Cecily Zamora
Hess's Law, or the Law of Constant Heat Summation, is a principle in physical chemistry and thermodynamics. It states that the total enthalpy change in a chemical reaction is independent of the sequence of steps taken. In other words, the total enthalpy change is the same whether the reaction occurs in one step or multiple steps. This law allows for the calculation of enthalpy change even when it cannot be directly measured. But can Hess's law be used for entropy? The answer is yes. While the calculation for entropy is slightly different, Hess's law can be used to determine entropy values that have not been directly measured.
| Characteristics | Values |
|---|---|
| Name | Hess's Law of Constant Heat Summation |
| Other Names | Hess's Law, Law of Constant Heat Summation |
| Field | Physical Chemistry and Thermodynamics |
| Named After | Germain Hess, a Swiss-born Russian chemist and physician |
| Year of Publication | 1840 |
| Description | The total enthalpy change during the complete course of a chemical reaction is independent of the sequence of steps taken |
| Calculation | The sum of the enthalpy changes for all equations in a sequence will be the enthalpy change for the net equation |
| Use Cases | Calculating entropy, Gibbs energy, and enthalpy |
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What You'll Learn
Hess's Law can be used to calculate entropy
Hess's Law, or Hess's Law of Constant Heat Summation, is a principle in physical chemistry and thermodynamics. It states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the total enthalpy change for a reaction is the sum of all changes, regardless of the number of steps or stages. This means that enthalpy can be calculated in one grand step or multiple smaller steps.
The law is useful in determining the enthalpies of heats of formation of unstable intermediates like CO(g) and NO(g). It can also be used to determine the overall energy required for a chemical reaction that can be divided into synthetic steps that are easier to characterise. This allows for the prediction of the enthalpy change in complex synthesis.
To calculate entropy using Hess's Law, one can use the formula:
> ΔSreaction = Σ νpSp − Σ νrSr
Where:
- ΔSreaction is the change in entropy during the reaction
- Νp is the stoichiometric coefficient of the product
- Sp is the absolute entropy of the product
- Νr is the stoichiometric coefficient of the reactant
- Sr is the absolute entropy of the reactant
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Hess's Law can be used to determine the overall energy required for a chemical reaction
Hess's Law, or Hess's Law of Constant Heat Summation, is a principle in physical chemistry and thermodynamics formulated by Swiss-born Russian chemist Germain Hess 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 identical. This is because enthalpy is a state function, and its value is proportional to the system size.
For example, consider the combustion of carbon and hydrogen. Hess's Law can be applied by taking the combustion of carbon and adding to it the reverse of the combustion of hydrogen. The heat of combustion of carbon minus the heat of combustion of hydrogen equals the heat of the overall reaction. This demonstrates that Hess's Law can be used to determine the overall energy required for a chemical reaction by manipulating the individual steps.
Additionally, Hess's Law can be used to calculate entropy and Gibbs' energy. While the calculation of entropy typically involves using the absolute entropies of products and reactants, Hess's Law can be helpful in determining entropy values that cannot be directly measured. By using the equation ΔG = ΔH - TΔS, where ΔG represents the change in Gibbs' free energy, ΔH represents the change in enthalpy, T represents the temperature, and ΔS represents the change in entropy, one can determine the spontaneity of a reaction. If ΔG is negative, the reaction is spontaneous, if positive, it is not spontaneous, and if zero, the reaction is at equilibrium.
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Hess's Law can be used to calculate enthalpy changes
Hess's Law, or Hess's Law of Constant Heat Summation, is a principle in physical chemistry and thermodynamics that allows for the calculation of enthalpy changes in chemical reactions. The law was formulated by Swiss-born Russian chemist Germain Hess in 1840 and 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 remain the same.
This law is particularly useful when the enthalpy change for a reaction cannot be measured directly. By using Hess's Law, we can perform basic algebraic operations based on the chemical equations of reactions and utilise previously determined values for the enthalpies of formation. The combination of these chemical equations yields a net or overall equation, and the sum of the enthalpy changes for each equation in the sequence will give us the enthalpy change for the net equation.
For example, consider the combustion of carbon and the combustion of hydrogen. By taking the combustion of carbon and subtracting the combustion of hydrogen (or adding the reverse of the combustion of hydrogen), we can determine the heat of the combustion of carbon minus the heat of the combustion of hydrogen, which equals the heat of the overall reaction. This observation led to the discovery of Hess's Law.
Hess's Law also has applications beyond just calculating enthalpy changes. It can be used to determine the overall energy required for a chemical reaction that can be divided into multiple synthetic steps, allowing for the compilation of standard enthalpies of formation. Additionally, Hess's Law can aid in the calculation of entropy and Gibbs' energy, as well as the determination of enthalpies of unstable intermediates like CO(g) and NO(g).
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Hess's Law can be used to calculate the heat of reaction
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. The law states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the total enthalpy change for a reaction is the sum of all changes, regardless of the multiple stages or steps of the reaction. This is based on the fact that enthalpy is a state function, meaning that the enthalpy of a chemical process is independent of the path taken from the initial to the final state.
The application of Hess's Law allows for the determination of the overall energy required for a chemical reaction that can be divided into synthetic steps that are individually easier to characterize. This enables the compilation of standard enthalpies of formation, which can be used to predict the enthalpy change in complex synthesis. Hess's Law is particularly valuable when studying reactions with multiple stages or steps, as it provides a method to calculate the total enthalpy change by summing up the changes for each step until the product is formed.
Additionally, Hess's Law can be expanded to include changes in entropy and Gibbs free energy, as these are also state functions. The values obtained with Hess's Law can be used to determine entropy values that have not been directly measured and need to be calculated through alternative paths. This is achieved by using the absolute entropies for products and reactants, as entropy can be measured as an absolute value rather than relative to the elements in their reference states.
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Hess's Law can be used to calculate Gibbs' energy
Hess's Law, or Hess's Law of Constant Heat Summation, is a relationship in physical chemistry and thermodynamics. Named after Swiss-born Russian chemist Germain Hess, the law states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the total enthalpy change for a reaction is the sum of all changes, regardless of the multiple stages or steps of a reaction.
Hess's Law can be used to determine the overall energy required for a chemical reaction that can be divided into synthetic steps that are individually easier to characterize. This allows for the calculation of the enthalpy change (ΔH) for a reaction even when it cannot be measured directly. This is particularly useful when the enthalpies of formation for some of the components in the reaction are unknown.
Hess's Law can also be used to calculate the change in Gibbs Free Energy, which is the sum of enthalpy (H), temperature in Kelvin (T), and entropy (S). The values for ΔG and ΔH are measured in kJ, while ΔS is measured in joules. The sign of the values for ΔG, ΔH, and ΔS can be used to determine whether the equation is spontaneous. When ΔH = TΔS, the value for ΔG = 0 and the reaction is at equilibrium.
For example, given the data for three combustion reactions, one can calculate the free energy of the reaction producing methanol (CH3OH) from carbon monoxide and hydrogen gas. The first step is to ensure that the equations are in the correct direction, with the same reactants and products as the target equation. Then, the equations are added together to get the net equation. The ΔG value for the net equation is the sum of the ΔG values for each equation in the sequence.
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Frequently asked questions
Hess's Law of Constant Heat Summation (or just Hess's Law) states that the total enthalpy change for a reaction is the sum of all changes, regardless of the multiple stages or steps of a reaction.
Yes, Hess's Law can be used to calculate entropy and Gibbs' energy in addition to enthalpy.
Hess's Law allows the determination of entropy values that have not been measured directly and need to be calculated through alternative paths.
Hess's Law allows the enthalpy change (ΔH) for a reaction to be calculated even when it cannot be measured directly. This is done by performing basic algebraic operations based on the chemical equations of reactions.
