Q.What do you understand by exothermic reaction and endothermic reaction? Give one example of each type.

Concept: Energy change during a chemical reaction — reactions either release heat to the surroundings (exothermic) or absorb heat from the surroundings (endothermic).

An exothermic reaction is one in which heat is given out to the surroundings. The enthalpy of the products is lower than that of the reactants, so $\Delta H$ is negative. The surroundings feel hotter.

Example: Burning of methane — $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} + \text{heat}$. …

The Concept: Energy Flow in Chemical Reactions

Every chemical bond stores energy. When bonds break, energy is absorbed; when new bonds form, energy is released. The net energy change — whether the reaction gives energy to the surroundings or takes it away — determines if it's exothermic or endothermic.

Think of a reaction as a transaction between the system (the chemicals) and the surroundings (everything else). If the system loses heat to the surroundings, you feel warmth — that's exothermic. If the system gains heat from the surroundings, you feel cooling — that's endothermic.

The quantity that measures this heat exchange at constant pressure is enthalpy ($H$). The change in enthalpy, $\Delta H$, tells the story:

• If $\Delta H < 0$: products have less enthalpy than reactants → heat is released → exothermic.
• If $\Delta H > 0$: products have more enthalpy than reactants → heat is absorbed → endothermic.

Step-by-Step Breakdown

1. Exothermic Reaction: Heat Flows Out

In an exothermic reaction, the total energy stored in the products is less than that in the reactants. The "extra" energy is released to the surroundings, often as heat, light, or sound.

Key signs you'd observe:

• The container feels hot.
• Temperature of the surroundings rises.
• $\Delta H$ is negative.

Example: Combustion of Methane

When methane ($CH_4$) burns in oxygen, it forms carbon dioxide and water. The reaction is:

The bonds in $CO_2$ and $H_2O$ are more stable (lower energy) than those in $CH_4$ and $O_2$. The difference in bond energy is released as heat — enough to cook food or heat a home.

2. Endothermic Reaction: Heat Flows In

In an endothermic reaction, the products have more stored energy than the reactants. That extra energy must come from somewhere — it's absorbed from the surroundings.

Key signs you'd observe:

• The container feels cold.
• Temperature of the surroundings drops.
• $\Delta H$ is positive.

Example: Photosynthesis

Plants use sunlight to convert carbon dioxide and water into glucose and oxygen:

$6CO_2(g) + 6H_2O(l) \xrightarrow{\text{sunlight}} C_6H_{12}O_6(s) + 6O_2(g)$ …