Question

How are enthalpy changes expressed in chemical equations?

Answer 1

First of all we will understand about enthalpy and according to the definition of enthalpy, it is a kind of energy which is involved in the chemical reactions in the form of heat energy.

Complete answer:
As we know that enthalpy is a heat energy which is involved in every chemical reactions and when there is absorption or removal of energy takes place in any chemical reaction, and the term used for expressing that process of changing in heat energy or enthalpy is “change in enthalpy”.
-Enthalpy is denoted by the symbol ‘${\text{H}}$’ and change in enthalpy is denoted by the symbol ‘$\Delta {\text{H}}$’.
-When in any chemical reactions heat energy is absorbed from the surroundings then value of change in enthalpy ($\Delta {\text{H}}$) is negative i.e. $\Delta {\text{H = - ve}}$.
-When in any chemical reactions heat energy is released to the surrounding then value of change in enthalpy ($\Delta {\text{H}}$) is positive i.e. $\Delta {\text{H = + ve}}$.
Change in enthalpy for any chemical reaction is written as follow:
Suppose 2 moles of carbon monoxide reacts with one mole of oxygen gas for the formation of 2 moles of carbon dioxide and energy released during this process is ${\text{245kJ}}$, so chemical reaction and change in enthalpy is written as below:
${\text{2CO + }}{{\text{O}}_{\text{2}}} \to {\text{2C}}{{\text{O}}_{\text{2}}}{{\Delta H = - 245kJ}}$

Note:
Here some of you may think that negative sign will convey the meaning of removal of energy, but that is not true because negative sign shows the addition of energy from surroundings. So never get confused with this concept.