There isn't a single specific formula for endothermic or exothermic reactions. Instead, the distinction lies in the energy change during the reaction.

Here's how we differentiate them:

Endothermic Reaction:

* Energy is absorbed from the surroundings. This means the system gains energy, and the surroundings lose energy.

* The enthalpy change (ΔH) is positive.

* Feels cold to the touch. (The reaction absorbs heat from its surroundings, making the surroundings feel colder.)

Exothermic Reaction:

* Energy is released to the surroundings. This means the system loses energy, and the surroundings gain energy.

* The enthalpy change (ΔH) is negative.

* Feels hot to the touch. (The reaction releases heat into its surroundings, making the surroundings feel hotter.)

Key Points:

* Enthalpy change (ΔH): This is the most important factor in determining whether a reaction is endothermic or exothermic. A positive ΔH indicates an endothermic reaction, and a negative ΔH indicates an exothermic reaction.

* Energy flow: Think about the direction of energy flow - from surroundings to system (endothermic) or from system to surroundings (exothermic).

Examples:

* Endothermic: Melting ice, photosynthesis, dissolving ammonium chloride in water.

* Exothermic: Burning wood, combustion, neutralization reaction (acid + base).

Remember: The specific formulas for the reactions themselves depend on the chemical species involved. The key is to understand the energy change and its impact on the surroundings.