1. Introduction to Chemical Reactions
A chemical reaction is a process in which one or more substances (called reactants) are transformed into new substances (called products) with different chemical properties. During a chemical change, electrons may be lost, gained, or shared, leading to the formation of new compounds.
Example: Rusting of iron, burning of wood, cooking rice, curd formation from milk, and photosynthesis.
A chemical equation is the symbolic representation of a chemical reaction, using symbols and formulas to show reactants and products.
Example:
- 2H2+O2→2H2O
- Reactants: H2 and O2
- Product: H2O
2. Types of Chemical Reactions
Chemical reactions are classified based on how reactants change to form products:
a) Combination (Synthesis) Reaction
Definition: A reaction in which two or more reactants combine to form a single product.
General form: A+B→C
Examples:
- 2Na+Cl2→2NaCl (Sodium + Chlorine → Sodium chloride)
- C+O2→CO2 (Carbon + Oxygen → Carbon dioxide)
- N2+3H2→2NH3 (Nitrogen + Hydrogen → Ammonia)
- 2Mg+O2→2MgO (Magnesium + Oxygen → Magnesium oxide)
b) Decomposition Reaction
Definition: A reaction in which a single reactant breaks down into two or more products. It usually requires heat, light, or a catalyst.
General form: AB→A+B
Examples:
- CaCO3→CaO+CO2 (Calcium carbonate → Calcium oxide + Carbon dioxide)
- 2NaOH→Na2O+H2O
- 2KClO3→2KCl+3O2
c) Displacement Reaction
Definition: A reaction in which an element or radical in a compound is replaced by another element or radical.
Types:
i) Single Displacement Reaction:
One element displaces another from a compound.
General form: A+BC→AC+B
Examples:
- Zn+H2SO4→ZnSO4+H2
- Cu+2AgNO3→2Ag+Cu(NO3)2
ii) Double Displacement Reaction:
Mutual exchange of ions between two compounds to form two new compounds.
General form: AB+CD→AD+CB
Examples:
- NaCl+AgNO3→AgCl+NaNO3
- BaCl2+Na2SO4→BaSO4+2NaCl
d) Acid-Base Reaction (Neutralization)
Definition: A reaction in which an acid reacts with a base to form salt and water.
This is a type of double displacement reaction.
Examples:
- HCl+NaOH→NaCl+H2O
- H2SO4+2NaOH→Na2SO4+2H2OH
- CaO+2HCl→CaCl2+H2O
3. Rate of Chemical Reactions
Rate of reaction refers to the speed at which reactants turn into products per unit time.
Reactions can be fast (e.g., acid-base reactions) or slow (e.g., rusting of iron).
At the start of a reaction, the concentration of reactants is highest and that of products is zero. Over time, products increase and reactants decrease.
4. Factors Affecting Reaction Rate
Several factors affect the speed of a chemical reaction:
a) Catalyst
A catalyst is a substance that increases or decreases the reaction rate without being permanently changed.
Positive catalyst: Speeds up reaction (e.g., MnO₂ in H₂O₂ decomposition)
Negative catalyst: Slows down reaction (e.g., H₃PO₄ in some reactions)
b) Temperature
Increasing temperature usually increases reaction rate because particles move faster and collide more frequently.
c) Surface Area
Higher surface area of reactants increases the rate of reaction because more particles are exposed to collisions.
d) Pressure (for gases)
Increasing pressure increases the reaction rate for gaseous reactions because particles are closer and collide more often.
e) Light
Some reactions (like photosynthesis) require light to proceed and may occur faster in its presence.
5. Observations and Activities
The chapter emphasizes practical activities such as:
Mixing acids and bases (to observe neutralization)
Reaction of zinc with HCl (displacement)
Effect of heat, light, and surface area on reaction rate
Copper plating on iron nail (displacement reaction)
These experiments help students understand concepts practically and reinforce theoretical knowledge.
6. Summary Table of Reaction Types
| Reaction Type | General Form | Example |
|---|---|---|
| Combination | A + B → C | 2Na + Cl₂ → 2NaCl |
| Decomposition | AB → A + B | 2KClO₃ → 2KCl + 3O₂ |
| Single Displacement | A + BC → AC + B | Zn + H₂SO₄ → ZnSO₄ + H₂ |
| Double Displacement | AB + CD → AD + CB | NaCl + AgNO₃ → AgCl + NaNO₃ |
| Acid-Base | Acid + Base → Salt + Water | HCl + NaOH → NaCl + H₂O |
Observation:
- Combination: Two or more reactants form one product.
- Decomposition: One compound breaks into simpler substances.
- Single Displacement: One element replaces another in a compound.
- Double Displacement: Exchange of ions between two compounds.
- Acid-Base: Special double displacement producing salt and water.
Physical Change & Chemical Change
Physical Change — Definition
A physical change is a change in form, shape, size, or state of a substance without forming a new substance.
Examples
Melting of ice
Dissolving sugar in water
Breaking glass
Chemical Change — Definition
A chemical change is a change in which one or more new substances are formed having different properties from the original substances.
Examples
Rusting of iron
Burning of wood
Milk turning into curd
Cooking food
2. Difference Between Physical & Chemical Change
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | No | Yes |
| Reversibility | Mostly reversible | Mostly irreversible |
| Type of change | Change in physical state | Change in chemical composition |
| Energy change | Small | Large |
3. Examples of Chemical Changes in Daily Life
| Process | Type of Change | Reason/Observation |
|---|---|---|
| Rusting of iron | Chemical | New substance (iron oxide) formed |
| Milk → Curd | Chemical | Action of bacteria |
| Cooking rice | Chemical | New food substance formed |
| Burning wood | Chemical | Forms charcoal, ash, CO₂ |
| Burning matchstick | Chemical | Combustion occurs |
| Photosynthesis | Chemical | Forms glucose & oxygen |
11. Differences (Important)
Single vs Double Displacement
| Feature | Single Displacement | Double Displacement |
|---|---|---|
| Definition | One element displaces another | Exchange of ions between compounds |
| General Form | A + BC → AC + B | AB + CD → AD + CB |
| Example | Zn + HCl → ZnCl₂ + H₂ | NaCl + AgNO₃ → AgCl + NaNO₃ |
Combination vs Decomposition
| Feature | Combination | Decomposition |
|---|---|---|
| Definition | Many → One | One → Many |
| General Form | A + B → AB | AB → A + B |
12. Sample Exercise Answers
Give reason
i. Hydrogen peroxide is stored in brown bottles
Because light decomposes it.
ii. Acid–base reaction is neutralization
Because acid + base forms neutral salt + water.
iii. Rate of reaction differs
Because reactions have different activation energies, reactants, conditions.
13. Important Reaction: Copper Sulphate & Iron
Type: Single displacement
Iron displaces copper.
Balanced reaction:
Fe+CuSO4→FeSO4+Cu
14. Magnesium Ribbon Burning
Substance ‘A’ = Magnesium oxide (MgO)
Reaction:
2Mg+O2→2MgO
Chapter Summary
Chemical reactions occur when reactants convert into products with new properties. They can be combination, decomposition, displacement, double displacement, or acid-base reactions. Reaction speed depends on temperature, catalyst, pressure, surface area, and light. Chemical equations represent reactions using symbols and must be balanced. Many natural and daily changes from rusting to photosynthesis—are examples of chemical reactions.