Types of Reactions
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Chemical Reaction
A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) through the breaking and forming of chemical bonds. Chemical reactions are represented by chemical equations.
The Five Main Types of Chemical Reactions
1. Synthesis (Combination) Reactions
Synthesis Reaction
A synthesis reaction occurs when two or more substances combine to form a single, more complex product.
General form: A + B --> AB
| Example | Equation | Description |
|---|---|---|
| Iron rusting | 4Fe + 3O2 --> 2Fe2O3 | Iron combines with oxygen to form iron(III) oxide |
| Water formation | 2H2 + O2 --> 2H2O | Hydrogen and oxygen combine to form water |
| Magnesium burning | 2Mg + O2 --> 2MgO | Magnesium burns in oxygen to form magnesium oxide |
2. Decomposition Reactions
Decomposition Reaction
A decomposition reaction occurs when a single compound breaks down into two or more simpler substances. This is the opposite of synthesis.
General form: AB --> A + B
| Example | Equation | Description |
|---|---|---|
| Water electrolysis | 2H2O --> 2H2 + O2 | Water decomposes into hydrogen and oxygen with electricity |
| Carbonate breakdown | CaCO3 --> CaO + CO2 | Calcium carbonate decomposes when heated |
| Hydrogen peroxide | 2H2O2 --> 2H2O + O2 | Hydrogen peroxide breaks down into water and oxygen |
3. Single Replacement (Displacement) Reactions
Single Replacement Reaction
A single replacement reaction occurs when one element replaces another element in a compound. The more reactive element displaces the less reactive one.
General form: A + BC --> AC + B or A + BC --> BA + C
Activity Series
The activity series ranks metals by their reactivity. A metal can only replace another metal that is below it in the series:
Most reactive to least reactive (metals): K > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb > H > Cu > Hg > Ag > Pt > Au
| Example | Equation | Description |
|---|---|---|
| Zinc in copper sulfate | Zn + CuSO4 --> ZnSO4 + Cu | Zinc replaces copper (Zn is more reactive than Cu) |
| Magnesium in acid | Mg + 2HCl --> MgCl2 + H2 | Magnesium replaces hydrogen in acid |
| Sodium in water | 2Na + 2H2O --> 2NaOH + H2 | Sodium replaces hydrogen in water |
4. Double Replacement (Metathesis) Reactions
Double Replacement Reaction
A double replacement reaction occurs when the cations and anions of two compounds switch places to form two new compounds.
General form: AB + CD --> AD + CB
Double replacement reactions typically occur when one of the products is:
- A precipitate (insoluble solid)
- A gas that escapes
- A molecular compound like water
| Example | Equation | Description |
|---|---|---|
| Precipitation | NaCl + AgNO3 --> NaNO3 + AgCl(s) | Silver chloride precipitates out of solution |
| Neutralization | HCl + NaOH --> NaCl + H2O | Acid + base produces salt + water |
| Gas formation | Na2CO3 + 2HCl --> 2NaCl + H2O + CO2(g) | Carbon dioxide gas is released |
5. Combustion Reactions
Combustion Reaction
A combustion reaction occurs when a substance reacts rapidly with oxygen, usually producing heat and light. Complete combustion of hydrocarbons produces carbon dioxide and water.
General form: Fuel + O2 --> CO2 + H2O + Energy
| Example | Equation | Description |
|---|---|---|
| Methane burning | CH4 + 2O2 --> CO2 + 2H2O | Natural gas combustion |
| Propane burning | C3H8 + 5O2 --> 3CO2 + 4H2O | Propane grill combustion |
| Ethanol burning | C2H5OH + 3O2 --> 2CO2 + 3H2O | Alcohol combustion |
Summary of Reaction Types
| Reaction Type | General Form | Key Identifier |
|---|---|---|
| Synthesis | A + B --> AB | Two or more reactants form one product |
| Decomposition | AB --> A + B | One reactant breaks into multiple products |
| Single Replacement | A + BC --> AC + B | Element replaces another element in compound |
| Double Replacement | AB + CD --> AD + CB | Compounds exchange ions |
| Combustion | Fuel + O2 --> CO2 + H2O | Substance burns in oxygen |
SAT/ACT Connection
Science sections may present chemical equations and ask you to identify the reaction type or predict products. Look for key patterns: number of reactants and products, presence of oxygen as a reactant, or whether elements are switching positions.
💡 Examples
Example 1: Identifying Synthesis Reactions
Problem: Classify the reaction: 2Na + Cl2 --> 2NaCl
Solution:
Step 1: Count reactants: 2 (sodium and chlorine gas)
Step 2: Count products: 1 (sodium chloride)
Step 3: Pattern: Two elements combining to form one compound (A + B --> AB)
Answer: This is a synthesis (combination) reaction. Two elements combine to form a single compound.
Example 2: Identifying Decomposition Reactions
Problem: Classify the reaction: 2KClO3 --> 2KCl + 3O2
Solution:
Step 1: Count reactants: 1 (potassium chlorate)
Step 2: Count products: 2 (potassium chloride and oxygen gas)
Step 3: Pattern: One compound breaking down into simpler substances (AB --> A + B)
Answer: This is a decomposition reaction. One compound breaks apart into two products.
Example 3: Predicting Single Replacement Products
Problem: Predict if a reaction occurs and write the products: Fe + CuSO4 --> ?
Solution:
Step 1: Check the activity series: Fe is above Cu, so iron is more reactive.
Step 2: Since Fe is more reactive, it will replace Cu in the compound.
Step 3: Write the products: Iron takes copper's place, forming FeSO4, and copper metal is released.
Step 4: Balance: Fe + CuSO4 --> FeSO4 + Cu
Answer: Yes, a reaction occurs. Fe + CuSO4 --> FeSO4 + Cu (single replacement reaction)
Example 4: Identifying Combustion Reactions
Problem: Classify and complete: C6H12O6 + O2 --> ?
Solution:
Step 1: Identify reactants: A carbon-containing compound (glucose) and oxygen
Step 2: This is combustion because an organic compound is reacting with O2.
Step 3: Complete combustion of organic compounds produces CO2 and H2O.
Step 4: Write products: C6H12O6 + O2 --> CO2 + H2O
Step 5: Balance: C6H12O6 + 6O2 --> 6CO2 + 6H2O
Answer: This is a combustion reaction. C6H12O6 + 6O2 --> 6CO2 + 6H2O
Example 5: Double Replacement with Precipitate
Problem: Classify and identify: Pb(NO3)2 + 2KI --> PbI2 + 2KNO3
Solution:
Step 1: Count reactants: 2 compounds (lead nitrate and potassium iodide)
Step 2: Count products: 2 compounds (lead iodide and potassium nitrate)
Step 3: Analyze the switch: Pb2+ switched from NO3- to I-; K+ switched from I- to NO3-
Step 4: Pattern: The cations exchanged anion partners (AB + CD --> AD + CB)
Step 5: Lead iodide (PbI2) is insoluble and precipitates as a bright yellow solid.
Answer: This is a double replacement (precipitation) reaction. The cations exchange anions, and PbI2 precipitates.
✏️ Practice
1. What type of reaction is: 2H2O --> 2H2 + O2?
A) Synthesis
B) Decomposition
C) Single replacement
D) Combustion
2. Which is an example of a synthesis reaction?
A) 2KClO3 --> 2KCl + 3O2
B) Zn + CuSO4 --> ZnSO4 + Cu
C) 2Na + Cl2 --> 2NaCl
D) NaOH + HCl --> NaCl + H2O
3. In a single replacement reaction, what determines if the reaction will occur?
A) The mass of the reactants
B) The activity series
C) The temperature
D) The color of the compounds
4. Complete combustion of a hydrocarbon always produces:
A) CO and H2
B) CO2 and H2O
C) CO and H2O
D) C and H2O
5. What type of reaction is: AgNO3 + NaCl --> AgCl + NaNO3?
A) Synthesis
B) Decomposition
C) Single replacement
D) Double replacement
6. Which reaction type always has only ONE reactant?
A) Synthesis
B) Decomposition
C) Single replacement
D) Combustion
7. If copper is placed in a solution of zinc sulfate, what happens?
A) Copper replaces zinc
B) No reaction occurs
C) Zinc replaces copper
D) Both metals dissolve
8. What type of reaction is: HCl + NaOH --> NaCl + H2O?
A) Synthesis
B) Combustion
C) Neutralization (double replacement)
D) Decomposition
9. The reaction 2Mg + O2 --> 2MgO is best classified as:
A) Single replacement only
B) Synthesis only
C) Both synthesis and combustion
D) Decomposition
10. Which products form when barium chloride reacts with sodium sulfate in solution?
A) BaSO4 and NaCl
B) Ba2O3 and NaCl
C) BaNa and ClSO4
D) No reaction occurs
Click to reveal answers
- B - One compound (water) breaks into two simpler substances (hydrogen and oxygen).
- C - Two elements (sodium and chlorine) combine to form one compound (sodium chloride).
- B - The activity series determines if a more reactive element can replace a less reactive one.
- B - Complete combustion produces carbon dioxide (CO2) and water (H2O).
- D - Two compounds exchange ions to form two new compounds.
- B - Decomposition reactions start with one compound that breaks apart.
- B - Copper is below zinc in the activity series, so copper cannot replace zinc.
- C - Acid + base --> salt + water is a neutralization reaction, which is a type of double replacement.
- C - Magnesium combining with oxygen is both a synthesis (two substances form one) and combustion (reaction with oxygen releasing energy).
- A - Ba2+ pairs with SO4 2- to form BaSO4 (precipitate); Na+ pairs with Cl- to form NaCl.
✅ Check Your Understanding
Question 1: Explain how you can quickly identify whether a reaction is synthesis or decomposition just by looking at the chemical equation.
Reveal Answer
Count the number of reactants and products. In a synthesis reaction, you have two or more reactants combining to form a single product (multiple --> one). In a decomposition reaction, you have one reactant breaking into two or more products (one --> multiple). If you see the arrow pointing from many substances to one, it's synthesis. If the arrow points from one substance to many, it's decomposition.
Question 2: Using the activity series, explain why gold jewelry doesn't tarnish in air while iron readily rusts.
Reveal Answer
Gold (Au) is at the bottom of the activity series, making it one of the least reactive metals. It is so unreactive that it doesn't combine with oxygen in the air under normal conditions. Iron (Fe), however, is much higher in the activity series and more reactive. Iron readily reacts with oxygen and moisture in the air to form iron oxide (rust): 4Fe + 3O2 --> 2Fe2O3. This is why gold has been prized for jewelry throughout history - its low reactivity means it maintains its luster indefinitely.
Question 3: A student adds a strip of zinc metal to a solution of silver nitrate. Describe what would be observed and classify the reaction type.
Reveal Answer
This is a single replacement reaction. Zinc is more reactive than silver (zinc is higher in the activity series), so zinc will replace silver. Observations: (1) Silver metal crystals would appear on the zinc strip as silver ions are reduced. (2) The zinc strip would gradually dissolve as zinc atoms become zinc ions. (3) The blue color of the solution might change as zinc nitrate forms. The reaction is: Zn + 2AgNO3 --> Zn(NO3)2 + 2Ag
Question 4: Baking soda (sodium bicarbonate) reacts with vinegar (acetic acid) producing vigorous bubbling. Identify the reaction type and explain why bubbles form.
Reveal Answer
This is a double replacement reaction: NaHCO3 + CH3COOH --> CH3COONa + H2CO3. However, the carbonic acid (H2CO3) formed is unstable and immediately decomposes: H2CO3 --> H2O + CO2. The bubbles are carbon dioxide gas escaping from the solution. This is actually a double replacement followed by decomposition. The overall equation is: NaHCO3 + CH3COOH --> CH3COONa + H2O + CO2. The driving force is the formation of a gas (CO2) that escapes from the reaction mixture.
🚀 Next Steps
- Review any concepts that felt challenging
- Move on to the next lesson when ready
- Return to practice problems periodically for review