Types of Chemical Reactions
Complete Notes — Combination, Decomposition, Displacement, Redox, Neutralization & Factors Affecting Reaction Rate
🔬 Introduction to Chemical Reactions
A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) with different properties. Chemical bonds are broken and new bonds are formed during this process.
Chemical Reaction: A process that involves rearrangement of atoms to form new substances with different chemical and physical properties.
Evidence of a Chemical Reaction
In any chemical reaction, the total mass of reactants = total mass of products. Matter is neither created nor destroyed.
Writing a Chemical Equation
A chemical equation uses symbols and formulae to represent a chemical reaction:
2Mg + O₂ → 2MgO (balanced)
(s) = solid | (l) = liquid | (g) = gas | (aq) = aqueous (dissolved in water)
🧪 Types of Chemical Reactions
Chemical reactions are classified into several main types based on how the reactants change to form products.
Two or more substances combine to form a single new product.
Examples
In a combination reaction, the number of products is always fewer than the reactants. Energy is usually released (exothermic).
A single compound breaks down into two or more simpler substances. It is the opposite of combination.
Types of Decomposition
Thermal Decomposition
- Uses heat energy
- CaCO₃ → CaO + CO₂
- 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
Electrolytic Decomposition
- Uses electric current
- 2H₂O → 2H₂ + O₂
- 2NaCl → 2Na + Cl₂
Photolytic Decomposition
- Uses light energy
- 2AgCl → 2Ag + Cl₂ (sunlight)
- 2AgBr → 2Ag + Br₂ (used in photography)
Examples in Life
- H₂O₂ → H₂O + O₂ (antiseptic breaks down)
- Cooking food uses thermal decomposition
Decomposition reactions require energy input (heat, light, or electricity) — they are generally endothermic.
A more reactive element displaces a less reactive element from its compound.
Examples
The reactivity series (K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au) determines which element can displace another. A metal can only displace metals below it in the series.
Two compounds exchange ions to form two new compounds. Also called metathesis reaction.
Examples
Precipitation: Forms an insoluble solid (precipitate) ↓
Neutralization: Acid + Base → Salt + Water
Gas formation: Products include a gas ↑
A redox reaction involves both oxidation and reduction occurring simultaneously. One substance loses electrons (oxidation) and another gains electrons (reduction).
Oxidation
- Loss of electrons
- Gain of oxygen
- Loss of hydrogen
- Increase in oxidation state
Reduction
- Gain of electrons
- Loss of oxygen
- Gain of hydrogen
- Decrease in oxidation state
Examples
OIL = Oxidation Is Loss (of electrons)
RIG = Reduction Is Gain (of electrons)
Oxidizing agent: Accepts electrons (gets reduced itself) — e.g., O₂, Cl₂, KMnO₄
Reducing agent: Donates electrons (gets oxidized itself) — e.g., H₂, C, Na
Exothermic
- Releases energy (heat/light)
- Products have less energy than reactants
- Temperature of surroundings increases
- ΔH is negative (−)
- Examples: Combustion, respiration, neutralization, burning of fuels
Endothermic
- Absorbs energy from surroundings
- Products have more energy than reactants
- Temperature of surroundings decreases
- ΔH is positive (+)
- Examples: Photosynthesis, thermal decomposition, dissolving ammonium nitrate
A neutralization reaction occurs when an acid reacts with a base to form a salt and water. The acidic and basic properties are neutralized.
Antacids neutralize excess stomach acid (HCl) | Lime applied to acidic soil | Toothpaste neutralizes mouth acids
A precipitation reaction forms an insoluble solid (precipitate) when two aqueous solutions are mixed.
(Yellow precipitate of Lead Iodide)
(White precipitate of Barium Sulfate)
Water purification | Testing for ions in solution | Making pigments and dyes | Separating substances in chemistry labs
📊 Comparison Tables
Types of Reactions — Quick Comparison
| Reaction Type | Pattern | Energy Change | Example |
|---|---|---|---|
| Combination | A + B → AB | Usually exothermic | 2Mg + O₂ → 2MgO |
| Decomposition | AB → A + B | Usually endothermic | CaCO₃ → CaO + CO₂ |
| Displacement | A + BC → AC + B | Varies | Zn + CuSO₄ → ZnSO₄ + Cu |
| Double Displacement | AB + CD → AD + CB | Varies | AgNO₃ + NaCl → AgCl↓ + NaNO₃ |
| Redox | Electron transfer | Varies | CuO + H₂ → Cu + H₂O |
| Neutralization | Acid + Base → Salt + H₂O | Exothermic | HCl + NaOH → NaCl + H₂O |
| Precipitation | Two solutions → precipitate | Varies | BaCl₂ + Na₂SO₄ → BaSO₄↓ |
Exothermic vs Endothermic
| Feature | Exothermic | Endothermic |
|---|---|---|
| Energy change | Releases heat | Absorbs heat |
| ΔH value | Negative (−) | Positive (+) |
| Surroundings | Get warmer | Get cooler |
| Product energy | Lower than reactants | Higher than reactants |
| Examples | Combustion, neutralization, respiration | Photosynthesis, thermal decomposition |
Oxidation vs Reduction
| Feature | Oxidation | Reduction |
|---|---|---|
| Electrons | Lost (LEO) | Gained (GER) |
| Oxygen | Gained | Lost |
| Hydrogen | Lost | Gained |
| Oxidation state | Increases | Decreases |
| Agent involved | Reducing agent (gets oxidized) | Oxidizing agent (gets reduced) |
⚡ Factors Affecting Rate of Reaction
The rate of reaction is how fast reactants are converted into products. Several factors can speed up or slow down a chemical reaction.
Rate of Reaction = Change in concentration of reactants or products ÷ Time taken
A reaction occurs when particles collide with sufficient energy (≥ activation energy) and the correct orientation. Any factor that increases the number of successful collisions increases the reaction rate.
📐 Rate Law & Rate Expression
The rate law (or rate equation) mathematically relates the rate of a reaction to the concentration of reactants.
Rate = speed of reaction (mol/L/s)
k = rate constant (depends on temperature)
[A], [B] = concentration of reactants
m, n = orders of reaction with respect to each reactant
Overall order = m + n
Orders of Reaction
| Order | Rate Equation | What It Means |
|---|---|---|
| Zero Order | Rate = k | Rate does not depend on concentration |
| First Order | Rate = k[A] | Rate doubles when [A] doubles |
| Second Order | Rate = k[A]² | Rate quadruples when [A] doubles |
Activation Energy
Activation energy (Eₐ) is the minimum energy that reactant particles must have for a collision to result in a reaction. A catalyst lowers the activation energy, increasing the number of successful collisions without raising temperature.
where A = frequency factor, R = gas constant, T = temperature (K)
📝 Summary
Key takeaways from chemical reactions in one place.
Combination: A + B → AB. Two or more substances form one product. Usually exothermic.
Decomposition: AB → A + B. One compound breaks into simpler substances. Requires energy input.
Displacement: More reactive element replaces less reactive one from its compound.
Double Displacement: Two compounds swap ions, forming precipitate or neutral products.
Redox: Simultaneous oxidation (loss of e⁻) and reduction (gain of e⁻). OIL RIG.
Rate Factors: Temperature, concentration, surface area, catalyst, light, pressure — all affect reaction speed.
⚡ Quick Revision Notes
Combination
- A + B → AB
- Two → one product
- Usually exothermic
Decomposition
- AB → A + B
- Thermal / electrolytic / photolytic
- Requires energy input
Displacement
- A + BC → AC + B
- More reactive replaces less reactive
- Uses reactivity series
Double Displacement
- AB + CD → AD + CB
- Ion exchange between two compounds
- Precipitation / neutralization / gas
Redox
- OIL RIG rule
- Oxidizing agent gets reduced
- Reducing agent gets oxidized
Exo vs Endo
- Exo: releases heat, ΔH negative
- Endo: absorbs heat, ΔH positive
- Catalyst lowers activation energy
Rate Factors
- ↑ Temp → ↑ rate
- ↑ Concentration → ↑ rate
- ↑ Surface area → ↑ rate
Rate Law
- Rate = k[A]ᵐ[B]ⁿ
- k increases with temperature
- Catalyst doesn’t change equation
🧠 Memory Tricks
RIG = Reduction Is Gain (of electrons)
LEO the lion says GER → Loss Electrons Oxidation / Gain Electrons Reduction
Combination · Decomposition · Displacement · Redox · Exo/Endo · Neutralization · Precipitation
“The Chemist Speaks Plainly — Listen Carefully”
K · Na · Ca · Mg · Al · Zn · Fe · Pb · H · Cu · Ag · Au
Decomposition = “Decomposes / falls apart” (AB → A + B) → breaks down
Opposite reactions!
🎯 Practice Quiz
📋 Practice Questions
Knowledge Based
- Define a chemical reaction. List four signs that indicate a chemical reaction has occurred.
- What is a combination reaction? Give two examples with equations.
- Name three types of decomposition reactions with one example each.
- State the law of conservation of mass.
- What is a redox reaction? Define oxidation and reduction.
Understanding Based
- Explain why decomposition reactions generally require energy input.
- How does temperature affect the rate of a chemical reaction? Explain using collision theory.
- What is a catalyst? How does it speed up a reaction without being consumed?
- Distinguish between a single displacement and double displacement reaction with examples.
- Why does powdered marble react faster with acid than marble chips?
Application Based
- Identify which type of reaction occurs: Zn + H₂SO₄ → ZnSO₄ + H₂↑. Justify your answer.
- In the reaction CuO + H₂ → Cu + H₂O, identify what is oxidized and what is reduced.
- Write a balanced equation for the neutralization of sulfuric acid with potassium hydroxide.
- Predict what happens when iron is placed in a solution of copper sulfate. Write the equation.
Higher Order Thinking
- A student noticed that food spoils faster in summer than in winter. Explain this using knowledge of reaction rates and temperature.
- Evaluate the advantages and disadvantages of using catalysts in industrial chemical processes.
- “All displacement reactions are redox reactions, but not all redox reactions are displacement reactions.” Discuss with examples.
- Design an experiment to show how surface area affects the rate of reaction between marble chips and hydrochloric acid.