Reaction Types

Why This Matters

In Honors Chemistry, you're not just memorizing what happens when chemicals mix—you're being tested on why reactions occur and how to predict their products. Reaction types are the foundation for balancing equations, predicting products, and understanding energy changes. Every reaction you encounter fits into a pattern, and recognizing that pattern lets you tackle unfamiliar problems with confidence.

The key concepts here include electron transfer, ion exchange, energy flow, and driving forces. Exams will ask you to classify reactions, predict products, and explain why certain reactions proceed while others don't. Don't just memorize the general forms—know what's actually happening at the particle level and what conditions make each reaction favorable.

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Reactions That Build: Synthesis

Synthesis reactions occur when simpler substances combine to form more complex products. These reactions decrease the total number of substances present and often release energy as new bonds form.

Synthesis (Combination) Reactions

• Two or more reactants combine to form a single product—the simplest reaction type to identify because you end with fewer substances than you started with
• General form: $A + B \rightarrow AB$—look for elements or simple compounds on the left, a single compound on the right
• Common examples include metal oxide formation—when metals burn in air, they combine with oxygen to form oxides like $2Mg + O_2 \rightarrow 2MgO$

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Reactions That Break Apart: Decomposition

Decomposition is the reverse of synthesis—a single compound breaks into simpler substances. These reactions typically require an energy input to break existing bonds, making them often endothermic.

Decomposition Reactions

• A single compound breaks down into two or more products—the opposite pattern of synthesis, with one reactant yielding multiple products
• Energy input is usually required—heat (thermal decomposition), electricity (electrolysis), or light (photolysis) provides the activation energy
• General form: $AB \rightarrow A + B$—classic example is electrolysis of water: $2H_2O \rightarrow 2H_2 + O_2$

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Reactions That Swap Partners: Displacement

Displacement reactions involve elements or ions trading places based on relative reactivity or stability. The driving force is always the formation of a more stable arrangement—either a more stable compound or a less reactive element.

Single Displacement Reactions

• An element replaces another element in a compound—the more reactive element "kicks out" the less reactive one
• General form: $A + BC \rightarrow AC + B$—requires checking the activity series to predict if the reaction occurs
• Reactivity determines feasibility—zinc replaces copper in $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ because zinc is more reactive than copper

Double Displacement Reactions

• Ions of two compounds exchange partners—also called metathesis reactions
• General form: $AB + CD \rightarrow AD + CB$—the cations and anions simply switch places
• Requires a driving force to proceed—formation of a precipitate, gas, or water pulls the reaction forward

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Reactions with Driving Forces: Precipitation and Neutralization

These are specific types of double displacement reactions where the driving force is clear and predictable. The formation of an insoluble solid, a gas, or water removes products from solution and drives the reaction to completion.

Precipitation Reactions

• Two soluble ionic compounds react to form an insoluble product—the precipitate (solid) is the driving force
• General form: $AB(aq) + CD(aq) \rightarrow AD(s) + CB(aq)$—use solubility rules to predict which product precipitates
• Essential for qualitative analysis—identifying ions in solution by the precipitates they form with known reagents

Neutralization Reactions

• An acid reacts with a base to produce water and a salt—a specific acid-base reaction with predictable products
• General form: $HA + BOH \rightarrow BA + H_2O$—the $H^+$ from the acid combines with $OH^-$ from the base
• Critical for titration calculations—the mole ratio at the equivalence point lets you determine unknown concentrations

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Reactions That Transfer Particles: Acid-Base and Redox

These reactions are defined by what gets transferred—protons for acid-base, electrons for redox. Understanding the transfer mechanism is essential for balancing these equations and predicting products.

Acid-Base Reactions

• Proton ($H^+$) transfer between reactants—the Brønsted-Lowry definition focuses on proton donors (acids) and acceptors (bases)
• Conjugate pairs form—when an acid donates a proton, it becomes a conjugate base, and vice versa
• Broader than neutralization—includes reactions without hydroxide ions, like $NH_3 + HCl \rightarrow NH_4Cl$

Oxidation-Reduction (Redox) Reactions

• Electron transfer between substances—oxidation is electron loss (OIL), reduction is electron gain (RIG)
• Oxidation states change—track these numbers to identify what's oxidized and what's reduced
• Requires both processes simultaneously—you can't have oxidation without reduction; electrons must go somewhere

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Reactions That Release or Absorb Energy: Combustion and Thermochemistry

Energy changes accompany all reactions, but combustion and the exothermic/endothermic classification focus specifically on heat flow. These concepts connect reaction types to thermodynamics and real-world applications.

Combustion Reactions

• Rapid reaction with oxygen producing heat and light—a specific type of highly exothermic redox reaction
• Hydrocarbons produce $CO_2$ and $H_2O$—general form: $C_xH_y + O_2 \rightarrow CO_2 + H_2O$ (must be balanced)
• Powers most of our energy systems—from car engines to power plants, combustion converts chemical energy to thermal energy

Exothermic and Endothermic Reactions

• Exothermic releases energy to surroundings—products have less energy than reactants, $\Delta H < 0$
• Endothermic absorbs energy from surroundings—products have more energy than reactants, $\Delta H > 0$
• Not a reaction type but an energy classification—any reaction type can be exothermic or endothermic depending on bond energies

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Quick Reference Table

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Self-Check Questions

1. Both single displacement and redox reactions involve electron transfer. How would you identify a single displacement reaction that is also a redox reaction? Give an example.

2. Which two reaction types are exact opposites in their general form, and how do their energy changes typically differ?

3. Precipitation and neutralization are both double displacement reactions. What is the driving force for each, and how would you use different reference tools to predict their products?

4. A hydrocarbon burns in excess oxygen. Classify this reaction in as many ways as possible (hint: it fits multiple categories).

5. FRQ-style: Given the reaction $Zn + 2HCl \rightarrow ZnCl_2 + H_2$, identify the reaction type, explain why it proceeds, and identify what is oxidized and what is reduced.