What does lighting a candle, digesting food, washing your hands, and driving a car all have in common? They are all types of chemical reactions in our daily life.
A chemical reaction is a conversion of one or more elements/compounds (called reactants) into one or more elements/compounds (called products). We illustrate this reaction using a chemical equation.
There are several types of chemical reactions: each with its own unique characteristics. In this article, we'll go into detail about the different types of chemical reactions and how to identify them.
- The article is about the types of chemical reactions.
- We will learn about and see examples of the 4 main types of chemical reactions.
- We will see how to tell these types of reactions apart based on their characteristics.
- We will also learn how to write reactions based on their type.
Different Types of Chemical Reactions
There are 4 types of chemical reactions: synthesis, decomposition, combustion, and replacement.
Synthesis reactions
The first type of reaction we will cover is the synthesis reaction.
A synthesis reaction involves two elements/compounds combining to form a singular compound.
The general form of this reaction is:
$$X + Y \rightarrow XY$$
Synthesis reactions are also known as combination reactions, since species are "combining" to form a product. Here are some examples of synthesis reactions:
$$2Na + Cl_2 \rightarrow 2NaCl$$
$$2H_2 + O_2 \rightarrow 2H_2O$$
$$Li_2O + H_2O \rightarrow 2LiOH$$
The key characteristic of a synthesis reaction is that there is always only one product.
Decomposition Reactions
The second type of chemical reaction is called a decomposition reaction.
A decomposition reaction is a reaction where a compound splits into two or more elements or compounds.
The general form of this reaction is:
$$XY \rightarrow X + Y$$
Since decomposition reactions involve breaking bonds, they typically require energy to complete. Decomposition is the opposite of synthesis. Here are some examples of decomposition reactions
$$2Al_2O_3 \rightarrow 4Al + 3O_2$$
$$Ca(OH)_2 \rightarrow CaO + H_2O$$
$$H_2SO_3 \rightarrow H_2O + SO_2$$
The key characteristic of a decomposition reaction is that you start with one reactant and end with 2 or more products.
Combustion Reactions
The third type of chemical reaction is a combustion reaction.
A combustion reaction occurs when a compound or element reacts with oxygen gas to release energy (typically in the form of fire). These reactions usually involve a hydrocarbon, which is a compound that contains only C and H.
The general reaction for a hydrocarbon combustion reaction is:
$$C_xH_y + O_2 \rightarrow aCO_2 + bH_2O$$
The products of a combustion reaction are in the gaseous state, since these reactions are very hot. Since the combustion of these can release a lot of heat energy, hydrocarbons are often used as fuel. Butane, for example, is used in lighters. Here are some other examples of combustion reactions:
$$2C_6H_{14} + 19O_2 \rightarrow 12CO_2 + 14H_2O$$
$$2CH_3OH + 3O_2 \rightarrow 2CO_2 + 4H_2O$$
$$2H_2 + O_2 \rightarrow 2H_2O
(Note: this is the combustion of hydrogen gas which produces water vapor, not the synthesis of liquid water. However, this is still also a synthesis reaction!)
The key component of these reactions is oxygen gas. It wouldn't be a combustion reaction without it!
Replacement reactions (single and double)
The fourth type of chemical reaction is the replacement reaction.
A replacement reaction involves the swapping of one or more elements between compounds. A single replacement reaction is the swapping of only one element, while a double replacement reaction is the swapping of two elements. The general formula for these reactions are (in order):
$$X + YZ \rightarrow XY + Z$$
$$XY + ZA \rightarrow XA + ZY$$
Note: The order of elements will stay the same when swapped, if "X" is the first element in "XY" then it will also be the first element in "XA"
Single replacement reactions typically involve the swapping of metals. The lone metal kicks out the other metal because it is more reactive.
We use a reactivity series to see if one metal can replace another. A reactivity series is a chart that ranks metals based on their reactivity. If a metal is less reactive, then it cannot swap with the metal in the compound.
For double replacement reactions, the cations (positively charged ions) are the ones that swap. They typically occur in an aqueous solution (solids have been dissolved in water). Below are some examples of both types of replacement reactions.
$$Zn + 2HCl \rightarrow ZnCl_2 + H_2$$
$$ZnCl_2 + MgSO_4 \rightarrow ZnSO_4 + MgCl_2$$
$$Li + MgCl_2 \rightarrow LiCl_2 + Mg$$
$$2KI + Pb(NO_3)_2 \rightarrow 2KNO_3 + PbI_2$$
A special type of double replacement reaction is called a precipitate reaction. In this type of reaction, two aqueous solutions form a solid called a precipitate and another aqueous solution.
We determine which product will be a solid based on solubility rules. When certain ions are combined, they can be either insoluble or soluble in water. Insoluble compounds form a precipitate. There are a lot of solubility rules, so chemists often use handy charts to help them remember them all!
Here are some examples of a precipitate reaction:
$$Pb(NO_3)_{2\,(aq)} + 2NaI_{(aq)} \rightarrow PbI_{2\,(s)} + 2NaNO_{3\,(aq)}$$
$$Li_2CO_{3\,(aq)} + Ca(NO_3)_{2\,(aq)} + 2LiNO_{3\,(aq)} + CaCO_{3\,(s)}$$
For precipitate reactions, only one of the products is a solid, the other will be aqueous.
Types of chemical reactions chart
Now that we have covered each of the 4 types of chemical reaction, we can identify the type of chemical reaction based on key characteristics. Here is a chart breaking down what we have learned so far:
| | Types of Chemical Reactions Chart | |
|---|
| Type of Reaction | Characteristics | General form(s) | Example(s) |
| Synthesis | Two or more species combine into one species | $$X + Y \rightarrow XY$$ | $$2H_2 + O_2 \rightarrow 2H_2O$$ |
| Decomposition | One species breaks down into two or more species | $$XY \rightarrow X + Y$$ | $$Ca(OH)_2 \rightarrow CaO + H_2O$$ |
| Combustion | A species reacts with oxygen gas, which releases energy. Typically done with a hydrocarbon (CH compound) | $$C_xH_y + O_2 \rightarrow aCO_2 + bH_2O$$(for hydrocarbons only) | $$2CH_3OH + 3O_2 \rightarrow 2CO_2 + 4H_2O$$ |
| Replacement | Single: One element swaps with another element of a different compoundDouble: An element from each compound swaps between them | $$X + YZ \rightarrow XZ + Y\,\text{(Single)}$$$$XY + ZA \rightarrow XA + ZY\,\text{(Double)}$$ | $$Li + MgCl_2 \rightarrow LiCl_2 +Mg\,\text{(Single)}$$$$2KI + Pb(NO_3)_2 \rightarrow 2KNO_3 + PbI_2\,\text{(Double)}$$ |
Identify the type of chemical reaction
Let's look at some equations and see if we can determine their type. Another important thing to note is that some reactions have an overlap. A previous example is the combustion of hydrogen gas, which is also a synthesis reaction.
$$2KClO_3 \rightarrow 2KCl + 3O_2$$
Since a molecule is being broken down, this is a decomposition reaction
$$2Mg + O_2 \rightarrow 2MgO$$
This reaction is both a synthesis (because two species are being combined) and a combustion reaction (because oxygen gas is involved)
$$AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$$
Since two species are being swapped (Ag and Na), then this is a double replacement reaction
Types of chemical reactions in our daily life
All the way back in the intro, we talked about different reactions in our daily life. Now that we've covered the types of chemical reactions, we can label these common reactions:
- Lighting a candle is a combustion reaction, since striking a match causes a reaction that produces a flame. Driving a car involves several reactions, but it also involves combustion as gasoline is burned.
- Digesting food is a set of complex reactions, but is overall a decomposition reaction, as the food we eat is broken down by our stomach acids.
- Lastly, washing your hands is also a complex decomposition reaction. The soap has two "ends": hydrophobic (water-hating) and hydrophilic (water-loving) end. The dirt on our hands is "attacked" by the hydrophobic end. The broken-down particles are released and go towards the hydrophilic end. This is then washed down the drain with the water.
Writing different types of chemical Reactions
Now that we have covered the characteristics of different chemical reactions, we can begin to learn how to write chemical reactions. There are 4 main steps to writing a chemical reaction:
- Determine the type of reaction.
- Determine the reactants and products.
- Write the basic equation.
- Balance the equation.
Let's start with an example:
Draw the reaction of nickel (III) oxide breaking down:
1. First, we need to determine the type of reaction. The key phrase here is "breaking down", which means we have a decomposition reaction.
Next, we need to figure out our key players.
2. The number next to nickel, in the name "nickel (III) oxide", refers to its charge; this means that nickel is +3. Oxide (O2-) is the anion of oxygen, which has a charge of -2, so our reactant is Ni2O3.
3. In a decomposition reaction, the compound splits into 2 or more simpler substances that are more stable than the reactant. So, our compound would break down into Ni metal and O2 (O3 is very reactive/unstable, while O2 is less so).
Here is our basic equation:
$$Ni_2O_3 \rightarrow Ni + O_2$$
4. Now for our last step, we need to balance this equation. We have 2 mols of Ni and 3 mols of O on the left, while there is 1 mol and 2 mols of O on the right. We must have an even amount of O on both sides, so we first multiply Ni2O3 by 2 to get:
$$2Ni_2O_3 \rightarrow Ni + O_2$$
Now we have on our left 4 mols of Ni and 6 mols of O. To finish balancing, we can multiply Ni by 4 and O2 by 3 to get:
$$2Ni_2O_3 \rightarrow 4Ni + 3O_2$$
It's important to look at the wording of a problem, since it can help you figure out what kind of reaction is happening. Phrases like "forms" and "creates" likely mean a synthesis reaction is happening, while phrases like "burning" and "explosion" mean a combustion reaction is happening. Replacement reactions don't really have such phrases, so if there is no clear phrasing, then it's probably a replacement reaction!
Types of Chemical Reactions - Key takeaways
- There are 4 types of chemical reactions: synthesis, decomposition, combustion, and replacement
- A synthesis reaction involves two elements/compounds combining to form a singular compound.
- A decomposition reaction is a reaction where a compound splits into two or more elements or compounds.
- A combustion reaction occurs when a compound or element reacts with oxygen gas to release energy (typically in the form of fire). These reactions typically involve a hydrocarbon, which is a compound that contains C and H.
- A replacement reaction involves the swapping of one or more elements between compounds. A single replacement reaction is the swapping of only one element, while a double replacement reaction is the swapping of two elements.
- A special type of double replacement reaction is called a precipitate reaction. In this type of reaction, two aqueous solutions (solids dissolved in water), form a solid, called a precipitate, and another aqueous solution.
- By understanding the different types of reactions, we can write chemical equations when given a description of the reaction.
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