Chemical Reactions: Replacement
Getting Started
We now have two kinds of reactions we can review, synthesis--putting two or more things together and analysis--taking something apart. Both have shown an energy transfer. The first showed an energy requirement to start the reaction and the second showed an energy output as a a result of the reaction. There are also reaction that absorb energy called endothermic reactions. See Endothermic reaction.
Another type of reaction is called a replacement type. In this example we will see that two molecules trade elements to form different compounds.
A Replacement Reaction Exploration
Copper Chloride Magnesium wire
CuCl2 Mg
1) Allow students to examine a strip of magnesium wire. Describe the characteristics
2) Give students some copper chloride to examine. Do not touch or taste. Describe the characteristics.
3) Have students place both of these materials in a plastic baggie. They can mix or jostle the ingredients. Do they change? Is a chemical reaction taking places at all?
4) Add water to the baggie. 50 ml should be plenty.
5) Observe. Feel the baggie. Watch the reaction.
A careful examination of the products of the reaction will yield a white powder that dissolves in water and copper that falls to the bottom.
Copper Magnesium Chloride
Cu MgCl2
Symbolically we have:
CuCl2 + Mg (s) --> MgCl2 + Cu (s)
In the replacement reaction, magnesium replaces copper. Magnesium is more active than copper as a metal so binds with chlorine instead of the copper. The copper falls out of solution and we see it as a greenish solid in the bottom of our reaction cup. Magnesium chloride is soluble, so the liquid will change from bluish green to clear.
Here is a video that helps show how a replacement reaction works:
Hunting the Elements Video. This is a larger view of the nature of elements and their characteristics.