Chapter 15: Chemical Reaction
A chemical reaction is a process in which reactants change into new substances called products. In this chapter we study chemical equations, types of chemical reactions, the rate of chemical reaction, and the factors that affect this rate.
Everyday examples of physical and chemical changes: battery, fire, melting ice, photosynthesis, digestion, rusting, boiling water.
1. Chemical Reaction and Chemical Equation
Chemical Reaction
During a chemical change, electrons are lost, gained, or shared between reactants. The reactants combine, decompose, or exchange ions to form new products. This process is called a chemical reaction.
Chemical Equation
A chemical reaction written using words, symbols, or chemical formulas of reactants and products is called a chemical equation.
SEE Focus: Reactants are always written on the left side and products on the right side of the arrow (→). The arrow means 'produces' or 'gives'.
2. Types of Chemical Reaction
On the basis of the types of reactants and products, chemical reactions are classified into four main types: combination, decomposition, displacement, and acid-base (neutralization) reaction.
2.1 Combination Reaction
The chemical reaction in which two or more reactants combine to form only one product is called a combination reaction.
General form: A + B → C
Two reactants A and B combining to form a single product C.
- 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)
- 4Fe + 3O2 → 2Fe2O3 (Iron + Oxygen → Ferric oxide)
2.2 Decomposition Reaction
The chemical reaction in which a single reactant breaks down into two or more products is called a decomposition reaction. A reactant decomposes due to the effect of heat, light, pressure, or other causes.
General form: AB → A + B
A single reactant AB breaking into two products A and B.
- CaCO3(s) → CaO(s) + CO2(g)
- 2NaOH(s) → Na2O(s) + H2O(l)
- H2CO3(aq) → CO2(g) + H2O(l)
- 2KClO3(s) → 2KCl(s) + 3O2(g)
Very Important for SEE: 2KClO3 → 2KCl + 3O2 is a very common decomposition reaction asked in exams, especially with the catalyst MnO2 and with heat.
2.3 Displacement Reaction
The chemical reaction in which an atom or radical of one reactant is displaced by the atom or radical of another reactant is called a displacement reaction. There are two types: single displacement and double displacement reaction.
i. Single Displacement Reaction
In a single displacement reaction, one element or radical displaces an element or radical of another compound.
General form: AB + C → A + BC (Compound + Element → Element + Compound)
An element C displacing element A from compound AB to form a new compound BC.
- Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
- Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
- Cu + 2AgNO3 → 2Ag + Cu(NO3)2
- Ca + 2H2O → Ca(OH)2 + H2
- Cl2 + 2NaBr → 2NaCl + Br2
- Br2 + 2KI → 2KBr + I2
Lab activity: an iron nail dipped in copper sulphate solution shows a single displacement reaction (copper plating on iron).
In this activity, iron displaces copper from copper sulphate solution: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s). A reddish-brown copper coating forms on the iron nail and the blue colour of the solution fades.
ii. Double Displacement Reaction
The chemical reaction in which two new compounds are formed by the mutual exchange of atoms or radicals between two reactants is called a double displacement reaction.
General form: AB + CD → AD + CB
Two compounds AB and CD exchanging partners to form two new compounds AD and CB.
- NaCl + AgNO3 → AgCl + NaNO3
- BaCl2 + Na2SO4 → BaSO4 + 2NaCl
- Pb(NO3)2 + 2KI → 2KNO3 + PbI2
2.4 Acid-Base Reaction (Neutralization Reaction)
The reaction in which an acid reacts with a base to form salt and water is called an acid-base reaction. Both the acid and the base lose their properties, so it is also known as a neutralization reaction. It is a special type of double displacement reaction.
General form: Acid + Base → Salt + Water
- HCl (acid) + NaOH (base) → NaCl (salt) + H2O (water)
- H2SO4 (acid) + 2NaOH (base) → Na2SO4 (salt) + 2H2O (water)
- KOH (base) + HNO3 (acid) → KNO3 (salt) + H2O (water)
- CaO (base) + 2HCl (acid) → CaCl2 (salt) + H2O (water)
3. Rate of Chemical Reaction
The change of reactants into products per unit time in any chemical reaction is called the rate of that chemical reaction.
At the start of a reaction, the concentration of reactants is maximum and the concentration of products is zero. As the reaction proceeds, the concentration of products increases while the concentration of reactants decreases.
Some reactions are fast (for example, the reaction between an acid and a base) while some reactions are slow (for example, rusting of iron).
4. Factors Affecting Rate of Chemical Reaction
4.1 Catalyst
A catalyst is a substance which increases or decreases the rate of a chemical reaction without undergoing any permanent chemical change itself.
| Type of Catalyst | Effect on Rate | Example |
|---|---|---|
| Positive catalyst | Increases the rate of reaction | MnO2, V2O5, Fe |
| Negative catalyst | Decreases the rate of reaction | H3PO4 (Phosphoric acid) |
- 2KClO3 --MnO2--> 2KCl + 3O2 (MnO2 is a positive catalyst)
- 2H2O2 --H3PO4--> 2H2O + O2 (H3PO4 is a negative catalyst)
4.2 Heat (Temperature)
Generally, the rate of a chemical reaction increases as the temperature increases. Example: 2KClO3 --Heat--> 2KCl + 3O2.
4.3 Pressure
The rate of reaction of gases depends on the pressure of the gases. Generally, the rate of reaction increases as the pressure increases. For example, ammonia is manufactured under high pressure (200-900 atm): N2(g) + 3H2(g) --200-900atm--> 2NH3(g).
4.4 Surface Area
If the surface area of contact of the reactants is more, the rate of reaction is also more. A powdered substance reacts faster than the same substance in a solid lump, because powder has a much larger surface area.
A powdered vitamin C tablet reacts faster in water than a whole (unbroken) tablet because it has more surface area.
4.5 Light
Some chemical reactions need light to take place. The presence of light increases the rate of such reactions.
Photosynthesis is a light-dependent chemical reaction that occurs in the presence of sunlight and chlorophyll.
- 6CO2 + 6H2O --light/chlorophyll--> C6H12O6 + 6O2 (Photosynthesis)
- 2AgNO3 --Light--> 2NO2 + O2 + 2Ag (Decomposition of silver nitrate in light)
Important Definitions
- Chemical Reaction: A process in which reactants change into new products through loss, gain, or sharing of electrons.
- Chemical Equation: A chemical reaction written using symbols/formulas of reactants and products.
- Combination Reaction: Two or more reactants combine to form one product (A + B → C).
- Decomposition Reaction: One reactant breaks down into two or more products (AB → A + B).
- Displacement Reaction: An atom or radical of one reactant is displaced by another.
- Neutralization Reaction: An acid reacts with a base to form salt and water.
- Rate of Chemical Reaction: Change of reactants into products per unit time.
- Catalyst: A substance that changes the rate of reaction without undergoing permanent chemical change itself.
Important Differences
Difference Between Combination Reaction and Decomposition Reaction
| Combination Reaction | Decomposition Reaction |
|---|---|
| Two or more reactants combine to form one product. | One reactant breaks down into two or more products. |
| General form: A + B → C | General form: AB → A + B |
| Example: 2Mg + O2 → 2MgO | Example: 2KClO3 → 2KCl + 3O2 |
Difference Between Single and Double Displacement Reaction
| Single Displacement Reaction | Double Displacement Reaction |
|---|---|
| Only one element or radical is displaced from a compound. | Two compounds exchange atoms or radicals with each other. |
| General form: AB + C → A + BC | General form: AB + CD → AD + CB |
| Example: Zn + H2SO4 → ZnSO4 + H2 | Example: NaCl + AgNO3 → AgCl + NaNO3 |
Important Diagrams
Diagram 1: Copper Sulphate and Iron Nail Apparatus
Draw and label a test tube fixed on a retort stand, containing blue copper sulphate solution, with an iron nail suspended inside by a thread. Label: test tube, thread, copper sulphate, iron nail.
Diagram 2: Models of Combination, Decomposition, Single and Double Displacement Reactions
Practice drawing the simple sphere models showing A + B → C, AB → A + B, AB + C → A + BC, and AB + CD → AD + CB, as these are frequently asked to illustrate reaction types.
Common Mistakes in SEE
- Do not confuse combination reaction (two reactants, one product) with decomposition reaction (one reactant, many products).
- Do not confuse single displacement (one element displaced) with double displacement (two radicals exchanged).
- Remember that acid-base reaction is also a type of double displacement reaction.
- Always balance chemical equations before writing the final answer.
- Do not forget to mention whether a catalyst is positive (increases rate) or negative (decreases rate).
| Image Reference | Where it is used |
|---|---|
| img_15_01_change_examples_collage | Collage of everyday changes for the chapter introduction |
| img_15_02_combination_reaction_model | Model of a combination reaction: A + B → C |
| img_15_03_decomposition_reaction_model | Model of a decomposition reaction: AB → A + B |
| img_15_04_single_displacement_model | Model of a single displacement reaction: AB + C → A + BC |
| img_15_05_double_displacement_model | Model of a double displacement reaction: AB + CD → AD + CB |
| img_15_06_photosynthesis_light_reaction | Photosynthesis as a light-dependent chemical reaction |
| img_15_07_copper_sulphate_iron_nail_apparatus | Lab set-up for the displacement reaction between copper sulphate and iron |
| img_15_08_surface_area_rate_comparison | Comparing reaction rate: whole tablet vs powdered tablet in water |
Quick Revision
- Chemical reaction = reactants change into products.
- Combination: A + B → C
- Decomposition: AB → A + B
- Single displacement: AB + C → A + BC
- Double displacement: AB + CD → AD + CB
- Acid + Base → Salt + Water (neutralization, a type of double displacement)
- Rate of reaction = change of reactants to products per unit time.
- Factors affecting rate: catalyst, heat, pressure, surface area, light.
- Positive catalyst increases rate; negative catalyst decreases rate.