Engage
Earlier in this course, you learned that a chemical reaction occurs when a substance reacts and forms one or more new substances. We can represent a chemical reaction by a chemical equation that shows the chemical formulas for the reactants (starting materials) and the products (ending materials).
Recall that a chemical property is the ability of a substance to undergo a chemical reaction. One of the chemical properties of matter is reactivity. Reactivity is the property that describes how readily a substance chemically combines with another substance.
One chemical reaction is that of metals in water. Some metals when put in water have no reaction, while others have quite a violent reaction. The following tabs show the reactions of two alkali metals with water. Recall that the alkali metals are located in the first column of the periodic table.
Group 1A of the periodic table containing the alkali metals--from top to bottom: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).
This picture shows sodium. Notice that it has a dull film on the outside. This metal is shiny when cut, but then a duller film forms on the surface.
Sodium will react with water as shown in the following video.
Alan Kuehner, CC BY 3.0, via Wikimedia Commons
When sodium is put in water, it produces heat, releases sparks, and burns with a yellow flame. The equation for this reaction is
2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2 (g).
This picture shows potassium. Notice that it has a bluish film on the outside. This metal is shiny when cut, but then a bluish film forms quickly on the surface.
Potassium will react with water as shown in the following video.
Ozone aurora / Philip Evans, CC BY-SA 3.0, via Wikimedia Commons
Potassium also produces heat and releases sparks. In fact, its reaction is even more vigorous than that of sodium. The equation for this reaction is:
2 K(s) + 2 H2O(l) → 2 KOH(aq) + H2 (g)
In this lesson you will construct an explanation as to why sodium and potassium react this way, and you will use that information to predict what other elements will do when they react with water.
Question
Recall that chemical reactions can be classified into five types based on how the reactants interact to form products. Look at the equation for the reaction of sodium with water and the equation for the reaction of potassium with water again. Identify the reactions as synthesis (combination), decomposition, single displacement (replacement), double displacement (replacement), or combustion.
The reactions are single replacement reactions.
In a single replacement reaction, one element takes the place of another element in a compound. Single replacement reactions occur between a pure element and a compound, and the products of these types of reactions are a different pure element and a different compound.
Reflection
Why do you think sodium and potassium react the way they do?
