Consider the following reactions. (Note: (s) = solid, () = liquid, and (g) = gas.)

V2H2 (g) + 1212 (9) - HI (g), AH = + 6.2 kcal/mole

21.0 kcal/mole + C (s) + 2S (s) - CS2 (1)

What type of reaction is represented by the previous two examples?

endothermic

exothermic

Both are endothermic reactions.

Explanation:

Chemical equation:

1/2H₂ (g) + 1/2I₂ (g) → HI (g) + 6.2 kcal/mol

Chemical equation:

21.0 kcal/mol + C (s) + 2S (s) → CS₂

Both reaction are endothermic because heat is added in both of reactions.

Endothermic reactions:

The type of reactions in which energy is absorbed are called endothermic reactions.

In this type of reaction energy needed to break the bond are higher than the energy released during bond formation.

For example:

C + H₂O → CO + H₂

ΔH = + 131 kj/mol

it can be written as,

C + H₂O + 131 kj/mol → CO + H₂

Exothermic reaction:

The type of reactions in which energy is released are called exothermic reactions.

In this type of reaction energy needed to break the bonds are less than the energy released during the bond formation.

For example:

Chemical equation:

C + O₂ → CO₂

ΔH = - 393 Kj/mol

it can be written as,

C + O₂ → CO₂ + 393 Kj/mol