Types of chemical reactions — combination, decomposition, displacement, double displacement, redox

Question

Classify the following reactions and give one example of each: combination, decomposition, displacement, double displacement, and redox reactions. How do you identify a redox reaction?

(CBSE 2023, similar pattern)

Solution — Step by Step

Two or more substances combine to form a single product.

2Mg + O_2 \rightarrow 2MgO

Pattern: $A + B \rightarrow AB$

Magnesium burns in air with a bright white flame to form magnesium oxide. This is also an exothermic reaction.

A single compound breaks down into two or more simpler substances.

2FeSO_4 \xrightarrow{\text{heat}} Fe_2O_3 + SO_2 + SO_3

Pattern: $AB \rightarrow A + B$

Decomposition can be triggered by heat (thermal), light (photolytic), or electricity (electrolytic).

A more reactive element displaces a less reactive one from its compound.

Zn + CuSO_4 \rightarrow ZnSO_4 + Cu

Zinc is more reactive than copper (check the reactivity series), so it displaces copper from copper sulphate solution. The blue solution turns colourless.

Two compounds exchange their ions to form two new compounds.

Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 \downarrow + 2NaCl

The $\downarrow$ arrow shows that $BaSO_4$ is an insoluble precipitate. Most double displacement reactions form a precipitate, gas, or water.

Oxidation = loss of electrons (or gain of oxygen). Reduction = gain of electrons (or loss of oxygen). Both happen simultaneously.

CuO + H_2 \rightarrow Cu + H_2O

Here, CuO is reduced (loses oxygen, Cu goes from +2 to 0) and $H_2$ is oxidised (gains oxygen, H goes from 0 to +1). CuO is the oxidising agent; $H_2$ is the reducing agent.

Why This Works

These five types cover almost every reaction you will see in Class 10 chemistry. The classification is based on what happens to the reactants:

Combination and decomposition are opposites (joining vs splitting)
Displacement and double displacement differ in how many exchanges happen
Redox is identified by tracking electron transfer or oxidation states

Many reactions fit multiple categories. For example, $2Mg + O_2 \rightarrow 2MgO$ is both a combination reaction AND a redox reaction (Mg is oxidised, O is reduced).

Alternative Method

To quickly identify redox reactions, track oxidation numbers. If any element’s oxidation number changes, it is a redox reaction. For displacement reactions: the free element always changes oxidation state, so all displacement reactions are also redox reactions.

CBSE loves asking: “Is this reaction endothermic or exothermic?” Quick rules — combination reactions are usually exothermic, decomposition reactions are usually endothermic. If a reaction needs continuous heating, it is endothermic.

Common Mistake

Students confuse displacement with double displacement. In single displacement, a free element replaces another ( $Zn + CuSO_4$ ) — one element is alone. In double displacement, two compounds swap ions ( $Na_2SO_4 + BaCl_2$ ) — there is no free element. Look for whether a free element is present among the reactants.

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

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