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Chapter 8 Chemical reactions 8.3 Reactions in aqueous solutions.

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Presentation on theme: "Chapter 8 Chemical reactions 8.3 Reactions in aqueous solutions."— Presentation transcript:

1 Chapter 8 Chemical reactions 8.3 Reactions in aqueous solutions

2 Aqueous solutions Earlier in the chapter we learned to write balanced equations of ionic compounds in aqueous solutions as if the formula units were intact. Here’s an example: AgNO 3(aq) + NaCl (aq) AgCl (s) + NaNO 3(aq)

3 However, Most ionic compounds dissociate into cations and anions when they are dissolved in water. So, the previous equation might be more correctly written as: Ag + (aq) + NO 3 - (aq) + Na + (aq) + Cl - (aq) AgCl (s) + Na + (aq) + NO 3 - (aq)

4 Now… We can cancel terms that appear on both sides of the equation. These are called spectator ions. Ag + (aq) + NO 3 - (aq) + Na + (aq) + Cl - (aq) AgCl (s) + Na + (aq) + NO 3 - (aq) This gives us a net ionic equation: Ag + (aq) + Cl - (aq) AgCl (s)

5 Sometimes When you create a net ionic equation from a balanced chemical equation, by eliminating spectator ions, the charges of the remaining ions do not balance. This needs to get fixed. Pb (s) + AgNO 3(aq) Ag (s) + Pb(NO 3 ) 2(aq)

6 Sometimes When you create a net ionic equation from a balanced chemical equation, by eliminating spectator ions, the charges of the remaining ions do not balance. Here’s the ionic equation: the charges are balanced Pb (s) + Ag + (aq) +NO3 - (aq) Ag (s) + Pb -2 (aq) +2NO 3 - (aq)

7 Sometimes When you create a net ionic equation from a balanced chemical equation, by eliminating spectator ions, the charges of the remaining ions do not balance. Here’s the spectator ions removed: the charges are now unbalanced Pb (s) + Ag + (aq) +NO3 - (aq) Ag (s) + Pb -2 (aq) +2NO 3 - (aq)

8 Sometimes When you create a net ionic equation from a balanced chemical equation, by eliminating spectator ions, the charges of the remaining ions do not balance. We have to add another Ag + to the reactant side of the equation: Pb (s) + Ag + (aq) Ag (s) + Pb +2 (aq) (unbalanced)

9 Sometimes When you create a net ionic equation from a balanced chemical equation, by eliminating spectator ions, the charges of the remaining ions do not balance. All better! Pb (s) + 2Ag + (aq) Ag (s) + Pb +2 (aq) (balanced)

10 Write the net ionic equation for HCl (aq) + ZnS (aq) H2S (g) + ZnCl (aq) Break the aqueous compounds into ions Eliminate the spectator ions Balance the charges

11 Write the net ionic equation for HCl (aq) + ZnS (aq) H2S (g) + ZnCl (aq) Break the aqueous compounds into ions H + (aq) + Cl - (aq) + Zn +2 (aq) + S -2 (aq) H 2 S (g) + Zn +2 + Cl - (aq)

12 Write the net ionic equation for HCl (aq) + ZnS (aq) H2S (g) + ZnCl (aq) Eliminate the spectator ions H + (aq) + Cl - (aq) + Zn +2 (aq) + S -2 (aq) H 2 S (g) + Zn +2 + Cl - (aq)

13 Write the net ionic equation for HCl (aq) + ZnS (aq) H2S (g) + ZnCl (aq) Balance the charges 2H + (aq) + S -2 (aq) H 2 S (g)

14 Predicting precipitates Here we have to do a bit of memorizing. It is always easier to have a list of the ionic compounds that are soluble or insoluble than to try to remember all the possibilities. A list of these will be supplied if we are asked to do a problem involving solubilities on a quiz or test.

15 Predicting precipitates

16 Should a precipitate form from a mixture of Na 2 SO 4(aq) and Ba(NO 3 ) 2(aq)? Break the aqueous compounds into ions Eliminate the spectator ions Balance the charges

17 Should a precipitate form from a mixture of Na 2 SO 4(aq) and Ba(NO 3 ) 2(aq)? Break the aqueous compounds into ions 2Na + (aq) + SO 4 -2 (aq) + Ba +2 (aq) + 2NO 3 - (aq)

18 Should a precipitate form from a mixture of Na 2 SO 4(aq) and Ba(NO 3 ) 2(aq)? Eliminate the spectator ions 2Na + (aq) + SO 4 -2 (aq) + Ba +2 (aq) + 2NO 3 - (aq) In this case, we have the possibilities of NaNO 3 or BaSO 4 precipitating. The solubility chart tells us all nitrates are soluble and that all sulfates are soluble except in the case of barium. So our spectator ions are Na and NO 3

19 Should a precipitate form from a mixture of Na 2 SO 4(aq) and Ba(NO 3 ) 2(aq)? Balance the charges (they are already balanced) Ba +2 (aq) + SO 4 -2 (aq) BaSO 4(s)


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