Kara Sears
Hess's law, also known as Hess's law of constant heat summation, is a principle in physical chemistry and thermodynamics. It was formulated by Germain Hess, a Swiss-born Russian chemist and physician, and published in 1840. The law states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the overall enthalpy change is the same whether a chemical change occurs in one step or several steps, as long as the initial and final conditions are the same. Hess's law allows for the calculation of the enthalpy change (ΔH) for a reaction, even when it cannot be directly measured, by performing algebraic operations on the chemical equations of reactions using known values for the enthalpies of formation. This involves manipulating equations by flipping, dividing, or multiplying them, and their corresponding ΔH values, to match the desired reaction.
| Characteristics | Values |
|---|---|
| Use | Determining the overall energy required for a chemical reaction that can be divided into synthetic steps that are individually easier to characterize |
| Named After | Swiss-born Russian chemist and physician Germain Hess |
| Date of Publication | 1840 |
| Equation | Enthalpy change, ∆H, can be defined as the amount of heat absorbed or released during a reaction |
| Enthalpy | An extensive property, meaning that its value is proportional to the system size |
| Enthalpy Change | Proportional to the number of moles participating in a given reaction |
| Enthalpy of a Chemical Process | Independent of the path taken from the initial to the final state |
| Enthalpy Changes | Known for all the equations in the sequence |
| Net Enthalpy Change | If negative, the reaction is exothermic and more likely to be spontaneous; positive values correspond to endothermic reactions |
| Entropy | Can be measured as an absolute value |
| Entropy of Formation | Not required |
| Lattice Energies of Ionic Substances | Can be determined by constructing Born–Haber cycles if the electron affinity to form the anion is known |
| Changes | Can be expanded to include changes in entropy and in Gibbs free energy |
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What You'll Learn
- Hess's Law allows for the calculation of enthalpy change (ΔH)
- Hess's Law can be used to determine the overall energy required for a chemical reaction
- Hess's Law states that the total enthalpy change of a reaction is the sum of all changes
- Hess's Law can be expanded to include changes in entropy and Gibbs free energy
- Hess's Law is useful in the determination of enthalpies of heats of formation
Hess's Law allows for the calculation of enthalpy change (ΔH)
Hess's Law, also known as Hess's Law of Constant Heat Summation, is a principle in physical chemistry and thermodynamics formulated by Swiss-born Russian chemist and physician Germain Hess in 1840. Hess's Law allows for the calculation of enthalpy change (ΔH) in a chemical reaction, even when it cannot be directly measured. This is achieved by performing basic algebraic operations on the chemical equations of reactions, using previously determined values for the enthalpies of formation.
The law states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the overall enthalpy change is the same regardless of the route by which the chemical change occurs, as long as the initial and final conditions are the same. This is because enthalpy is a state function, and its value depends only on the state of the materials, including their temperature, pressure, and composition.
By applying Hess's Law, the overall equation for a chemical reaction can be determined by combining multiple chemical equations. If the enthalpy changes for all the individual equations are known, their sum will be the enthalpy change for the overall equation. This allows for the prediction of enthalpy change in complex synthesis by compiling standard enthalpies of formation.
Hess's Law can be used to determine the enthalpy change in various types of reactions, including the formation of unstable intermediates, phase transitions, and allotropic transitions. It can also be applied to determine the lattice energies of ionic substances by constructing Born-Haber cycles when the electron affinity to form the anion is known. Additionally, the concepts of Hess's Law can be expanded to include changes in entropy and Gibbs free energy, as they are also state functions.
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Hess's Law can be used to determine the overall energy required for a chemical reaction
Hess's law, also known as Hess's law of constant heat summation, is a principle in physical chemistry and thermodynamics. It was formulated by Germain Hess, a Swiss-born Russian chemist and physician, and published in 1840.
Mathematically, Hess's law states that the enthalpy change (ΔH) for a reaction can be calculated by summing the enthalpy changes for each individual reaction in a series of reactions. This is true even when the enthalpy change cannot be directly measured. By manipulating chemical equations and their enthalpy changes, we can determine the overall enthalpy change for a reaction. This is particularly useful for reactions that are challenging to measure directly, such as those involving unstable intermediates or phase transitions.
The law's applicability extends beyond enthalpy and can be used to calculate other state functions, including changes in Gibbs' energy and entropy. For example, the Bordwell thermodynamic cycle utilises easily measured equilibria and redox potentials to determine Gibbs free energy values that are otherwise experimentally inaccessible. Thus, Hess's law is a valuable tool in thermodynamics, allowing for the determination of overall energy requirements in chemical reactions.
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Hess's Law states that the total enthalpy change of a reaction is the sum of all changes
Hess's law, also known as Hess's law of constant heat summation, is a principle in physical chemistry and thermodynamics formulated by Germain Henri Hess, a Swiss-born Russian chemist and physician, in 1840. Hess's law states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the overall enthalpy change is the same regardless of the route by which the chemical change occurs, as long as the initial and final conditions are the same. This means that if a chemical equation can be written as the sum of several other chemical equations, the enthalpy change of the first chemical equation equals the sum of the enthalpy changes of the other chemical equations.
Hess's law is particularly useful when a reaction cannot be examined directly in a laboratory due to high activation energy requirements, making it tricky, expensive, or dangerous to perform. By applying Hess's law, one can determine the enthalpy change for a reaction by performing basic algebraic operations based on the chemical equations of reactions using previously determined values for the enthalpies of formation. This involves manipulating multiple equations by reversing their direction, multiplying them by appropriate factors, and adding them together to obtain a net equation. The enthalpy changes of the individual equations are summed up to calculate the enthalpy change for the net equation.
The concepts of Hess's law can be expanded to include changes in entropy and Gibbs free energy, as they are also state functions. For example, the Bordwell thermodynamic cycle takes advantage of easily measured equilibria and redox potentials to determine experimentally inaccessible Gibbs free energy values. By combining ΔG values from the Bordwell cycle and ΔH values from Hess's law, it is possible to determine entropy values that have not been directly measured and must be calculated through alternative paths.
Hess's law is valuable in determining the enthalpies of various processes, such as the heats of formation of unstable intermediates, heat changes in phase transitions, lattice energies of ionic substances, and more. It also enables the prediction of the overall energy required for a chemical reaction by dividing it into simpler synthetic steps, making it a versatile tool in the field of chemistry and thermodynamics.
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Hess's Law can be expanded to include changes in entropy and Gibbs free energy
Hess's law, also known as Hess's law of constant heat summation, is a relationship in physical chemistry and thermodynamics formulated by Germain Hess, a Swiss-born Russian chemist, in 1840. The law states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the change in enthalpy in a chemical reaction remains the same whether the reaction occurs in one step or several steps, as long as the initial and final states of the reactants and products are the same. This law is particularly useful when the enthalpy change for a reaction cannot be measured directly, as it allows for the calculation of this value through basic algebraic operations using previously determined values for the enthalpies of formation.
Hess's law can be expanded beyond just enthalpy changes to include changes in entropy and Gibbs free energy. This is because entropy and Gibbs free energy are also state functions, meaning they depend on the initial and final states of the system and not on the path taken between these states. By combining ΔG° values from Bordwell thermodynamic cycles with ΔH° values found using Hess's law, it is possible to determine entropy values that have not been directly measured and must be calculated through alternative methods. This expansion of Hess's law provides a valuable tool for understanding and predicting the behaviour of complex chemical systems.
The change in Gibbs free energy (ΔG) can be determined using the equation ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy. The values for ΔG and ΔH are typically measured in kJ, while ΔS is measured in joules. By plugging in the values for these variables, the change in Gibbs free energy can be calculated. This calculation is essential in determining whether a reaction is spontaneous or not at standard conditions. If ΔG is negative, the reaction is spontaneous, whereas if it is positive, the reaction is not spontaneous.
Furthermore, Hess's law can be applied to determine the overall energy required for a chemical reaction that can be divided into multiple synthetic steps. This allows for the compilation of standard enthalpies of formation, which are useful in predicting the enthalpy change in complex synthesis. Enthalpy, being an extensive property, has a value proportional to the system size and the number of moles involved in a reaction. Thus, Hess's law provides a versatile tool for understanding and manipulating chemical reactions, making it a fundamental concept in the field of chemistry.
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Hess's Law is useful in the determination of enthalpies of heats of formation
Hess's law, also known as Hess's law of constant heat summation, is a principle in physical chemistry and thermodynamics formulated by Germain Henri Hess in 1840. This law is particularly useful in determining the enthalpies of heats of formation, especially when dealing with unstable intermediates like CO(g) and NO(g).
Hess's law states that the total enthalpy change during a chemical reaction remains constant, regardless of whether the reaction occurs in a single step or multiple steps. In other words, the enthalpy change is independent of the path taken, as long as the initial and final states of the reactants and products remain the same. This is because enthalpy is a state function, and its value is proportional to the system size.
The law's applicability extends to situations where direct measurement of enthalpy change is challenging or impossible. In such cases, basic algebraic operations can be performed on the chemical equations of reactions, utilising previously determined values for the enthalpies of formation. By manipulating and combining these equations, the overall enthalpy change for the net equation can be calculated.
Hess's law is valuable in predicting the enthalpy change in complex synthesis processes. It enables the determination of standard enthalpies of formation, which can be used to anticipate the energy requirements of a chemical reaction that can be broken down into simpler, more manageable steps. This is especially useful when dealing with reactions that cannot be directly measured, such as the standard enthalpy change of formation of propane, where carbon and hydrogen do not directly react to form propane.
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Frequently asked questions
Yes, you can divide in Hess' Law. For example, you can divide an equation by 2 to get the desired number of a certain compound on one side of the equation.
You would divide in Hess' Law to manipulate an equation so that it matches the specific final reaction you want.
Hess' Law is a relationship in physical chemistry and thermodynamics named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. It states that the total enthalpy change during the complete course of a chemical reaction is independent of the sequence of steps taken.
In the equation 2B(s) + 3H2(g) → B2H6(g) (ΔH = 36 kJ/mol), you can divide by 2 to get 3/2 O2(g) on the left-hand side.
You can also multiply, flip, and add equations in Hess' Law.
