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Hess's Law, also known as Hess's Law of Constant Heat Summation, is a scientific law in physical chemistry and thermodynamics. It is based on the first law of thermodynamics and states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the enthalpy change is the same whether the reaction occurs in one step or several steps. Hess's Law allows for the calculation of enthalpy changes even when they cannot be directly measured. This is achieved through algebraic operations based on the chemical equation of reactions using previously defined values for the formation of enthalpies. The law was formulated by Swiss-born Russian chemist Germain Hess in 1840.
| Characteristics | Values |
|---|---|
| Hess's Law | Hess's law of constant heat summation, also known as Hess's law, states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. |
| First Law of Thermodynamics | The first law of thermodynamics states that the total energy of substances before and after any physical or chemical change should be equal. |
| Similarities | Both laws state that the total energy of a system remains constant throughout a process and that energy is finite. |
| Differences | Hess's law focuses on enthalpy changes during chemical reactions, while the first law of thermodynamics emphasizes the conservation of energy in any physical or chemical change. |
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What You'll Learn
- Hess's Law is based on the first law of thermodynamics
- Hess's Law allows for the calculation of non-measurable enthalpy changes
- Hess's Law is useful for determining other state functions
- Hess's Law is derived from the first law's statement that energy cannot be created or destroyed
- Hess's Law is an expression of the fact that enthalpy is a state function
Hess's Law is based on the first law of thermodynamics
Hess's law, also known as Hess's law of constant heat summation, is a scientific principle in physical chemistry and thermodynamics. It was named after Germain Hess, a Swiss-born Russian chemist and physician, who formulated it in 1840.
Hess's law can be expressed as:
> ΔH° = ΣΔHn
Where ΔH° is the heat absorbed or evolved, and ΣΔHn is the sum of the heat absorbed or evolved in the individual n steps of the reaction. This equation demonstrates that the heat absorbed or evolved in a chemical reaction is a fixed quantity, regardless of the path of the reaction or the number of steps involved.
The concept of Hess's law can be expanded to include changes in entropy and Gibbs free energy, as these are also state functions. By combining ΔG° and ΔH° values from different thermodynamic cycles, it is possible to calculate entropy and Gibbs free energy values that cannot be directly measured.
In summary, Hess's law is based on the first law of thermodynamics, as it relies on the principle that the total energy of a system remains constant during any chemical reaction. Hess's law provides a valuable tool for calculating enthalpy changes and understanding the behaviour of energy in chemical reactions.
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Hess's Law allows for the calculation of non-measurable enthalpy changes
Hess's law, also known as Hess's law of constant heat summation, is a scientific principle in physical chemistry and thermodynamics. It was formulated by Swiss-born Russian chemist and physician Germain Hess, who published it 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 enthalpy change in a system due to a reaction at constant pressure is equal to the heat absorbed or the negative of the heat released. This is in line with the first law of thermodynamics, which states that the total energy of the substances before and after any physical or chemical change should be equal.
Hess's law is based on the state function of enthalpy, which means that the enthalpy of a chemical process is independent of the path taken from the initial to the final state. Enthalpy is an extensive property, meaning its value is proportional to the system size. Therefore, the enthalpy change is proportional to the number of moles participating in a given reaction.
The concepts of Hess's law can be expanded to include changes in entropy and Gibbs free energy, as these 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.
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Hess's Law is useful for determining other state functions
Hess's law, also known as Hess's law of constant heat summation, is a scientific law named after Swiss-born Russian chemist and physician Germain Hess, who published it 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 regardless of the route by which the chemical change occurs, as long as the initial and final conditions are the same.
Hess's law is based on the first law of thermodynamics, which states that the enthalpy change in a system due to a reaction at constant pressure is equal to the heat absorbed or the negative of the heat released. This can be determined by calorimetry for many reactions, and the values are usually stated for reactions with the same initial and final temperatures and pressures.
Now, let's discuss how Hess's law is useful for determining other state functions:
The concepts of Hess's law can be expanded beyond just enthalpy changes to include changes in entropy and Gibbs free energy, as these are also state functions. For example, the Bordwell thermodynamic cycle uses easily measured equilibria and redox potentials to determine experimentally inaccessible Gibbs free energy values. By combining ΔG° values from the Bordwell cycle with ΔH° values found using Hess's law, we can determine entropy values that have not been directly measured and must be calculated through alternative paths. This demonstrates the versatility of Hess's law in determining other state functions beyond just enthalpy.
In summary, Hess's law is useful for determining other state functions because it recognizes that enthalpy is a state function, and this understanding can be extended to other state functions such as entropy and Gibbs free energy. By applying Hess's law, we can calculate overall changes in these state functions and gain valuable insights into chemical reactions, even when direct measurements are not feasible.
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Hess's Law is derived from the first law's statement that energy cannot be created or destroyed
Hess's Law, also known as Hess's Law of Constant Heat Summation, is a scientific principle in thermodynamics named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. It is a fundamental principle in chemistry that allows us to calculate the enthalpy change of a chemical reaction by using the enthalpy changes of other reactions. Enthalpy is a measure of the heat energy involved in a chemical reaction.
Hess's Law states that the change of enthalpy in a chemical reaction is the same regardless of whether the reaction takes place in one step or several steps, as long as the initial and final states of the reactants and products are the same. In other words, the total enthalpy change during the complete course of a chemical reaction is independent of the sequence of steps taken. This is based on the concept that enthalpy is a state function, meaning it only depends on the initial and final states of a system, not the path taken.
The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed but can only be transformed from one form to another. In other words, the total energy of the substances before and after any physical or chemical change should be equal. This law distinguishes two principal forms of energy transfer: heat and thermodynamic work.
Hess's Law is derived from the First Law of Thermodynamics. If the First Law were not true, then Hess's Law could be violated. Hess's Law can be understood as an application of the First Law to chemical reactions. It allows for the calculation of the enthalpy change (ΔH) for a reaction even when it cannot be measured directly. By using known enthalpy changes of reactions, we can combine them algebraically to determine the enthalpy change of an unknown reaction.
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Hess's Law is an expression of the fact that enthalpy is a state function
Hess's law, also known as Hess's law of constant heat summation, is a scientific law named after Germain Hess, a Swiss-born Russian chemist and physician. 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 change in enthalpy in a chemical reaction remains the same whether the reaction occurs in a single step or multiple steps, as long as the initial and final states of the reactants and products are identical. Enthalpy is an extensive property, and its value is proportional to the size of the system. Therefore, the enthalpy change is proportional to the number of moles involved in a given reaction.
Hess's law is based on the first law of thermodynamics, which states that the total energy of the substances before and after any physical or chemical change should be equal. According to the first law of thermodynamics, the enthalpy change in a system due to a reaction at constant pressure is equal to the heat absorbed or the negative of the heat released. This can be determined by calorimetry for many reactions. Enthalpy change, denoted by the symbol ΔH, is a crucial concept in thermodynamics and is fundamental to understanding how energy is exchanged in chemical reactions. It can be either endothermic or exothermic, depending on whether the system absorbs or releases heat, respectively.
The concepts of Hess's law can be extended to include changes in entropy and Gibbs free energy, as these are also state functions. For example, the Bordwell thermodynamic cycle utilises easily measured equilibria and redox potentials to determine experimentally inaccessible Gibbs free energy values. Combining ΔG° values from Bordwell thermodynamic cycles and ΔH° values from Hess's law can facilitate the calculation of entropy values that have not been directly measured.
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Frequently asked questions
Hess's Law, also known as Hess's Law of Constant Heat Summation, states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken.
The First Law of Thermodynamics states that the total energy of substances before and after any physical or chemical change should be equal.
No, they are not the same, but Hess's Law is based on the First Law of Thermodynamics. Hess's Law is a consequence of the First Law and can be considered an expression or outcome of the First Law.
The First Law of Thermodynamics talks about the conservation of energy, stating that energy cannot be created or destroyed. Hess's Law talks about the change in enthalpy during a chemical reaction, stating that this change is independent of the path taken.
Hess's Law can be used to calculate the heat of formation of carbon dioxide from carbon and oxygen. It allows for the calculation of enthalpy changes even when they cannot be directly measured.
