Lesson Plan - Get It!
Why do you shuffle a deck of cards between rounds of play?
Shuffling the cards allows you to mix them up so they are in a new random order for the next hand.
Chemical elements shuffle during chemical reactions called "double replacement reactions," also known as "double displacement reactions."
In a double replacement reaction, the cations and anions in each compound switch. This creates two new substances. The pattern for a double replacement reaction is AB + XY —> AY + XB. Notice that the first element in the reactant compound stays the same in the product compounds. Cations must always be written first in the chemical formula.
In the reaction above, the two reactant compounds are shuffled into two new substances as products. Sodium and copper are metals, so they need to written first in the compound formulas.
- What do you notice about the Cu(OH)2 substance that forms after the reaction?
It appears to be a solid, sitting at the bottom of the container. In a double replacement reaction, a precipitate must form. A precipitate is a solid that must fall out of reaction, or no reaction has occurred.
Precipitates can be white or colored, but they usually look like small particles forming as the two solutions mix.
- How do you know if a solid will form during a reaction?
You use a chart called the "table of solubility rules" to determine which compounds form solids during reaction (see below*). Solubility is defined as whether a substance will dissolve, usually in the context of water. If a compound is soluble, it will dissolve and not precipitate out of solution. If a compound is insoluble, it will precipitate out.
For a double replacement reaction, you must predict the products formed during the reaction first. Then, use the solubility rules to determine which one of the two products formed will precipitate out of solution. Once you have identified the correct substance, you put a special notation, (ppt), after the compound in the chemical equation. This notation communicates that the compound forms a solid during reaction. It stands for "precipitate."
An example of a double replacement reaction is silver nitrate reacting with potassium chloride:
AgNO3 + KCl —> AgCl (ppt) + KNO3
Notice that the compounds actually swap partners. Silver (Ag) is bonded with nitrate before the reaction, but chlorine after the reaction. Potassium bonds with the nitrate ion, creating potassium nitrate. When you review the solubility rules outlined below, you will find that silver chloride forms a solid in reaction, or a precipitate.
*Reference the Table of solubility from periodic-table-of-elements.org (Requires Adobe Flash Player). Notice that most of the rules are based on the anions, or negatively charged particles, in the compound.
- Do you think that NaNO3 would be soluble or insoluble?
It is soluble, because nitrates (NO3) are soluble.
It is insoluble, because it is calcium carbonate. It takes a lot of practice to feel comfortable with the solubility rules. You will practice identifying precipitates in the Got It? section.
Double replacement reactions are pretty complex. Watch Chemistry Lesson: Double Displacement Reactions, from GetChemistryHelp, to practice several reactions. Keep notes throughout the video, working each example as the instructor works it. This will help you predict products later in the lesson:
- Summarize what you have learned about double replacement reactions in a visual image.
- Use symbols or pictures to communicate what a double replacement is and how they run.
Double replacement reactions are some of the most important reactions in chemistry because they are responsible for creating new substances and precipitates. A solid precipitate must form in order for a double replacement reaction to run.
In the Got It? section, you will review the formation of precipitates using an interactive.