What is Enthalpy?

Enthalpy is the heat absorbed by the system at constant pressure. Why do we need this thermodynamic function?  What is its significance? Enthalpy is introduced to study profit and loss of heat of reactions those are carried out at constant pressure. In this post we will study how you can make a balance sheet of a reaction with the help of enthalpy of reactants and products and predict the heat status of a reaction, whether it will be endothermic or exothermic.

What is the Enthalpy of a reaction?

You have learnt the first law of thermodynamics in previous post. Let’s write it for an expansion reaction at constant pressure:

      

      ΔU = q + w

Now consider the previous example of work done on ideal gas by piston. This time we keep pressure constant. What will happen now? Gas will expand. Work will be done by the gas so it will get negative sign. Let’s write the equation for expansion reaction at constant pressure:

            ΔU = q - w

            ΔU = q_p - pΔV

            U₂- U₁ = q_p – p(V₂- V₁)

            q_p = U₂- U₁ + p(V₂- V₁)

            q_p = (U₂+ pV₂)-(U₁ + pV₁)

Enthalpy is the heat absorbed by the system at constant pressure. It is denoted by symbol “H”.

            q_p = (H₂)-(H₁)

            q_p = ΔH

           ΔH = ΔU + p ΔV

How is enthalpy different from heat?

You have seen that enthalpy is a special kind of heat (heat absorbed at constant pressure q_p). Heat depends on the conditions of the reaction. When reaction occurs at constant volume is becomes q_v and it becomes q_p when reaction occurs at constant pressure. That means it depends on the path taken by the system. So it is a path dependent function.

Enthalpy depends on internal energy, pressure and volume. Internal energy U, pressure p and volume V are all state functions and so is the enthalpy H.

What is enthalpy change of a reaction?

You can get the enthalpy change of a reaction by subtracting the sum of enthalpy of reactants from the sum of enthalpy of products.

aA + bB ⟶ cC + dD

ΔH_reaction = (cH_mC+ dH_mD) - (aH_mA+ bH_mB)

Where H_m is the molar enthalpy. Molar enthalpy is the heat of one mole of substance at constant pressure.

H_m = (q / n) _p

Where n is the number of moles.

ΔH_reaction = ƩH_products– ƩH_reactants

Enthalpy of a reaction gives us important information about the reaction. If it is positive it means that the reaction will be endothermic and negative enthalpy results in exothermic reaction.

ΔH_reaction = ‘-ve’ ⟶ ƩH_reactants > ƩH_products⟶products liberate excess of heat = exothermic reaction.

ΔH_reaction = ‘+ve’ ⟶ Ʃ_Hreactants < ƩH_products ⟶ reactants absorb heat to get converted into products = endothermic reaction.

Now you can predict the effect of temperature change to the reaction. And by applying Le-Chatelier’s principleyou can adjust the yield of products.

Exothermic and Endothermic Reaction

Extensive and Intensive Properties

Enthalpy of a reaction depends on the number moles of reactants and products. It means its value depends on the quantity of matter present in the system; it isn’t the property of the system. Such properties are called extensive properties, for example enthalpy; internal energy, mass, volumes, heat capacity all are extensive properties.

Properties which are independent of quantity or size of system are known as intensive properties e.g. Temperature, density, pressure.

But when you divide an extensive property by number of moles then it becomes the property of a mole of substance which is independent of size or amount of the matter.

Extensive property/ n = Intensive property

For example enthalpy H is an extensive property while molar enthalpy H_m is an intensive property. Heat capacity is an extensive property and molar heat capacity is an intensive property.

Now you can understand why we use molar enthalpy to calculate the enthalpy of a reaction. Although enthalpy of reaction is an extensive property which varies with the amount of matter and reaction conditions but by putting some special conditions we can get the standard value of enthalpy of reaction.

In the next post we will study standard enthalpy of reaction of different kind of reactions, learn about enthalpy of formation and see how we can calculate heat of reaction with the help of enthalpy of formation.

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