Classify reactions as synthesis, decomposition, single replacement, double replacement, or combustion, and predict the products from the reactants (MA STE HS-PS1-2, predicting reaction outcomes).

What this topic is asking

Standard HS-PS1-2 asks you to explain and predict the outcome of a reaction. A powerful shortcut is to recognize that most reactions fall into a few patterns. If you can spot the pattern, you can often predict the products without memorizing the reaction. Massachusetts high school chemistry expects you to classify a reaction into one of five common types and use that classification to predict what forms.

Synthesis and decomposition

Synthesis: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$, where magnesium and oxygen combine into magnesium oxide. The clue is many reactants, one product.

Decomposition: $2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2$ when water is electrolyzed, or $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ when limestone is heated. The clue is one reactant, many products.

Single and double replacement

Single replacement follows $\text{A} + \text{BC} \rightarrow \text{AC} + \text{B}$. A more reactive metal pushes out a less reactive one: $\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}$, because zinc is above copper in the activity series (a ranking of metals by reactivity). If the free element is less reactive than the one already in the compound, no reaction occurs.

Double replacement follows $\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$. The cations swap anions: $\text{AgNO}_3 + \text{NaCl} \rightarrow \text{AgCl} + \text{NaNO}_3$, where solid silver chloride precipitates. A double replacement "goes" only if one product leaves the solution as a precipitate, escapes as a gas, or forms water (a neutralisation, covered in neutralization and titration).

Combustion

For any hydrocarbon: fuel plus oxygen gives carbon dioxide plus water, for example $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$. Because combustion releases energy, it is also exothermic, a link to energy changes in chemical reactions.

Try this

Q1. Classify $2\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2$. [1]

• Cue. Decomposition (one compound splits into simpler substances).

Q2. Will silver react with zinc nitrate solution? Explain. [2]

• Cue. No; silver is less reactive (lower in the activity series) than zinc, so it cannot displace zinc from the compound.