Chapter 8: Chemical Reaction Types

Q :In a single replacement rx if both reactants are solids is there no rx. In a double replacement rx if one of the compounds that is the reactants is a solid- not (aq) does that mean that there is no rx?

A: In a single replacement reaction,  the ionic compound on the reactant side must be in solution  for the switch to take place. Otherwise the ions are locked in a crystalline lattice and cannot move.In a double replacement reaction the two ionic compounds on the reactant side must be in solution for the same reason as stated above. -Mr. Gilliland

Q:I really don't understand the double replacement reactions.  I see that two elements or ions switch places, but I don't know why.  Could you please give an example and explain?  In class you were saying that they had to be dissolved in water and that one needed to not be dissolved or something like that.  Obviously I don't understand.  Thanks for helping.

A: When you place a soluble ionic compound in water it dissociates (breaks down into individual ions) and the ions are free to move about. You use the Solubility Chart on the back of your program sheet to determine if a compound is soluble or insoluble. If it is insoluble it will not dissolve in water and will therefore remain a visible solid. Put salt in water and it dissociates into Na+ ions and Cl- ions. It disappears because you cannot see individual ions. Put sand in water and because it is insoluble it remains a solid and sinks to be bottom of the container.

When you take a solution of sodium chloride and a solution of silver nitrate (both soluble in water) and pour them together they create the products sodium nitrate (which is soluble in water) and silver chloride (which is insoluble). Because silver chloride is insoluble it produces a white solid when the Ag+ ions and the Cl- ions come together. This solid sinks to the bottom of the container because it is more dense than water. So you take two clear solutions, pour them together and produce a white solid that sinks the bottom of the test tube. This is a double replacement reaction.

If you need more assistance there are book marked resources in NetTutor, your textbook and your computer program on Chemical Reaction types. If you still don't understand double replacement reactions, please ask in class or come in outside of class for help.

Q: Mr. G-How do you know WHEN an element in a double replacement reaction replaces the other?  I'm having trouble on #4 on the study sheet and don't understand if or why Li would replace Cu or well.. I just don't understand.  Please help me.

A: Think of these reactions as a dance with the cations (+) being guys and the anions (-) being girls. In a single replacement reaction an element attempts to replace another element that is already part of a compound. Kind of like a guy walking into a dance and seeing his girl dancing with another guy. If he is strong enough he can displace the other guy, if not he will remain single (and have no reaction). The same is true of elements - a more chemically active element will replace a less chemically active element in a compound. The reaction looks like this: A + BC  -->  AC + B You can see that cation A replaced cation B in the reaction because he is more chemically active. Anion C is a spectator ion - nothing happened to it. If  A were an anion (a non-metallic element) in this reaction it would attempt to replace the non-metal in the compound. The reaction would look like this:  A + BC --> BA + C. You have to consult the Activity Series to see if one element is more active than another. If so a reaction will take place. The element that is attempting to "butt in" must be more reactive (above on the table) than the element it is replacing. So you use the Activity Series for Single Replacement Rxs. In a double replacement reaction two cations (in two different compounds) switch places. It's like two couples dancing and the two guys switching places (or the two girls - the result would be the same). What you are looking for in the end is if one of the two products come out of solution as an insoluble solid (precipitate). For single or double replacement reactions to occur, the compounds must be in aqueous solution (dissolved in water). So in a double replacement reaction you pour two solution together and look for a solid* to form. You can tell before you do the experiment by looking at the Solubility Table. If either of the two products are insoluble you get a reaction. If both products are soluble you get no reaction. In #4 on the study sheet solutions of copper(II) nitrate and lithium hydroxide are poured together. You must determine what the two products are (by switching cations) and then, using the Solubility Table, determine if either is insoluble in water. If so place a down arrow infront of it and place a subscript (s) in the below right behind it. Hope this helps. Look in your book, computer program and/or NetTutor if you need any further help. -Mr. Gilliland

* you can also get a gas instead of a solid but at this point we are looking for solids.