DAT General Chemistry: Synthesis, Decomposition, and Combustion Reactions

DAT High-Yield Concept

Synthesis (Combination): Multiple reactants combine into a single product ( $\ce{A + B -> AB}$ ). Metal oxides plus water yield basic hydroxides, while nonmetal oxides plus water yield acidic oxoacids.
Decomposition: A single compound breaks down into two or more simpler species ( $\ce{AB -> A + B}$ ). Carbonates yield metal oxides plus $\ce{CO2}$ ; chlorates yield metal chlorides plus $\ce{O2}$ .
Combustion: A compound reacts rapidly with oxygen gas ( $\ce{O2}$ ) to yield energy. Complete combustion of hydrocarbons or alcohols produces $\ce{CO2}$ and $\ce{H2O}$ .
Oxygen Accounting: When balancing alcohol combustion, subtract the oxygen atom already present in the substrate before balancing $\ce{O2}$ coefficients.

Synthesis (Combination) Reactions

In a synthesis reaction, two or more substances react to form a single combined product. The general form is $\ce{A + B -> AB}$ . DAT questions often test inorganic synthesis pathways involving water.

Worked Example: Metal vs. Nonmetal Oxides with Water

Problem: Predict the product when solid sodium oxide reacts with liquid water.

Reasoning: Metal oxides act as basic anhydrides. Reacting sodium oxide with water produces sodium hydroxide: $\ce{Na2O + H2O -> 2NaOH}$ . In contrast, nonmetal oxides (acidic anhydrides) react with water to form oxoacids, such as $\ce{SO3 + H2O -> H2SO4}$ .

Common Exam Trap: Students often mistake nonmetal oxides for bases or fail to balance hydroxide ions correctly. Remember: metal oxides give basic hydroxides, nonmetal oxides give acids.

Decomposition Reactions

Decomposition is the reverse of synthesis: a single reactant breaks down into two or more products according to $\ce{AB -> A + B}$ . These reactions frequently require thermal energy (pyrolysis) or a catalyst.

Worked Example: Thermal Decomposition of Chlorates and Carbonates

Problem: Predict the products of heating solid potassium chlorate in the presence of heat.

Reasoning: Metal chlorates decompose upon heating to form metal chlorides and oxygen gas: $\ce{2KClO3 -> 2KCl + 3O2}$ . Similarly, metal carbonates undergo thermal decomposition to yield metal oxides and carbon dioxide gas, such as $\ce{CaCO3 -> CaO + CO2}$ .

Common Exam Trap: Do not confuse decomposition with single-replacement reactions (e.g., $\ce{Zn + 2HCl -> ZnCl2 + H2}$ ), which involve an uncombined element displacing another in a compound and are covered in depth in a separate lesson. Decomposition starts with exactly one reactant compound.

Mnemonic - Oxide Products in Water:
Basic = Metal oxide +

\ce{H2O}

(think Big Mac)
Acidic = Nonmetal oxide +

\ce{H2O}

(think All Night)

Combustion Reactions

A combustion reaction involves a reactant rapidly combining with molecular oxygen ( $\ce{O2}$ ) in an exothermic process. For hydrocarbons and oxygenated hydrocarbons (alcohols), complete combustion yields carbon dioxide ( $\ce{CO2}$ ) and water ( $\ce{H2O}$ ).

Visual of how the law of conservation of mass means that there must be same number of atoms of each element for the reactants and the products. (Source: Yulo1985 · CC BY-SA 4.0 · Wikimedia Commons)

Key Distinction: Complete vs. Incomplete Combustion

Complete Combustion: Occurs with stoichiometric or excess $\ce{O2}$ to produce $\ce{CO2}$ and $\ce{H2O}$ .

Incomplete Combustion: Occurs when $\ce{O2}$ is limiting, yielding carbon monoxide ( $\ce{CO}$ ) or solid elemental carbon ( $\ce{C}$ , soot) alongside water: $\ce{2CH4 + 3O2 -> 2CO + 4H2O}$ .

Worked Example: Balancing Alcohol Combustion

Problem: Write the balanced chemical equation for the complete combustion of ethanol ( $\ce{C2H5OH}$ ).

Reasoning:

Write unbalanced reaction: $\ce{C2H5OH + O2 -> CO2 + H2O}$ .
Balance carbon: 2 C on left $\rightarrow$ $\ce{2CO2}$ .
Balance hydrogen: 6 H on left ( $5+1$ ) $\rightarrow$ $\ce{3H2O}$ .
Count oxygen on product side: $2(2) + 3(1) = 7$ oxygen atoms. Ethanol supplies 1 O atom, leaving 6 O atoms needed from $\ce{O2}$ , which requires $3\ce{O2}$ .

Balanced Equation: $\ce{C2H5OH + 3O2 -> 2CO2 + 3H2O}$

Common Exam Trap: Forgetting the single oxygen atom in the ethanol formula ( $\ce{C2H5OH}$ ) leads to overcounting $\ce{O2}$ coefficients, ruining the stoichiometric ratio.

Summary Comparison of Reaction Types

Reaction Type	General Pattern	Reactant Key Feature	Product Key Feature
Synthesis	$\ce{A + B -> AB}$	Two or more substances	Exactly one single compound
Decomposition	$\ce{AB -> A + B}$	Exactly one compound	Two or more simpler substances
Combustion	$\ce{Substrate + O2 -> CO2 + H2O}$	Hydrocarbon/alcohol + $\ce{O2}$	$\ce{CO2}$ and $\ce{H2O}$ (complete)

Critical Thinking Questions

If solid sulfur trioxide ( $\ce{SO3}$ ) gas is bubbled through water, what is the pH profile of the resulting solution, and which class of reaction occurred?
How does the balanced coefficient of $\ce{O2}$ compare between the complete combustion of 1 mole of propane ( $\ce{C3H8}$ ) and 1 mole of propanol ( $\ce{C3H7OH}$ )?
When calcium carbonate ( $\ce{CaCO3}$ ) is heated strongly in an open crucible, its mass decreases. Does this violate the law of conservation of mass? Explain based on the decomposition products.

References

Sources returned by Google Search grounding during lesson verification.

4.5: Composition, Decomposition, and Combustion Reactions - Chemistry LibreTexts — chem.libretexts.org
Types of Chemical Reactions - Let's Talk Science — letstalkscience.ca

Synthesis, Decomposition, and Combustion Reactions

Summary

Practice Questions

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