Precipitation Reactions & Solubility Rules Thursday, February 18 th, 2016

Definition: Precipitate: A solid that forms as the result of a chemical reaction in aqueous solution.

Practice Using Solubility Rules – Are These Compounds Soluble or Insoluble? Barium Nitrate Potassium Carbonate Sodium Sulfate Copper (II) Hydroxide Mercury (I) Chloride Ammonium Phosphate Chromium (III) Sulfide Lead (II) Sulfate * Use your solubility reference sheet

Solubility – Why does it matter? A precipitation reaction (double displacement reaction) only occurs if one of the products is insoluble.

Precipitation Reaction: A reaction in which a solid is formed from a solution is called a precipitation reaction Solid = precipitate = insoluble What is the precipitate in the reaction below? Pb(NO 3 ) 2 (aq) + 2NaI (aq) PbI 2 (s) + 2NaNO 3 (aq)

What Happens When an Ionic Compound Dissolves in Water? NaCl (s) NaCl (aq) But what does NaCl (aq) mean? NaCl particles in water ? Na and Cl atoms in water? Na + and Cl - ions in water ?

When an ionic compound dissolves in water, it breaks apart into ions: NaCl (aq)  Na+ + Cl-

Predicting whether a reaction happens What happens when solutions of BaCl 2 and Na 2 SO 4 are made? The ionic compounds break up into their cations and anions: BaCl 2 becomes Ba 2+ and Cl - Na 2 SO 4 becomes Na + and SO 4 2-

When the two solutions are mixed together, there will be 4 ions floating around: Ba 2+ Cl - Na + SO 4 2- A double replacement reaction will occur in this case because when the ions switch partners, there is a precipitate: BaCl 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (s) + NaCl(aq) How do we know that it was the barium sulfate that was the precipitate?? From the solubility rules!

Another example: Will a reaction occur between solutions of NaCl and AgNO 3 ? NaCl (aq) + AgNO 3 (aq)  ? The reactants separate into ions.

Using the solubility rules, we see that NaNO 3 is soluble, so those ions will continue to float around in the water. However, AgCl is insoluble:

Because we got a new substance (AgCl), a chemical reaction took place: NaCl (aq) + AgNO 3 (aq)  NaNO 3 (aq) + AgCl (s)

Practice: Will this reaction occur? 2 NaNO 3 (aq) + Li 2 SO 4 (aq)  Na 2 SO LiNO 3 (Think: Is Na 2 SO 4 soluble in water? Is LiNO 3 soluble in water?) Will the reaction occur?

Steps for writing a precipitation reaction Step 1: Identify the reactants Step 2: Split 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 Solubility Rules to find if one product is insoluble. If one product is insoluble, the reaction occurs. If both products are soluble, no reaction occurs.

Practice: Switch the partner ions and write balanced equations for each possible reaction. Use your Solubility Rules to see whether a precipitate forms. NaOH + CaCl 2 CuBr 2 + (NH 4 ) 2 CO 3 K 2 SO 4 + Fe(NO 3 ) 3

Practice: A precipitate forms when a colourless solution of lead II 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.

Na+ Al3+ S2– 2Ca2+ PO 4 3– 3Cl–

Equations can be divided into 3 types 1) Molecular, 2) Ionic, 3) Net ionic Here is a typical molecular equation: Cd(NO 3 ) 2 (aq) + Na 2 S(aq)  CdS(s) + 2NaNO 3 (aq)

We can write the molecular formula as an ionic equation (all compounds that are (aq) are written as ions): Cd 2+ (aq) + 2NO 3 – (aq) + 2Na + (aq) + S 2– (aq)  CdS(s) + 2Na + (aq) + 2NO 3 – (aq) To get the NET ionic equation we cancel out all terms that appear on both sides: Net: Cd 2+(aq) + S 2–(aq)  CdS (s )

NaOH(aq) + MgCl 2 (aq)  Mg(OH) 2 (s) + NaCl(aq) First write the skeleton equation 2 2 Next, balance the equation NaOH(aq) + MgCl 2 (aq)  Mg(OH) 2 (s) + NaCl(aq)

Then, Write the Ionic equation: 2Na+(aq) + 2OH-(aq) + Mg2+(aq) + 2Cl-(aq)  Mg(OH) 2 (s) + 2Na+(aq) + 2Cl-(aq) And the Net ionic equation: 2OH-(aq) + Mg2+(aq)  Mg(OH) 2 (s) What are the spectator ions?