In this lesson, you learned that in a single replacement reaction the atoms of one element replace the atoms of another element in a compound. However, an element will not always replace another element in a compound. This is because elements have different reactivities. Recall that reactivity is a chemical property that describes how readily a substance chemically combines with another substance. Generally, a more reactive element replaces an element that is less reactive, and the less reactive element is set free from the compound.
Sodium metal has a violent reaction when it contacts water.
The reaction is represented by this equation:
\(2 \text{Na}_{(\text{s})} + 2\text{H}_{2}\text{O} _{(\text{l})} → 2 \text{NaOH}_{(\text{s})} + \text{H}_{2\text{(g)}}\)
Writing the water as HOH helps to visualize the single replacement reaction:
\(2 \text{Na}_{(\text{s})} + 2 \text{HOH}_{(\text{l})} → 2 \text{NaOH}_{(\text{s})} + \text{H}_{2\text{(g)}}\)
Because the sodium is more reactive than the hydrogen, it can replace the hydrogen in the water to form the new compound, \(\text{NaOH}\).
Silver metal is so unreactive with water that it can be made into drinking vessels.
In this case, no reaction takes place:
\(\text{Ag}_{(\text{s})} + \text{H}_{2}\text{O}_{(\text{l})} → \text{no reaction}\)
The letters NR (no reaction) are commonly used to indicate that no reaction occurs:
\(\text{Ag}_{(\text{s})} + \text{H}_{2}\text{O}_{(\text{l})} → \text{NR}\)
Because the silver is not more reactive than hydrogen, it cannot replace the hydrogen in water to produce a new compound. Therefore, no reaction takes place.
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 that ranks the elements in order of their relative reactivity.
An activity series is a list of elements in decreasing order of their reactivity. It is useful for determining if a reaction will take place. A partial activity series of some common metals and hydrogen is shown here.
| Potassium \((\text{K})\) | Most Reactive |
| Sodium \(\text{(Na)}\) |
|
| Calcium \(\text{(Ca)}\) | |
| Magnesium \(\text{(Mg)}\) | |
| Aluminum \(\text{(Al)}\) | |
| Carbon \(\text{(C)}\) | |
| Zinc \(\text{(Zn)}\) | |
| Iron \(\text{(Fe)}\) | |
| Tin \(\text{(Sn)}\) | |
| Lead \(\text{(Pb)}\) | |
| Hydrogen \(\text{(H)}\) | |
| Copper \(\text{(Cu)}\) | |
| Silver \(\text{(Ag)}\) | |
| Gold \(\text{(Au)}\) | Least Reactive |
The mnemonic device “Please Stop Calling Me A Careless Zebra; Instead Try Learning How Copper Saves Gold” can help you remember the reactivity of some common metals.
| Potassium | Please |
|---|---|
| Sodium | Stop |
| Calcium | Calling |
| Magnesium | Me |
| Aluminum | A |
| Carbon | Careless |
| Zinc | Zebra |
| Iron | Instead |
| Tin | Try |
| Lead | Learning |
| Hydrogen | How |
| Copper | Copper |
| Silver | Saves |
| Gold | Gold |
For Example
Examine the example on each slide to learn how to use the activity series to determine if a single replacement reaction will take place.
Question
Which element, potassium \(\text{(K)}\) or magnesium \(\text{(Mg)}\), would replace the other in a reaction?
Potassium is above magnesium in the activity series because it is more reactive than magnesium; therefore, potassium replaces magnesium in a reaction.
