1rst law of thermodynamics for chemical reactions

 For short, for a set of reactants to be transformed into products certain amount of energy is required. If such a reaction needs to be preformed in a series of steps then the summation of the energy spent for each step equals that as if the reaction were made in a single stage.

As you may see, the above is in fact in agreement with the first law of thermodynamics which is also stated as: the energy can neither be created nor destroyed - only converted from one form of energy to another.

Heat of reaction (enthalpy)

As mentioned before, conversion of reactants into products require energy. This energy is called the heat of reaction and is related to enthalpy changes. Also, as you may forsee these enthalpy changes are necessarily related to a thermodynamic process at constant volume because of its definition itself,

$\Delta H= Q_P$        Eq. (01)

or

$\Delta H = \Delta U + P\Delta V$        Eq. (02)

See: Processes at constant volume and constant pressure.

In Eq. (01), $Q_P$ represents the thermal energy at constant pressure. You should remember that Eqs. (02) are derived for an isobaric process (constant pressure). On the other hand, since the chemical reaction is linked to a change in energy $\Delta H$ this idea applies, equally, for endothermal or exothermal reactions. The only difference one would notice in $\Delta H$ is the sign.

The idea of a change of enthalpy $\Delta H$ comes from the need to understand and quantify the energy required for the reaction to occur. This the base for more complex subjects such as: reactor design and chemical process simulations.

The enthalpy applied for reactants and products in a chemical reaction

For short, reactants will have a given temperature $T_i$ and pressure $P$ at which they do not react but may do so if certain amount of energy is applied. Reactants will be stable with their initial enthalpy $H_{initial}$.

However, if energy is supplied in the form of heat so that the temperature changes to $T_2$ the enthalpy will vary as well to $H_{final}$. Pressure remains constant or almost constant, which is a typical condition for several chemical reactions.

Fig. 1 Schematics of enthalpy as related to chemical reactions

The next step in our reasoning is to consider that the amount of thermal energy supplied $Q_P$ is the one required for the reactants to transform into products. Of course, this is already expressed in Eq. (01).

Remember that neither $H_{initial}$ nor $H_{final}$ are useful by themselves alone since these cannot be measured. Only $\Delta H$ have a thermodynamical sense.

If we go deeper into the Reactants and Products of the sketch in Fig. 1 we will find that every reactant and product have its own enthalpy $H$. In other words, $\Delta H$ is the result of the contribution of these little parts (this assertion needs to be demonstrated).

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• Some basic concepts on thermodynamics
• Who was Clausius?
• Processes at constant volume and pressure
• Composition variables in mixtures
• The first law of thermodynamics and some concepts
• Partial and total derivatives (for thermodynamics)
• About pilot lights
• Solving the Colebrook equation

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Ildebrando.