Somebody’s got to leave the dance! • Remember how we compared double-displacement reactions to a dance in which the couples switch partners? • A double-displacement reaction only occurs if one of the new couples gets to sneak out! We’re outta here!
What’s that mean in chemistry? • The ions that form one of the possible products has to stay together in some way/ • If both products remain dissolved, nothing really happens!
Double-Displacement Reactions • The ions of two ionic compounds exchange places (often in an aqueous solution) to form two new compounds, one of which is: • A precipitate • A gas that bubbles out (effervesces) • A molecular compound, usually water
What does this mean? • When ionic substances dissolve in water, the cations and anions separate. They exist as separate ions surrounded by water. • A reaction happens if something occurs that changes this. • FORMATION OF A PRECIPITATE • FORMATION OF A GAS • FORMATION OF WATER
Formation of a Precipitate • When the cations of one reactant react with the anions of another reactant to form an insoluble or slightly soluble compound. Check the solubility chart: S = soluble I = insoluble = slightly soluble If one of the products is insoluble or slightly soluble, a reaction happens. AgNO3 (aq) + NaCl(aq) AgCl(s) + NaNO3(aq)
Formation of a Gas • If one of the products is an insoluble gas that bubbles out of the mixture, the reaction happens. FeS(s) + 2HCl (aq) H2S(g) + FeCl2 (aq)
Formation of Water • If water, a very stable molecular compound, is formed, the reaction happens. HCl (aq) + NaOH(aq) NaCl(aq) + H2O(l)
Sometimes, nothing happens • If none of the conditions described above happens, the ions remain separated, and there is no reaction. • Remember: A double-displacement happens if the products of ion exchange include one of the following: • Precipitate • Gas • Water