Exothermic reaction and Endothermic process and Exothermic process and Endothermic reaction
1. Exothermic Reaction
An exothermic reaction is a chemical reaction in which energy is released to the surroundings. This energy is mostly released as heat, but sometimes also as light, sound, or electricity.
Why does heat come out?
During a chemical reaction, old bonds break and new bonds form.
Breaking bonds requires energy
Forming new bonds releases energy
In an exothermic reaction, the energy released while forming new bonds is greater than the energy used to break old bonds.
The extra energy is released as heat.
Effect on surroundings:
Temperature of surroundings increases
Reaction mixture feels hot
Examples of exothermic reactions:
Burning of coal, petrol, LPG
Reaction between acids and bases (neutralization)
Respiration in living organisms
Combustion of magnesium ribbon
Example equation:
CH₄ + 2O₂ → CO₂ + 2H₂O + heat
2. Reactions of Metals with Water (In Detail)
Metals react with water depending on their position in the reactivity series.
General reaction:
Metal + Water → Metal hydroxide + Hydrogen gas
(a) Highly Reactive Metals (Sodium, Potassium)
React violently with cold water
Reaction is exothermic
Hydrogen gas may catch fire
Example (Sodium):
2Na + 2H₂O → 2NaOH + H₂ + heat
Observations:
Sodium melts into a ball
Moves rapidly on water
Produces fizzing sound
Sometimes burns with yellow flame
(b) Moderately Reactive Metals
Calcium:
Reacts slowly with cold water
Ca + 2H₂O → Ca(OH)₂ + H₂
Magnesium:
Very slow with cold water
Faster with hot water
Reacts strongly with steam
Mg + H₂O (steam) → MgO + H₂
Iron:
Does not react with cold or hot water
Reacts with steam only
3Fe + 4H₂O (steam) → Fe₃O₄ + 4H₂
(c) Low Reactive Metals
Copper, silver, gold
Do not react with water
3. Reactions of Metals with Acids (In Detail)
Most metals react with dilute acids to produce a salt and hydrogen gas.
General reaction:
Metal + Acid → Salt + Hydrogen gas
Example reactions:
Zn + HCl → ZnCl₂ + H₂ Mg + H₂SO₄ → MgSO₄ + H₂
Important points:
Reaction is usually exothermic
Faster for more reactive metals
Hydrogen gas is tested using a burning splint (pop sound)
Exceptions:
Copper, silver, gold do not react with dilute acids
Nitric acid usually does not produce hydrogen gas
4. Exothermic Process (Very Detailed)
An exothermic process is any physical or chemical process in which heat is released to the surroundings.
Difference from reaction:
A reaction changes substances chemically
A process may be physical or chemical
Examples of exothermic processes:
(a) Combustion
Burning of fuels releases large amounts of heat
(b) Respiration
Glucose reacts with oxygen in cells
Energy is released for life activities
(c) Freezing of water
Water releases heat while turning into ice
(d) Condensation
Steam releases heat when it becomes water
5. Endothermic Process (Very Detailed)
An endothermic process is one that absorbs heat energy from its surroundings.
Why heat is absorbed:
Energy is required to overcome forces between particles
Heat is taken from surroundings
Effect:
Surroundings become cooler
Temperature decreases
Examples:
(a) Melting of ice
Ice absorbs heat to become water
(b) Evaporation
Water absorbs heat to change into vapor
(c) Sublimation
Solid directly turns into gas (e.g., camphor)
6. Endothermic Reactions (In Detail)
An endothermic reaction is a chemical reaction that absorbs energy from the surroundings.
Energy explanation:
More energy is needed to break bonds than released during bond formation
Extra energy is absorbed as heat
Example:
CaCO₃ + heat → CaO + CO₂
Other examples:
Photosynthesis
Thermal decomposition reactions
Characteristics:
Temperature of surroundings decreases
Heat is written on the reactant side of equation