DAT General Chemistry: Replacement Reactions and Predicting Products

DAT High-Yield Concept

Single-Replacement Reactions: A free element displaces another element from a compound only if the free element is higher in the activity series (redox process).
Double-Replacement Reactions: Metathesis involves partner-swapping between aqueous ionic compounds and requires a thermodynamic driving force: formation of an insoluble precipitate, weak electrolyte ( $\ce{H2O}$ ), or gas ( $\ce{CO2}$ , $\ce{SO2}$ , $\ce{H2S}$ ).
Solubility Essentials: Group 1 metals, $\ce{NH4^+}$ , $\ce{NO3^-}$ , and $\ce{C2H3O2^-}$ salts are always soluble. Halides ( $\ce{Cl^-}$ , $\ce{Br^-}$ , $\ce{I^-}$ ) are insoluble with $\ce{Ag^+}$ , $\ce{Pb^{2+}}$ , and $\ce{Hg2^{2+}}$ .
Common DAT Trap: If no driving force is present in a metathesis reaction or if an uncombined metal is lower in activity than the metal cation in solution, record "No Reaction".

Overview of Reaction Types

Chemical transformations reorganize chemical bonds to yield distinct products. Standard reaction classes include synthesis, single-replacement, and double-replacement (metathesis) reactions. (Decomposition reactions, where a single substance breaks down into simpler products, are covered in depth in a dedicated lesson.)

Single-Replacement Reactions

Single-Replacement Reaction: A reaction in which an uncombined element displaces another element from an aqueous compound:

\ce{A + BC -> AC + B}

These reactions are oxidation-reduction (redox) processes governed by oxidation potentials.

To predict whether a single-replacement reaction proceeds, compare the free element to the ion it would replace using the activity series. An element higher on the series readily oxidizes and displaces any ion below it. Halogen activity follows periodic group trends ( $\ce{F2}$ > $\ce{Cl2}$ > $\ce{Br2}$ > $\ce{I2}$ ).

Worked Example: Metal and Halogen Displacements

Problem: Predict the products and write balanced chemical equations when: (1) solid zinc is added to aqueous copper(II) sulfate, and (2) chlorine gas is bubbled into aqueous sodium bromide.

Step-by-Step Solution:

Metal Displacement: Zinc ( $\ce{Zn(s)}$ ) is above copper ( $\ce{Cu}$ ) in the activity series. Zinc oxidizes to $\ce{Zn^{2+}(aq)}$ , reducing $\ce{Cu^{2+}(aq)}$ to elemental copper: $\ce{Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)}$ Check: Atoms (Zn:1, Cu:1, S:1, O:4) and charges (0) are conserved.
Halogen Displacement: Elemental $\ce{Cl2(g)}$ is more electronegative and reactive than $\ce{Br2(aq)}$ . Chlorine oxidizes bromide ions to elemental bromine: $\ce{Cl2(g) + 2NaBr(aq) -> 2NaCl(aq) + Br2(aq)}$ Check: Atoms (Cl:2, Na:2, Br:2) and charges (0) are conserved.

Exam Trap: Placing solid copper into aqueous zinc sulfate ( $\ce{Cu(s) + ZnSO4(aq)}$ ) yields No Reaction because copper is less active than zinc. Never force a single-replacement reaction backward.

Double-Replacement Reactions

Double-Replacement (Metathesis) Reaction: An exchange of cations and anions between two aqueous ionic compounds:

\ce{AB + CD -> AD + CB}

Oxidation numbers remain constant throughout metathesis reactions.

Metathesis reactions only occur if removal of ions from solution provides a thermodynamic driving force:

Precipitate Formation: An insoluble ionic salt forms according to solubility rules.
Weak Electrolyte Formation: Neutralization produces liquid water ( $\ce{H2O(l)}$ ).
Gas Evolution: Unstable intermediates decompose into gases, such as carbonic acid decomposing to $\ce{CO2(g) + H2O(l)}$ .

Worked Example: Precipitation and Gas Evolution

Problem: Predict products and write net ionic equations for mixing: (1) aqueous silver nitrate and aqueous sodium chloride, and (2) aqueous sodium carbonate and hydrochloric acid.

Step-by-Step Solution:

Precipitation: Partner exchange gives $\ce{AgCl}$ and $\ce{NaNO3}$ . Nitrates and alkali metals are soluble, but $\ce{AgCl}$ is insoluble: $\ce{AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)}$ Canceling spectator ions ( $\ce{Na^+}$ and $\ce{NO3^-}$ ) yields the net ionic equation: $\ce{Ag^+(aq) + Cl^-(aq) -> AgCl(s)}$
Gas Evolution: Partner exchange gives $\ce{NaCl(aq)}$ and $\ce{H2CO3(aq)}$ . Carbonic acid spontaneously decomposes into $\ce{H2O(l)}$ and $\ce{CO2(g)}$ : $\ce{Na2CO3(aq) + 2HCl(aq) -> 2NaCl(aq) + H2O(l) + CO2(g)}$

Exam Trap: Mixing $\ce{NaCl(aq)}$ and $\ce{KNO3(aq)}$ produces no precipitate, gas, or weak electrolyte. All ions remain spectator ions in solution, resulting in No Reaction.

Comparing Replacement Reactions

Feature	Single-Replacement	Double-Replacement
General Formula	$\ce{A + BC -> AC + B}$	$\ce{AB + CD -> AD + CB}$
Primary Driving Force	Difference in redox potential / Activity series ranking	Formation of precipitate, gas, or weak electrolyte ( $\ce{H2O}$ )
Oxidation States	Change (Redox reaction)	Unchanged (Non-redox process)

Mnemonic for DAT Solubility:

References

Sources returned by Google Search grounding during lesson verification.

4.2 Classifying Chemical Reactions - Chemistry 2e | OpenStax — openstax.org
Solubility Rules - Chemistry LibreTexts — chem.libretexts.org

Replacement Reactions and Predicting Products

Summary

Practice Questions

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