What’s the difference between the two pictures above?
We see above two metals that can be exposed to water. The picture on the left is of sodium, which gives a violent reaction when it comes in contact with water. The picture on the right is of silver, a metal so unreactive with water that it can be made into drinking vessels. Both metals have a single s electron in their outer shell, so you would predict similar reactivities. However, we have a better tool that allows us to make better prediction about what will react with what.
The Activity Series
Single-replacement reactions only occur when the element that is doing the replacing is more reactive than the element that is being replaced. Therefore, it is useful to have a list of elements in order of their relative reactivities. The activity series is a list of elements in decreasing order of their reactivity. Since metals replace other metals, while nonmetals replace other nonmetals, they each have a separate activity series. Table below is an activity series of most common metals and of the halogens.
|Activity of Metals||Activity of Halogens|
K React with cold water, replacing
Al React with steam, but not cold
Zn water, replacing hydrogen.
Ni Do not react with water. React
Sn with acids, replacing hydrogen.
Hg Unreactive with water or acids.
For a single-replacement reaction, a given element is capable of replacing an element that is below it in the activity series. This can be used to predict if a reaction will occur. Suppose that small pieces of the metal nickel were placed into two separate aqueous solutions: one of iron(III) nitrate and one of lead(II) nitrate. Looking at the activity series, we see that nickel is below iron, but above lead. Therefore, the nickel metal will be capable of replacing the lead in a reaction, but will not be capable of replacing iron.
In the descriptions that accompany the activity series of metals, a given metal is also capable of undergoing the reactions described below that section. For example, lithium will react with cold water, replacing hydrogen. It will also react with steam and with acids, since that requires a lower degree of reactivity.
Sample Problem: Single-Replacement Reactions
Use the activity series to predict if the following reactions will occur. If not, write NR. If the reaction does occur, write the products of the reaction and balance the equation.
Step 1: Plan the problem.
For A, compare the placements of aluminum and zinc on the activity series. For B, compare the placements of silver and hydrogen.
Step 2: Solve.
Since aluminum is above zinc, it is capable of replacing it and a reaction will occur. The products of the reaction will be aqueous aluminum nitrate and solid zinc. Take care to write the correct formulas for the products before balancing the equation. Aluminum adopts a 3+ charge in an ionic compound, so the formula for aluminum nitrate is Al(NO3)3. The balanced equation is:
Since silver is below hydrogen, it is not capable of replacing hydrogen in a reaction with an acid.
- Metals and halogens are ranked according to their ability to displacement other metals or halogens below them in the series.
- What does the activity series tell us?
- Can a metal undergo any of the reactions listed below it in the series?
- List two metals that cobalt will displace and two that will displace it.