Types of Chemical Reactions

By the end of this lesson, you will be able to classify a reaction from its pattern, predict the products of each type, use an activity series and a solubility table, and recognize the same reaction under two different names.

Five Patterns

Classification is worth doing because it lets you predict products for a reaction you have never seen.

$$\textbf{synthesis} \qquad \mathrm{A} + \mathrm{B} \rightarrow \mathrm{AB}$$

$$\textbf{decomposition} \qquad \mathrm{AB} \rightarrow \mathrm{A} + \mathrm{B}$$

$$\textbf{single replacement} \qquad \mathrm{A} + \mathrm{BC} \rightarrow \mathrm{AC} + \mathrm{B}$$

$$\textbf{double replacement} \qquad \mathrm{AB} + \mathrm{CD} \rightarrow \mathrm{AD} + \mathrm{CB}$$

$$\textbf{combustion} \qquad \text{fuel} + \mathrm{O_2} \rightarrow \mathrm{CO_2} + \mathrm{H_2O}$$

Count the reactants and products first, and most classifications fall out immediately. Two in and one out is synthesis; one in and two or more out is decomposition.

The categories overlap, and that is not a flaw. Burning magnesium, $2\mathrm{Mg} + \mathrm{O_2} \rightarrow 2\mathrm{MgO}$, is both a synthesis and a combustion. Both names are correct, and they emphasize different things: that two substances combined, and that something burned in oxygen.

Sort It: Which Type

Synthesis and Decomposition

Synthesis combines two or more things into one, and the product's formula comes from balancing the charges, exactly as in the ionic bonding lesson.

$$2\mathrm{Na} + \mathrm{Cl_2} \rightarrow 2\mathrm{NaCl}$$

$$\mathrm{CaO} + \mathrm{H_2O} \rightarrow \mathrm{Ca(OH)_2}$$

Decomposition is the reverse, and it usually needs energy supplied as heat, light or electricity, because you are undoing something stable.

$$2\mathrm{H_2O} \xrightarrow{\text{electricity}} 2\mathrm{H_2} + \mathrm{O_2}$$

$$\mathrm{CaCO_3} \xrightarrow{\text{heat}} \mathrm{CaO} + \mathrm{CO_2}$$

Three decomposition patterns are worth recognizing:

• Metal carbonates give the metal oxide and $\mathrm{CO_2}$. That reaction, run on limestone, is how cement is made and is a major source of industrial $\mathrm{CO_2}$
• Metal hydroxides give the metal oxide and water
• Binary compounds give their elements

Those three cover most of what you will be asked to predict.

Single Replacement Needs an Activity Series

$$\mathrm{Zn} + \mathrm{CuSO_4} \rightarrow \mathrm{ZnSO_4} + \mathrm{Cu}$$

Zinc replaces copper. The reverse does not happen at all.

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