Get the definition of a single replacement reaction or single displacement reaction. Get examples of single replacement reactions and learn how to use the metal reactivity series to predict whether a reaction will occur and the products.
Single Replacement Reaction Definition
A single replacement reaction is a chemical reaction where one element replaces another in a compound. It is also known as a single displacement reaction. The general form of a single replacement reaction chemical equation is:
A + BC → B + AC
Single replacement reactions occur when A is more reactive than B or product AC is more stable than BC. A and B can be either two metals (including hydrogen; C is an anion) or else two halogens (C is a cation). If BC and AC are in aqueous solutions, C acts as a spectator ion.
Single Replacement Reaction Examples
There are two different scenarios for single replacement reactions. In one form of the reaction, one cation replaces the other. In the other form of the reaction, one anion replaces the other.
Cation Replacement Examples
Usually the cation is a metal, but it doesn’t have to be. Here are examples of single replacement reactions involving the cations:
- Zn(s) + 2 HCl(aq) → ZnCl2(aq) + H2(g)
- 2 K + 2H2O → 2 KOH + H2 (note how the anion is written differently because we don’t write water as HOH)
- Cu + 2 AgNO3 → 2 Ag + Cu(NO3)2
- Ca + 2 H2O → Ca(OH)2 + H2
But, if the reactant in element form is not more reactive than the other cation, no reaction occurs. In some cases, the reverse reaction is favored, but the forward reaction is not.
Anion Replacement Examples
Instead of cation replacement, a single replacement reaction may involve the anion. In practice, the only anions participating in single replacement reactions are the halogens (fluorine, chlorine, bromine, iodine). The general form of the reaction is:
A + BC → BA + C
In addition to being a single replacement reaction, this is also an oxidation-reduction or redox reaction. Examples of anion replacement reactions include:
- Cl2 + 2 NaBr → 2 NaCl + Br2
- Br2 + 2 KI → 2 KBr + I2
Again, if the elemental reactant is not more reactive than the other anion, no reaction will occur. For example, the following reaction does not occur:
I2 + 2 KBr → no reaction
How to Determine the Products of Single Replacement Reactions
The product of a single replacement reaction is easy enough to predict. If the pure element is a halogen, it takes the place of the other halogen in the compound. All halogens have the same oxidation state (-1), so it’s a simple substitution.
But, if the elemental reactant is not a halogen, it replaces the cation in the compound. The two cations don’t always have the same oxidation state. You may need to balance the charge of the cation and anion and then balance the chemical equation to get what you need.
For example, consider the reaction:
Mg(s) + AlPO4(aq) →
Magnesium is more reactive than aluminum, so the replacement is favorable. However, the aluminum cation has a charge of +3 (balancing the PO43- anion), while the magnesium ion (as a rare earth metal) has a charge of +2.
First, find the formula of the product by balancing the cation and anion charges to get:
Mg(s) + AlPO4(aq) → Al(s) + Mg3(PO4)2(aq)
Then, adjust the coefficients in front of the reactants and products to balance the chemical equation:
3 Mg(s) + 2 AlPO4(aq) → 2 Al(s) + Mg3(PO4)2(aq)
Using the Reactivity Series to Predict Whether a Reaction Will Occur
Use the reactivity series to determine whether a single replacement reaction will occur.
For anion replacement, the reactivity series for the halogens is:
Most reactive F2 > Cl2 > Br2 > I2 Least Reactive
This is the order of the halogens going down their group on the periodic table, so it’s easy to remember. The higher the halogen is on the periodic table, the more reactive it is. So, Cl2 replaces I2 in a single replacement reaction, but it won’t react if the anion has fluoride ions.
The reactivity series for cations is longer and not as obvious. The least reactive metals won’t react with the H+(aq) ion, while the most reactive metals not only react with the ion, but can even pull the hydrogen ion off liquid water. Elements in-between can react with the H+(aq) ion and sometimes pull the hydrogen off water vapor.
But, for a general chemistry class, you mainly need to know which metals can replace each other and which ones can’t. For example, zinc (Zn) can replace tin (Sn) as the cation in a compound, but it can’t replace potassium (K). In general, alkali metals are the most reactive, followed by alkaline earth metals. Noble metals, in contrast, are relatively unreactive.
- Barke, Hans-Dieter; Hazari, Al; Yitbarek, Sileshi (2008). Misconceptions in Chemistry Addressing Perceptions in Chemical Education. Berlin: Springer. ISBN 3540709894.
- Brown, Theodore; et al. (2017) Chemistry: The Central Science (14th ed.). Pearson. ISBN 9780134414232
- Myers, Richard (2009). The Basics of Chemistry. Greenwood Publishing Group. ISBN 978-0-313-31664-7.