Gavin Howe
Hess's Law, discovered by Germain Henri Hess in 1840, is a principle in physical chemistry and thermodynamics that can be used to calculate the change in Gibbs free energy during a chemical reaction. It states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken, and it can be calculated by summing the enthalpy changes of individual steps. This law can be applied to reactions with multiple pathways to the same product, such as the formation of water, to determine the overall enthalpy change. By using Hess's Law, we can calculate the standard Gibbs free energy change (ΔG∘rxn) for a reaction by considering the sum of the standard Gibbs free energy changes for each step. This is particularly useful when direct measurement of the enthalpy change is challenging.
| Characteristics | Values |
|---|---|
| Hess's Law | Hess' Law, also known as Hess's Law of constant heat summation, is a relationship in physical chemistry and thermodynamics. |
| Named After | Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. |
| Use | Used to calculate the enthalpy of a chemical reaction, as well as changes in Gibbs' Energy, Entropy, and other state functions. |
| Application | Applicable when a chemical equation can be written as the sum of several other chemical equations. |
| Enthalpy Change | The enthalpy change of the first chemical equation equals the sum of the enthalpy changes of the other chemical equations. |
| State Function | Enthalpy is a state function, so the total enthalpy change is independent of the sequence of steps taken in a chemical reaction. |
| First Law of Thermodynamics | The enthalpy change in a system at constant pressure is equal to the heat absorbed or released, which can be determined by calorimetry. |
| Conditions | Values are usually stated for reactions with the same initial and final temperatures and pressures, while conditions can vary during reactions. |
| Example | Calculating the enthalpy changes in reactions leading to the formation of water, which can occur through multiple pathways. |
| Calculation | By summing the ΔG° values of the individual reactions, the ΔG° for the overall reaction can be determined. |
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What You'll Learn
- Hess's Law can be used to calculate the standard Gibbs free energy change for a reaction
- The pathway doesn't affect the total energy change
- Hess's Law can be used to calculate other state functions, like entropy
- Enthalpy is a state function, so it doesn't matter what route the conversion takes
- Hess's Law can be used to calculate heats of formation of unstable intermediates
Hess's Law can be used to calculate the standard Gibbs free energy change for a reaction
Hess's Law, named after Germain Henri Hess, who discovered it in 1840, 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 enthalpy of a chemical process is independent of the path taken from the initial to the final state. This is because enthalpy is a state function, and so it only depends on the initial and final states.
For example, let's consider the reaction: CO (g) → C (s) + 1/2 O2 (g). Using Hess's Law, we can calculate the ΔG∘rxn for this reaction by considering a series of reactions whose sum gives the overall reaction. We can use the following two reactions to calculate ΔG∘rxn: CO2(g) → C (s) + O2(g) and CO (g) + 1/2 O2 (g) → CO2(g). The ΔG∘rxn values for these reactions are +394.4 kJ and -257.2 kJ, respectively. Adding these two equations together, we get the desired reaction: CO (g) → C (s) + 1/2 O2 (g). The ΔG∘rxn for this reaction is equal to the sum of the ΔG∘rxn values of the previous two reactions, which is 137.2 kJ.
This showcases that the pathway does not affect the total energy change, as per Hess's Law. By using Hess's Law, we can calculate the standard Gibbs free energy change for a reaction by summing the Gibbs free energy values from each step.
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The pathway doesn't affect the total energy change
Hess's Law, named after Germain Henri Hess, who discovered it in 1840, states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the pathway does not affect the total energy change. This is because enthalpy is a state function, and so it only depends on the initial and final states, not on how those states were achieved.
For example, if two hikers meet at a 500 ft altitude, it doesn't matter if one is on their way up, and the other is on their way down. Similarly, the enthalpy change is the same if molecules A and B are converted to C, and then D, compared to if they go directly to D.
Hess's Law can be used to calculate the standard Gibbs free energy change (ΔGrxn∘) for a given reaction by considering a series of reactions whose sum gives the overall reaction. The standard Gibbs free energy change for the overall reaction is equal to the sum of the standard Gibbs free energy changes for each step. For instance, the formation of water can occur through multiple pathways, and the total enthalpy change will remain consistent regardless of the route taken.
Hess's Law can also be used to calculate the enthalpy change for a reaction that cannot be measured directly. This is done by performing basic algebraic operations based on the chemical equations of reactions using previously determined values for the enthalpies of formation.
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Hess's Law can be used to calculate other state functions, like entropy
Hess's Law, discovered by Germain Henri Hess in 1840, is a principle in physical chemistry and thermodynamics. It states that the enthalpy of a given chemical reaction is constant, regardless of whether the reaction occurs in a single step or multiple steps. In other words, the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. This is because enthalpy is a state function, and in general, state functions only depend on the initial and final states, not on how those states were achieved.
Hess's Law can be used to calculate the standard heat of a reaction for a specific reaction by summing the heats of reaction for any set of reactions that, when combined, are equivalent to the overall reaction. This is particularly useful when the enthalpy change of a reaction cannot be measured directly, as it allows for the calculation of enthalpy changes using previously determined values for the enthalpies of formation.
While Hess's Law is most commonly associated with calculating enthalpy changes, it can also be used to calculate other state functions, such as changes in Gibbs' energy and entropy. This is because the concepts of Hess's Law can be expanded to include these state functions. By combining ΔG° values from Bordwell thermodynamic cycles and ΔH° values found with Hess's Law, it is possible to determine entropy values that have not been directly measured and must be calculated through alternative paths.
For example, consider the reaction: CO (g) → C (s) + 1/2 O2 (g). Using Hess's Law, we can calculate the ΔG°rxn value for this reaction by summing the ΔG° values of two given reactions: CO (g) + 1/2 O2 (g) → CO2 (g) and CO2 (g) → C (s) + O2 (g). By reversing the second reaction and summing the Gibbs free energy values from both reactions, we can derive the ΔG°rxn for the overall reaction.
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Enthalpy is a state function, so it doesn't matter what route the conversion takes
Hess's Law, named after Swiss-born Russian chemist and physician Germain Hess, was published in 1840. This law is a relationship 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, enthalpy is a state function.
For instance, the formation of water can occur through multiple pathways, but the total enthalpy change will remain consistent regardless of the route taken. This is because the net heat evolved or absorbed during a reaction is independent of the path connecting the reactant to the product.
Hess's Law can be used to calculate other state functions like changes in Gibbs' energy and entropy. For example, by using Hess's Law, we can calculate the standard Gibbs free energy change (ΔGrxn∘) for a given reaction by considering a series of reactions whose sum gives the overall reaction.
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Hess's Law can be used to calculate heats of formation of unstable intermediates
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. It states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken and remains constant regardless of whether the reaction occurs in one step or many steps. In other words, the overall enthalpy change for a reaction is the sum of all the changes in each step. This principle is based on the fact that enthalpy is a state function.
Hess's Law is particularly useful when calculating the heats of formation of unstable intermediates. Unstable intermediates are substances that are difficult to measure directly due to their short lifetimes or challenging synthesis. By applying Hess's Law, we can determine the overall energy required for a chemical reaction that can be divided into multiple steps or pathways. Each step may involve stable substances whose enthalpy changes are known and can be measured independently.
For example, consider the formation of water from its elements, which can occur through multiple pathways. Using Hess's Law, we can calculate the enthalpy change for the overall reaction by summing the enthalpy changes for each individual step, regardless of the specific pathway taken. This approach is especially valuable when dealing with unstable intermediates that may not be easily measured or characterized directly.
Additionally, Hess's Law can be applied to calculate the standard enthalpy of formation for various substances. By manipulating chemical equations and utilizing previously determined values for the enthalpies of formation, we can determine the enthalpy change for a reaction even when it cannot be measured directly. This method is illustrated in several examples, such as determining the standard enthalpy of formation for methyl bromide and calculating the enthalpy of vaporization for ethanol.
In conclusion, Hess's Law provides a powerful tool for calculating heats of formation, especially for unstable intermediates. By considering the overall reaction and manipulating the associated equations, we can determine the enthalpy change for each step and sum them to find the total enthalpy change. This approach allows us to work around the challenges posed by unstable intermediates and gain valuable insights into the energy requirements of complex chemical reactions.
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Frequently asked questions
Hess's Law, discovered by Germain Henri Hess in 1840, states that the total enthalpy change during a chemical reaction is independent of the sequence of steps taken. In other words, the pathway does not affect the total energy change.
Hess's Law can be used to calculate changes in Gibbs' Energy (Delta G). This is because enthalpy is a state function, so it only depends on the initial and final states, not the path taken. This means that the change in Gibbs' Energy for a reaction can be calculated by summing the changes for individual steps.
The formula for calculating Delta G using Hess's Law is:
Delta G = sum of Delta G values for each step
