: Write the equation and balance it so that equal numbers of all atoms occur on both sides. Step 5: Use the table on page 199 from your text to deduce which product is insoluble and which is not.

Worked Example 12.1a A precipitate forms when a colourless solution of lead nitrate (Pb(NO 3 ) 2 ) is added to a colourless solution of potassium iodide (KI). A dense yellow colour appears that is solid lead iodide. Write a balanced chemical equation, identifying the precipitate. A precipitate forms when a colourless solution of lead nitrate (Pb(NO 3 ) 2 ) is added to a colourless solution of potassium iodide (KI). A dense yellow colour appears that is solid lead iodide. Write a balanced chemical equation, identifying the precipitate.

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Non-Ionic compounds The steps for writing precipitation reactions also work for equations that involve polar covalent compounds that ionise such as HCl instead of an ionic compound. The steps for writing precipitation reactions also work for equations that involve polar covalent compounds that ionise such as HCl instead of an ionic compound.

Worked example 12.1 Turn to page 221 and lets go through this together. Turn to page 221 and lets go through this together.

Ionic Equations In the reaction In the reaction BaCl 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (s) + NaCl(aq) However the ionic substances dissolve in water, the ionic compounds dissociate into their ions in solution. However the ionic substances dissolve in water, the ionic compounds dissociate into their ions in solution. This means that it is more free ions that are reacting to form a precipitate, and the equation is more accurately expressed as: This means that it is more free ions that are reacting to form a precipitate, and the equation is more accurately expressed as: Ba 2+ (aq) + 2Cl - (aq) + 2Na + (aq) + SO 4 2- (aq) BaSO 4 (s) + 2Na + (aq) + 2Cl - (aq) BaSO 4 (s) + 2Na + (aq) + 2Cl - (aq)

Ionic Equations Barium sulfate is insoluble and does not dissociate into its ions. Barium sulfate is insoluble and does not dissociate into its ions. So we leave it as is. So we leave it as is.

Ionic Equations On a closer look, the only change that has occurred in this reaction is the combination of barium ions and sulfate ions to form the barium sulfate precipitate. On a closer look, the only change that has occurred in this reaction is the combination of barium ions and sulfate ions to form the barium sulfate precipitate. The sodium and chloride ions have not reacted or changed state. The sodium and chloride ions have not reacted or changed state. Ions such as these are called spectator ions and do not have to be included in the equation Ions such as these are called spectator ions and do not have to be included in the equation

Spectator Ions Therefore: Ba 2+ (aq) + 2Cl - (aq) + 2Na + (aq) + SO 4 2- (aq) BaSO 4 (s) + 2Na + (aq) + 2Cl - (aq) BaSO 4 (s) + 2Na + (aq) + 2Cl - (aq) This equation can now be simplified to: Ba 2+ (aq) + SO 4 2- (aq) BaSO 4 (s) By removing the spectator ions we have written an ionic equation. Ionic equations are simpler than the full equation and focus on the actual reaction that takes place. However full equation show you what compounds dissolve in water to provide a source of an ion.

Worked Example 12.1b Write an ionic equation for the reaction between the solutions of lead nitrate and potassium iodide. Write an ionic equation for the reaction between the solutions of lead nitrate and potassium iodide.

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