Thermochemistry

In general, chemical reactions are accompanied by energy change which is in the form of evolution or absorption of heat. The branch of chemistry which deals with energy change in chemical reactions is called thermochemistry. The chemical equations which represent the heat change involved in the reaction is called thermochemical equation.

E. g.

$C + O_2 \to CO_2 + 94.3 kcal$

This equation indicates that 12 g of carbon reacts with 32 g of oxygen to produce 44 g of carbon dioxide and 94.3 kcal of heat is evolved.

All substances are associated with certain amount of energy which is liberated as heat during chemical change. This energy is called internal or intrinsic energy. It may be defined as, “The additional amount of energy in calories which one mole of the compound possesses above that of its constituent elements”.

Thus, for above equation;

$E_C +E_{O_2} = E_{CO_2} + 94300 cal \\[3mm] E_{CO_2} = \Delta H = -94300 cal$

Because for convenience, the intrinsic energies of the elements are arbitrarily fixed as zero i.e. $E_C = 0 \text{and} E_{o_2}= 0$.

Since in the chemical reactions the heat may be evolved or absorbed, hence they are termed as exothermic or endothermic reactions respectively.

These terms may be defined as;

Exothermic Reactions: Exothermic reactions are those chemical reactions which are accompanied by the evolution of heat, and the compounds so formed are (known as exothermic compounds.

E.g.

$C + O_2 \to CO_2 + 94,300 cal \\[3mm] Cu + S \to CuS + 11,600 cal$

The above reactions are exothermic reactions and $CO_2$ and CuS are known as exothermic compounds.

Exo-thermic reaction

$\Delta H = -ve$                             $\Delta H = +ve$

$A_S H_R > H_P$                         $A_SH_R < H_P$

Exothermic                                                       Endothermic

Endothermic Reactions: Endothermic reactions are those chemical reactions which are accompanied by the absorption of heat, and the compounds so formed are known as endothermic compounds e.g.

$C+ 2S \to CS_2- 22,000 cal \\[3mm] 2C+ H_2 \to C_2H_2- 59,200 cal$

These reactions are endothermic reactions, and $CS_2$ and $C_2H_2$ are known as endothermic compounds.

In terms of heat content the above reactions may be represented as :

$C + O_2 \to CO_2 \hspace{2mm} \Delta H = =-94300 cal$

$Cu + S \to CuS \hspace{2mm} \Delta H = -11600 cal$

$C + 2S \to CS_2 \hspace{2mm} \Delta H = + 22000 cal$

$2C + H_2 \to C_2 H_2 \hspace{2mm} \Delta H = + 59200 cal$

If $H_P \text{and} H_R$ are the heat contents of products and reactants, respectively, then $\Delta H = H_P- H_R$. Thus For exothermic reactions$H_R > H_p or \Delta H$ is negative . For endothermic reactions $H_R < H_P or \Delta H$ is positive.

Heat of Reaction: It may be defined as “The quantity of heat evolved or absorbed when the numbers of moles of substances expressed in the chemical equation have completely reacted.”

E. g.

$C + O_2 \to CO_2 \hspace{2mm} \Delta H = -94.3 kcal$

$C + 2S \to CS_2 \hspace{2mm} \Delta H = +22.0 kcal$

The heat of reaction is effected by the temperature, physical state of reactants and products and whether the reaction is carried out at constant pressure or at constant volume, enthalpy of solution etc.

(i)    Physical state of reactants and products:

E.g.

$H_2 + \dfrac{1}{2} O_2 \to H_2 O + 68.32 kcal \\[3mm] H_2 + \dfrac{1}{2} O_2 \to H_2 O + 57.8 kcal$

I.e., $\Delta H$ = 57.8 – 68.32 -10.52 kcal

I.e., When steam condenses heat is evolved.

(ii)                Enthalpies of solution :

$H_2S(gas) + I_2 (gas) \leftrightharpoons 2HI + S + 17.2 kcal \\ H_2S (g) + I_2 (solution) \leftrightharpoons 2HI (Solution) + S + 21.92 kcal$

(iii)               Allotropic forms:

$\underset{Amorphous}{C} + \underset{g}{O_2} \to \underset{g}{CO_2} + 97.6 kcal \\[3mm] \underset{Diamond}{C} + \underset{g}{O_2} \to CO_2 + 94.5 kcal$

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