BlogNCERTImportant Topic of Chemistry: Hess’s Law

Important Topic of Chemistry: Hess’s Law

In general, the science of the relationship between heat, work, temperature, and energy is known as thermodynamics. Thermodynamics, in its broadest sense, is concerned with the transfer of energy from one location to another and from one form to another. We can say that heat is a type of energy that corresponds to a specific quantity of mechanical work, which is the central notion. Even though thermodynamics grew rapidly in the nineteenth century in response to the need to improve the performance of steam engines, the rules of thermodynamics’ vast universality make them relevant to all physical and biological systems. The rules of thermodynamics, in particular, provide a comprehensive explanation of all changes in a system’s energy state and its ability to do beneficial work on its surroundings. Hess’s Law is an outcome of the first rule of thermodynamics and hence does not need to be regarded as a separate thermodynamic law; yet, it retains its identity in thermochemistry due to its usefulness in calculating reaction heats.

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    Overview

    Hess’s Law is named after Russian chemist and doctor Germain Hess. Hess played a pivotal role in the early development of thermochemistry concepts. His most famous work, Hess’s Law, is named after Russian chemist and doctor Germain Hess. Hess played a pivotal role in the early development of thermochemistry concepts. In his most famous work, published in 1840, he contained his Law on thermochemistry. Because enthalpy is a state function, Hess’s Law allows us to calculate the overall change in enthalpy by combining the changes for each step until the product is created. The equations for each stage must be balanced, and all processes must be finished at the same temperature.

    Since Hess’s Law holds, a chemical reaction may be broken down into many steps, and the overall energy of the reaction can be calculated using normal enthalpies of formation. Empirical data, usually obtained through calorimetry, is used to generate standard enthalpy tables. It is feasible to calculate whether a more complex reaction is thermodynamically advantageous or not using these tables.

    Hess’s Law of constant heat summation with example

    According to Hess’s Law, the overall enthalpy change for a reaction equals the sum of all changes, regardless of how many stages or steps are involved. This Law demonstrates the notion that enthalpy is a state function.

    Energy exists in every substance (atom/molecule), and the internal energy is calculated by the type of force present in the substance and the temperature. When a substance goes through chemical reactions, some bonds between atoms are broken, and others are formed. Energy is used in the breaking and forming of bonds.

    And finally, the product substances may have less, the same, or more energy than reacting substances in reactions. As an outcome, reactions can either release heat and become exothermic or absorb heat and become endothermic. Reactants may react further to produce the final product in a single step, multiple steps, or in combination with other items.

    The Law can be expressed as:

    H=ΣΔHn

    Here, H is said to be the heat absorbed or evolved, and ΣΔHn is said to be the sum of the heat absorbed or evolved in the individual n steps of the reaction.

    Knowing the energy changes in every reaction is critical for manipulating the reactants and products in a chemical process to meet our needs. Internal energy change ΔE denotes heat energy changes measured at constant volume, while enthalpy change ΔH denotes heat energy changes measured at constant pressure. The experimental data only determine the net value of all reactions or products generated. An intermediate reaction step’s enthalpy change, or any intermediate product’s enthalpy change, cannot be measured experimentally.

    Carbon, for example, reacts with oxygen in excess to produce carbon dioxide. Carbon and oxygen can react directly to form carbon dioxide, or they can react in two phases to form carbon monoxide and then carbon dioxide. Here, the energy changes associated with the production of carbon dioxide will be measured, but not those associated with the production of carbon monoxide.

    Likewise, calculating the enthalpy of the creation of benzene from carbon and hydrogen is impossible since, in the given conditions, carbon and hydrogen can combine to generate not only benzene but also other forms of hydrocarbons.

    Hess’s Law is helpful in determining non-measurable enthalpy changes in physical and chemical transformations, and it is the sole way to do so.

    Forms of Hess Law

    For multi-step reactions:

    If reactants react to form products in a sequence of phases, including several intermediary products, the total of all reactants, products, and accompanying energy changes will yield the reactant, product, and heat energy changes of the overall reaction. Heat energy changes, like molecules, can be subjected to mathematical processes.

    For multi-different reactions:

    If the enthalpies of several other chemical reactions can be used to obtain the reactants and products of a required chemical reaction, then the enthalpy of the required reactant-to-product reaction may likewise be calculated by adding the enthalpies of all those chemical processes.

    Applications of Hess Law

    The enthalpies of the following compounds can be calculated using Hess’s Law:

    • Heats are related to the formation of unstable intermediates like CO(g) and NO (g).
    • Heat changes during phase transitions and allotropic transitions.
    • The lattice energies of ionic compounds can be computed using Born–Haber cycles if the electron affinity to generate the anion is known.
    • Electron affinities are calculated using a Born–Haber cycle with theoretical lattice energy.

    As per Hess’s Law, whether reactants A and B are converted in one, two, or more steps, the overall enthalpy change will be the same.

    Also read: Important Topic of Chemistry: Enthalpy

    Frequently Asked Questions

    What is the Significance of Hess's Law?

    It is clear that each and every substance, such as atoms or molecules contains energy. The nature of the force that exists in the substance, as well as the temperature, influences the internal energy of these substances. When these compounds are subjected to chemical reactions, some of the bonds that connect some atoms are destroyed, while others are formed. This connection breaking and forming requires energy. As a result, the resultant compounds in reactions may have less, greater, or the same energy as the responding substances. As a result, the reactions can either release heat or absorb heat, making them exothermic or endothermic. Knowing how energy changes in a reaction can manipulate the reactants and products in a chemical process to meet our needs.

    Is it possible to measure the enthalpy change for a reaction?

    In the laboratory, measuring the enthalpy change of a reaction can be challenging, if not impossible. Direct measurement is impossible for some reactions because they occur at such a slow rate. Fortunately, the enthalpy change for a reaction may be calculated using an indirect method. According to Hess's Law of heat summation, if two or more thermochemical equations can be joined together to produce a final equation, the heats of reaction can be summed to produce a heat of reaction for the final equation.

    What is the most important application of Hess's Law?

    Hess's Law is being used to compute the temperatures of various non-direct processes. It is useful to determine the heat of production, neutralization, and other processes. Finding the temperatures of exceedingly slow reactions is useful.

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