Chemistry

Enthalpy


As we have learned, enthalpy (H) is the amount of energy found in substances that can be altered through chemical reactions. From now on, we will study some types of enthalpy.

Enthalpy Types

Through some reactions, it is possible to calculate the value of the enthalpy variation.

- Enthalpy of formation
- Enthalpy of combustion
- Enthalpy of binding
- neutralization enthalpy
- Enthalpy of dissolution

Enthalpy of formation or heat of reaction

The enthalpy of formation is the energy of the reaction when it forms 1 mol substance from chemicals (element in its standard state).

Default state: is the most stable form of a substance at 25 ° C and 1atm pressure. They are the simple substances.

Substances participating in the formation reaction should be simple and should inform the physical state. Its standard formation enthalpy range is zero.

Example of simple substance:

Ç(graphite), O2 (g), N2 (g), H2 (g), At(s), S(s).

Example of formation reaction:

This means that to form 1 mol of NH3, the reaction produces 11 kcal of energy.

This calculation can be done using the enthalpy variation formula and some tabulated data.

Note the table with the standard formation enthalpy values ​​of some substances:

Substance

H ° f kJ / mol

Substance

H ° f kJ / mol

Ç2H2(g)

226,8

ÇDiamond

+2,1

CH4(g)

-74,8

NH3 (g)

-45,9

CO (g)

-110,3

NaCl (s)

-412,1

CO2(g)

-393,3

O3 (g)

+143

H2O (v)

-242

ONLY2 (g)

-297

H2O (l)

-286

ONLY3 (g)

-396

Example:

Write the formation reaction for each substance below, indicating the value of the SO formation enthalpy3(g):

1 °) Assemble forming reaction:

2 °) Apply the formula: