Double Replacement Reactions (Double Displacement Reactions)

General equation: AB + CD  AD + BC Double replacement General equation: AB + CD  AD + BC  Like “Wife Swap”

3 Double Replacement Reactions All double replacement reaction will either form: 1. a precipitate (solid) 2. a gas 3. water OR no reaction (NR) will occur because both products are (aq)

Driving forces Formation of… How do you recognize it? Precipitate Results in the formation of an insoluble product; use the solubility chart Gas You must memorize the combinations that decompose into gases (there are 4). You must also memorize the gases that form. Water Reaction between an acid and base that results in a salt and water. You must memorize the short list of strong acids and strong bases so you will recognize all the weak acids and bases that dissolve, but do not dissociate into ions.

#1 Precipitate reactions

DR-#1 forming precipitate A solid is formed in a DR when the product is not soluble in water 2NaOH(aq) + CuCl2(aq)  2NaCl(aq) + Cu(OH)2(s) Write the skeleton, then balance the equation, then look at the solubility chart for states of matter to see if there is a reaction making a solid, gas, water or NR (aq and aq).

Example of Double Replacement Reactions that form Solid(s) K2CrO4 + 2AgNO3  Ag2CrO4 + 2KNO3 2KI + Pb(NO3)2  PbI2 + 2KNO3

I will give you a chart of these rules on the test!

DR-#2 forming gas Gas is formed in a DR in an aqueous solution We will cover these in more depth to see the most common gases produced are: CO2 (carbon dioxide) SO2 (sulfur dioxide) NH3 (ammonia or nitrogen trihydride) H2S (dihydrogen sulfide –covalent name, or hydrogen sulfide-ionic name) HCN (hydrogen cyanide)

Gas formation (one day lesson to follow!) Certain salts react with acids to produce gaseous products Any Acid + ionic sulfur compound  H2S(g) Any Acid + ionic cyanide compound  HCN(g) Any acid + any sulfite  H2SO3  breaks down into H2O(l) + SO2(g) Any acid + any carbonate  H2CO3  breaks down into H2O(l) + CO2(g) Any ammonium salt + any base (OH)  NH4OH  breaks down into H2O(l) + NH3(g) If you get one of these as a product in your molecular equation, they immediately breakdown as above Gases do not ionize

#3 Acid-base reactions

Strong acids Strong bases HI HBr HClO4 HCl H2SO4 HNO3 NaOH KOH LiOH RbOH CsOH         Ca(OH)2 Ba(OH)2 Sr(OH)2 Strong acids/bases are strong electrolytes and will completely dissociate in water.

DR-#3 forming water Water and “salt” is formed in a DR when we have a neutralization of a strong acid and strong base (It can be a combo of weak acid/base, but we are not going to discuss these now) Ca(OH)2(aq) +2HCl(aq)  CaCl2(aq) + 2H2O(l) This is because the SA and SB completely “dissociate” or “ionize” which is not the same as dissolve! Sugar dissolves but does not ionize

HCl(l) H+(aq) + Cl−(aq)

Double Replacement Rxns Review Driving Force to keep the compound together How do you recognize it? Precipitate You must use the solubility rules. Any compound formed from two ions can be recognized as soluble (written as separate ions) or as a precipitate (written as a molecule). Gas formed You must memorize the combinations that decompose into gases (there are 4). You must also memorize the gases that form. For example, when you have H2SO3 as a product, you must know it decomposes into H2O and SO2 gas. Weak electrolyte You must memorize the short list of strong acids and strong bases so you will recognize all the weak acids and bases that dissolve, but do not dissociate into ions. The weak base ammonia, NH3, is in this category. It exits in water as NH3(aq) and only slightly forms the ions NH4+ + OH−