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Precipitation Reactions

Precipitation Reactions. Result when two soluble compounds mix and an insoluble (solid) compound is produced. AgNO 3 (aq) + NaCl (aq) . AgCl (s) + NaNO 3 (aq). This compound is soluble. The (aq) designates that it dissolves in water.

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Precipitation Reactions

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  1. Precipitation Reactions • Result when two soluble compounds mix and an insoluble (solid) compound is produced. AgNO3 (aq) + NaCl (aq) AgCl (s) + NaNO3 (aq) This compound is soluble. The (aq) designates that it dissolves in water. This is the insoluble compound (precipitate). Precipitation is established by looking at the solubility table. Insoluble compounds are designated (s) for solid.

  2. Precipitation Reactions • We predict the products by matching the cation of one reactant to the anion in the other reactant, and vice versa. AgNO3 (aq) + NaCl (aq) silver cation + chloride anion AgCl (s) + NaNO3 (aq) nitrate anion + sodium cation

  3. Writing Formula Unit Equations • Step 1. Check to see if a precipitate forms using the solubility table. this is insoluble Ca(NO3)2 (aq) + NaF (aq) this is soluble

  4. Writing Formula Unit Equations • Step 2. Write the products with cations and anions in a 1:1 ratio. this is insoluble Ca(NO3)2 (aq) + NaF (aq) CaF (s) + NaNO3 (aq) this is soluble One sodium to one nitrate ion. Nitrate is a polyatomic ion; we do not alter it’s formula. The (aq) is used for the soluble compounds. One calcium ion to one fluoride ion. The (s) is added because this is the insoluble compound.

  5. Writing Formula Unit Equations • Step 3. Determine the charges of each ion on the product side, and balance the products using subscripts. 2 Ca(NO3)2 (aq) + NaF (aq) CaF (s) + NaNO3 (aq) 2 Since these charges do balance out, no subscript is needed. Since these charges did not balance out, a subscript of 2 was added to fluoride so the total charges would add to zero. -1 -1 +1 +2

  6. Writing Formula Unit Equations • Step 4. Balance the equation using coefficients. 2 Ca(NO3)2 (aq) + NaF (aq) 2 CaF (s) + NaNO3 (aq) 2 2 -1 -1 +1 These are added to balance the equation. +2

  7. Writing Total Ionic Equations • The total ionic equations represents what each compound looks like when dissolved in water. • Strong electrolytes break apart 100% in water to form ions, so these are shown as ions in the equation. • NaCl (aq)  Na+ (aq) + Cl- (aq) • Weak electrolytes break apart ~5% in water, so the predominant form of these species is the neutral molecule. • HF (aq) is shown as HF (aq) • Non-electrolytes do not break apart at all. Like weak electrolytes, they are shown as is. H2O (l) is shown as H2O (l)

  8. Writing Total Ionic Equations • The bottom line: Separate into ions everything with an (aq) except weak acids. This is a solid; it will not break apart. Ca(NO3)2 (aq) + 2 NaF (aq) CaF2 (s) + 2 NaNO3 (aq) These are soluble and will split into ions.

  9. Writing Total Ionic Equations • The bottom line: Separate into ions everything with an (aq) except weak acids. This subscripts will become a coefficient. It is not part of the polyatomic ion’s formula. Ca(NO3)2 (aq) + 2 NaF (aq) CaF2 (s) + 2 NaNO3 (aq) These subscripts will not be moved because they are part of the polyatomic ion.

  10. Writing Total Ionic Equations • The bottom line: Separate into ions everything with an (aq) except weak acids. Ca(NO3)2 (aq) + 2 NaF (aq) CaF2 (s) + 2 NaNO3 (aq) The total ionic equation is: Ca2+(aq) + 2 NO3-(aq) + 2 Na+(aq) + 2 F-(aq) CaF2(s) + 2 Na+(aq) + 2 NO3-(aq)

  11. Writing Net Ionic Equations • Eliminate the spectator ions from the total ionic equation. The spectator ions are the species that look identical in every respect on both sides of the equation. Ca2+(aq) + 2 NO3-(aq) + 2 Na+(aq) + 2 F-(aq) CaF2(s) + 2 Na+(aq) + 2 NO3-(aq) The net ionic equation is: Ca2+(aq)+ 2 F-(aq) CaF2(s)

  12. Summary of a precipitation reaction • The formula unit equation • The total ionic equation • The net ionic equation Ca(NO3)2 (aq) + 2 NaF (aq) CaF2 (s) + 2 NaNO3 (aq) Ca2+(aq) + 2 NO3-(aq) + 2 Na+(aq) + 2 F-(aq) CaF2(s) + 2 Na+(aq) + 2 NO3-(aq) Ca2+(aq)+ 2 F-(aq) CaF2(s)

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