Enthalpy Change Formula: Definition, Methods, Solved Example

Enthalpy Change Formula: Enthalpy serves as a measure of the energy associated with chemical reactions within a thermodynamic system. It quantifies the total heat content within the system and is expressed as the sum of the system’s internal energy, product of volume, and pressure. Enthalpy provides insight into the internal energy required to establish a system. This article aims to facilitate students’ comprehension of enthalpy and its formula through illustrative examples.

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Enthalpy Formula

In the study of certain chemical reactions, it is crucial to grasp the concepts of enthalpy and standard enthalpy. Calculating the total internal energy of a system can be challenging, but it is possible to assess the changes related to heat transfer. Consequently, the enthalpy of a reaction is denoted as ΔH, where the symbol Δ signifies a change.

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The Formula for Enthalpy Change

When a process initiates at a constant pressure, heat is either absorbed or released, and this quantity is equivalent to the change in enthalpy. Therefore, the change in enthalpy is the summation of internal energy (E) and the product of volume and pressure, represented as P × V.

H = E + PV

Enthalpy is also described as a state function, dependent on state variables such as pressure (P), temperature (T), and internal energy (E). The change in enthalpy, ΔH, between initial and final states, corresponds to the heat flow (q) under constant pressure, as per the equation:

ΔH = q

The relationship between q and ΔH distinguishes between endothermic and exothermic reactions. In endothermic reactions, heat is absorbed, indicating a positive q (>0), while exothermic reactions release heat to the surroundings, yielding a negative q (<0).

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Calculating ΔH for a Chemical Reaction

There are various methods for computing ΔH:

Method 1: When the work done by or on a system is zero, and the container’s volume remains constant, the change in enthalpy equals the heat transfer (q). This relationship is expressed as:

q = m × s × ΔT

In this equation, m represents mass, s is specific heat, and ΔT signifies the temperature change.

Method 2: For known reactions, a table of heat change values, denoted as ΔHf (heat of formation), can be employed to calculate ΔH. ΔHf quantifies the heat required to form a substance from its constituent elements. Thus, the formula is:

ΔH_reaction = ∑ΔHf(products) – ∑ΔHf(reactants)

Method 3: Hess’s Law proves valuable in determining the enthalpy of a reaction.

Method 4: ΔH_reaction can also be computed using bond energies of reactants and products, represented as:

ΔH_reaction = ∑ΔH_bonds_broken – ∑ΔH_f(bonds_formed)

Solved Examples on Enthalpy Formula

Q.1: Calculate the heat of the following reaction using the table of values.

C + O2 → CO2

Substance              ΔHf (kJ per mole)

C                               0

O2                             0

CO2                         -393.5

Solution: ΔHf represents the heat of formation, signifying the energy needed to form a substance from its elemental components. In this context, ΔHf for C and O2 is 0 since they exist in their elemental forms.

Therefore,

ΔH_reaction = ∑ΔHf(products) – ∑ΔHf(reactants)

ΔH_reaction = (0 – 393.5) – (0 + 0)

ΔH_reaction = -393.5 kJ

Enthalpy plays a pivotal role in understanding the energy dynamics of chemical reactions within thermodynamic systems. It encapsulates the total heat content and is expressed as the sum of internal energy, volume, and pressure. Enthalpy change, denoted as ΔH, is a fundamental concept, signifying the energy difference associated with a reaction.

We explored the formula for calculating enthalpy change, which involves both heat transfer and the internal characteristics of a system. Enthalpy is regarded as a state function, relying on variables like pressure, temperature, and internal energy. The relationship between heat flow (q) and ΔH distinguishes between endothermic and exothermic reactions.