A chemical reaction is a process in which one or more substances (reactants) change into new substances (products), with different properties. Chemical reactions can be grouped into four main types based on how the reactants change into products.
| N₂ + 3H₂ → 2NH₃ |
| C + O₂ → CO₂ |
| C + 2H₂ → CH₄ |
| 2Na + Cl₂ → 2NaCl |
| Fe + S → FeS |
| CaO + H₂O → Ca(OH)₂ |
| CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ |
| 2Mg + O₂ → 2MgO |
| CaCO₃ →(Δ)→ CaO + CO₂ |
| 2KClO₃ →(Δ)→ 2KCl + 3O₂ |
| CuCO₃ →(Δ)→ CuO + CO₂ |
| 2Ag₂O →(Δ)→ 4Ag + O₂ |
| 2HgCO₃ →(Δ)→ 2Hg + 2CO₂ + O₂ |
| 2Pb(NO₃)₂ →(Δ)→ 2PbO + 4NO₂ + O₂ |
| 2Cu(NO₃)₂ →(Δ)→ 2CuO + 4NO₂ + O₂ |
| 2AgNO₃ →(Δ)→ 2Ag + 2NO₂ + O₂ |
| Zn + 2HCl → ZnCl₂ + H₂ |
| Zn + H₂SO₄ → ZnSO₄ + H₂ |
| 2KBr + Cl₂ → 2KCl + Br₂ |
| 2KI + Cl₂ → 2KCl + I₂ |
| Zn + CuSO₄ → ZnSO₄ + Cu |
| Mg + ZnCl₂ → MgCl₂ + Zn |
| Fe + CuSO₄ → FeSO₄ + Cu |
| Cu + 2AgCl → CuCl₂ + 2Ag |
| AgNO₃ + NaCl → NaNO₃ + AgCl |
| CaCl₂ + 2AgNO₃ → Ca(NO₃)₂ + 2AgCl |
| HgCl₂ + 2KI → 2KCl + HgI₂ |
| Pb(NO₃)₂ + Na₂SO₄ → PbSO₄ + 2NaNO₃ |
| FeCl₂ + 2NaOH → Fe(OH)₂ + 2NaCl |
| HCl + NaOH → NaCl + H-OH |
| H₂SO₄ + CaO → CaSO₄ + H₂O |
| H₂SO₄ + 2NaOH → Na₂SO₄ + 2H-OH |
| 2HCl + CuO → CuCl₂ + H₂O |
| 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H-OH |
| H₂SO₄ + 2KOH → K₂SO₄ + 2H-OH |
Note: water is written as H-OH (instead of H₂O) in the table above only to show, by colour, that its H comes from the acid and its OH comes from the base — both forms mean exactly the same molecule, H₂O.
In an acid–base reaction, both the acid and the base lose their individual properties. The result is a neutral substance — salt and water. This loss of acidic and basic character is why the reaction is called neutralization.
The positive change in the concentration of a reactant or a product per unit time is called the rate of chemical reaction. It is calculated using the given formula.
Rate of chemical reaction = Change in concentration of a reactant or a product ÷ Time taken for the change
The SI unit of the rate of chemical reaction is moles per litre per second (mol/l·s).
Various factors affect the rate of chemical reaction. Some major factors are:
| Factor | Effect on Rate of Reaction |
|---|---|
| Concentration | Higher concentration → more collisions → faster reaction |
| Temperature | Higher heat → more kinetic energy → faster reaction |
| Catalyst | Speeds up (positive) or slows down (negative) the reaction; itself unchanged |
| Physical nature | Larger contact area / common solvent → faster reaction |
| Chemical nature | Ionic compounds react faster than covalent compounds |
| Surface area | Powdered form reacts faster than solid lumps |
| Pressure | Higher pressure → gas molecules packed closer → faster reaction |
| Solution | Ions form only in solution, enabling many reactions |
| Light | Some reactions (e.g. photosynthesis) occur only with light energy |
| Electricity | Can split compounds, e.g. electrolysis of water |
| Contact | Direct contact between reactants can speed up a reaction |
The rate of chemical reaction increases on increasing the concentration of reactants. It is because increasing the concentration of reactants increases the number of reacting molecules. There will be more collision between the reacting molecules, and hence the rate of chemical reaction increases.
Heat increases the thermal energy and kinetic energy of reactants. This increases the probability of collisions between reactants molecules. An increase in temperature increases the rate of a chemical reaction, whereas a decrease in temperature decreases the rate of the chemical reaction.
For example,
2KClO₃ →(Δ)→ 2KCl + 3O₂
A catalyst is a chemical substance which increases or decreases the rate of chemical reaction but itself remaining chemically unchanged. There are two types of catalysts.
2H₂O₂ →(MnO₂ catalyst)→ 2H₂O + O₂↑
2H₂O₂ →(Glycerine)→ 2H₂O + O₂↑
Phosphoric acid is called a negative catalyst during laboratory preparation of oxygen gas without heating because when phosphoric acid is mixed with hydrogen peroxide, oxygen gas is produced slowly. It decreases the rate of reaction.
Examples of positive catalyst are: MnO₂, V₂O₅, Fe, etc.
Examples of negative catalyst are: H₃PO₄, H₂O₂ mixtures, glycerine, etc.
The reaction takes place in the contact area of the reacting molecules. So the rate of chemical reaction can be increased by increasing the area of contact between the reactants. Similarly, the rate of chemical reaction increases by using a common solvent if the reactants are not soluble in one another.
Chemical nature of reactants also determines the rate of chemical reaction. Some reactants are more active and some are less reactive. The elements of group IA and VIIA are more reactive than the elements of other groups. Similarly, the reactions between ionic compounds are faster than the reactions between covalent compounds.
The surface area of a reactants can affect the rate of a chemical reaction. If surface area of reactants increases, it increases the rate of collisions between reactant molecules and increases the rate of reaction. For example, powdered calcium carbonate reacts faster with hydrochloric acid to produce carbon dioxide. Powdered vitamin C capsules react faster than solid capsules.
Pressure can affect the rate of a chemical reaction if at least one of the reactants is a gas. When pressure is increased, the gas molecules are compressed and more densely packed. It increases the frequency of collisions between gas molecules and possibly increases the rate of reaction.
For example, the synthesis of ammonia by Haber's process needs high pressure, i.e., about 200-500 atmospheric pressure.
N₂ + 3H₂ →(200-500 atm, 600°C, Fe/Mo)→ 2NH₃↑
Acid doesn't show any effect without water. Similarly, salt doesn't give ions without water. This shows that a solution also plays an important role in many chemical reactions.
When silver nitrate and calcium chloride are brought in contact, they do not show any reaction. But if they are dissolved in water, they give corresponding ions; and after exchanging ions, products are formed.
AgNO₃(aq) + NaCl(aq) → NaNO₃(aq) + AgCl↓
2AgNO₃(aq) + CaCl₂(aq) → Ca(NO₃)₂(aq) + 2AgCl↓
Some reactions occur in the presence of light only. For example, decomposition of silver bromide and hydrogen peroxide. Light energy increases the kinetic energy of molecules of reactants and helps them to react faster. Photosynthesis occurs only in the presence of sunlight.
6CO₂ + 6H₂O →(Chlorophyll, Light)→ C₆H₁₂O₆ (Glucose) + 6O₂
Electricity is also one of the important factors that brings about chemical reaction. When electricity is passed in acidified water, it decomposes and gives hydrogen (H₂) and oxygen (O₂) gas. This is called Electrolysis.
H₂O →(Electricity)→ 2H₂ + O₂
Some chemical reactions occur faster when reactants come in direct contact. Sodium comes in contact with water to produce sodium hydroxide and hydrogen gas.
2Na + 2H-OH → 2NaOH + H₂
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