Which type of chemical reaction occurs when two reactants exchange their ions to form new compounds?

Question

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The correct answer is Option D: Double displacement reaction. A double displacement reaction is a chemical process where two ionic compounds in aqueous solution exchange their ions to form two new compounds. This process typically involves the formation of a precipitate, a gas, or water.

Step-by-Step Solution & Concepts:
A double displacement reaction is a chemical process where two ionic compounds in aqueous solution exchange their ions to form two new compounds. This process typically involves the formation of a precipitate, a gas, or water.

Working Principle: The mechanism relies on the mutual exchange of cations and anions between two ionic reactants. If one of the products is insoluble in water, it precipitates out of the solution ($AB + CD \rightarrow AD + CB$). This is often driven by the decrease in free energy through the formation of a stable ionic solid (precipitate) or a covalent molecule like water.

Key Formulas: $AgNO_{3(aq)} + NaCl_{(aq)} \rightarrow AgCl_{(s)} \downarrow + NaNO_{3(aq)}$ — Precipitation reaction; $HCl_{(aq)} + NaOH_{(aq)} \rightarrow NaCl_{(aq)} + H_{2}O_{(l)}$ — Neutralization reaction

Key Points: Commonly observed as precipitation reactions.; Usually involves aqueous solutions of ionic compounds.; The net charge of the ions remains conserved.; Driven by the formation of more stable products like precipitates or water.

Advantages: Used for the qualitative analysis of salts in chemistry laboratories.; Essential for industrial extraction of metals and chemical synthesis.

Disadvantages: Often difficult to separate the resulting products if both remain in solution.

Applications: Precipitation of silver chloride in analytical chemistry.; Neutralization of acids and bases in wastewater treatment.

Additional Info: Precipitate formation is often indicated by a downward arrow ($\downarrow$) in chemical equations.. Option A (Combination) involves joining two substances to form one. Option B (Decomposition) involves breaking one substance into two or more. Option C (Displacement) involves a more reactive element replacing a less reactive one in a single compound.

Answer: D is correct — Double displacement reactions involve the mutual exchange of ions between two reactants to form new products.

Exam Tip: Always check for the state symbols ($aq$ vs $s$) in the equation; if you see two aqueous reactants producing a solid, it is almost certainly a double displacement/precipitation reaction.

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Option A: Combination reaction

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Option B: Decomposition reaction

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Option C: Displacement reaction

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