Enthalpies For Different Types Of Reactions

The standard enthalpy of different reactions change when reactants are converted in their respective standard states (where p equals 1 bar, whereas T=298K) into products in their standard conditions. It refers to the standard heat of reaction measured at constant temperature and pressure. It can be estimated with calorimetric methods despite the change in temperature throughout the measurement process, given that the initial pressure and temperature are equal to the final pressure and temperature. 

Introduction To Different Types Of Enthalpy In Thermodynamics

Enthalpy can be calculated by adding the internal energy and the product of the system’s pressure and its volume. It is also termed heat content. This means –

H = E + P.V

Here, E= Internal energy of the system

P= Pressure of the system

V= Volume of the system

The absolute enthalpy value cannot be calculated because it is a state function. On the other hand, a change in enthalpy (ΔH) associated with a process can be accurately measured and is represented by the following formula.

ΔH = Hproducts–Hrectants

= Hp–Hr

Suppose ΔV is the change in volume in a reaction at constant temperature and pressure. In that case, the difference in enthalpy will equal the sum of the internal energy change (ΔE) and the work done in expansion or contraction. 

ΔH = ΔE + P.ΔV

Importance of Enthalpy

The principle of enthalpy is crucial for every chemical reaction that requires temperature and pressure. To calculate the overall amount of heating and cooling needed for commercial production of any material, we need to understand the enthalpy.

For example, we need to know the enthalpy of mass-produced substances like ammonia, oxygen, or calcium carbonate. So, how do we find out the enthalpy change for any reaction?

Change in Enthalpy

The standard enthalpy of formation equals enthalpy change. For several substances, we’ve determined this out. The reactants go through several chemical changes and get combined to generate different products in each particular chemical reaction.

The change in enthalpy is represented as ΔrH for any such reaction.  It is known as the reaction enthalpy. By deducting the sum of all the reactant’s enthalpies from the enthalpies of the given products, we can find out about the reaction of enthalpy. Mathematically,

ΔrH = Sum of enthalpies of products –Sum of reactant’s enthalpies of reactants 

Above is the summary of what enthalpy is and how it works. However, our primary goal in the chapter is about learning the enthalpies of various reactions. Let’s have a look at them now.

Different Types Of Enthalpies For Reactions

We’ll look at enthalpies for different types of reactions in the sections below.

Enthalpy of Formation

It can be defined as the amount of heat emitted or absorbed when one mole is formed from its constituent elements. It can be expressed as ΔfH, for example.

It’s significant to remember that the thermochemical equation should be balanced such that it only represents the creation of a single mole of the substance. The heat of formation at 298 K and 1 atm pressure is the standard heat of formation. In the case of the free state of elements, we take it to be zero.

Enthalpy of Combustion

We can define it as the amount of heat produced when one mole of a substance is fully oxidised. For example, (ΔH = – ve) is always exothermic.

The heat of combustion is used to compute formation heat, which can be challenging in some circumstances, such as when calculating the calorific value of fuels. Combustion heat can also clarify or explain the structure of certain organic compounds.

Enthalpy of Solution

We can define it as the amount of heat emitted or absorbed when one mole of a solute completely dissolves in a significant amount of water. Further dilution of the solution does not affect the amount of heat emitted or absorbed. Example-

Enthalpy of Neutralisation

The amount of heat released when one equivalent (or equivalent mass) of acid in a dilute solution is entirely neutralised by one match (or equal mass) of a base. e.g.

The heat of neutralising a solid acid and a strong base is calculated to be 13.7 kcal or 57 kJ. This heat of neutralisation can be explained using the electrolytic dissociation theory, which states that it is just the heat of water production from H+ of an acid and OH- of a base.

Some weak acids or weak bases have a heat of neutralisation lower than 13.7 kcal. This is because dissociating the weak electrolyte consumes some energy. The difference in the measurements can be used to calculate the dissociation energy of a weak acid or basic.

The dissociation of two weak electrolytes requires 1.8 kcal of heat. In the lab, we can use polythene or polystyrene bottles to monitor the heat of neutralisation.

Enthalpy of Dissociation

The quantity of heat absorbed when one mole of a substance completely dissociates into its ions is the enthalpy of dissociation. Example-

Dilution Enthalpy

The amount of heat emitted or that is absorbed at a time when a one mole containing solution gets diluted from a given concentration to another is known as the enthalpy of dilution.

The heat of dilution =

Precipitation Enthalpy

The amount of heat released in one mole’s precipitation in a scarcely soluble substance when appropriate electrolytes’ diluted solutions are mixed is known as enthalpy of rainfall. Example-

Enthalpy of Hydration

The quantity of heat emitted or absorbed is specified as the enthalpy of hydration. When one mole of an anhydrous or partially hydrated salt is mixed with the appropriate number of moles of water to create a specified hydrated product. Example-

Fusion Enthalpy

The enthalpy change during the conversion of a substance’s 1 mole from the solid state into the liquid state when at melting point is known as enthalpy of fusion (mostly endothermic).

Vaporisation Enthalpy

The enthalpy change during the conversion of a substance’s 1 mole from the liquid state to gas when at boiling point is known as enthalpy of vaporisation.

In conclusion

the enthalpy of a system is equal to the sum of its internal energy and the product of its pressure and volume. The absolute value of enthalpy cannot be established because it is a function of the state. On the other hand, a change in enthalpy (ΔH) associated with a process can be precisely measured. Enthalpy or enthalpy changes accompanying chemical reactions are expressed in a variety of ways, depending on the nature of the reaction.