Hess's Law: A Practical Guide To Calculating Enthalpy Changes

Hess's Law, also known as the Hess 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. It 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 law allows for the calculation of the enthalpy change (ΔH) for a reaction even when it cannot be directly measured, by performing basic algebraic operations based on the chemical equations of reactions and previously determined values for the enthalpies of formation. Hess's Law is particularly useful in determining the enthalpies of formation for unstable intermediates, heat changes in phase transitions, and lattice energies of ionic substances, among other applications.

Hess's Law allows for the calculation of enthalpy changes in phase transitions and allotropic transitions

Hess's Law, also known as Hess's Law of Constant Heat Summation, is a fundamental principle in physical chemistry and thermodynamics. It was formulated by Swiss-born Russian chemist and physician Germain Hess and published in 1840. This law states that the total enthalpy change during a chemical reaction remains constant, regardless of the number of steps or the specific route taken. In other words, the overall enthalpy change is the same whether a reaction occurs in one step or multiple steps, as long as the initial and final states of the reactants and products are identical.

Hess's Law is based on the concept that enthalpy is a state function, meaning it only depends on the current state of the system and not on the path taken to reach that state. This law allows for the calculation of enthalpy changes in various processes, including phase transitions and allotropic transitions.

Phase transitions refer to changes in the state of matter, such as from solid to liquid or liquid to gas. These transitions involve changes in enthalpy, which can be calculated using Hess's Law. For example, consider the phase transition of water from liquid to gas. By applying Hess's Law, we can determine the enthalpy change associated with this transition.

Allotropic transitions, on the other hand, involve changes between different structural forms of the same element. For instance, the transition between graphite and diamond, which are both forms of carbon, can be analysed using Hess's Law to calculate the enthalpy change during this transition.

By utilising Hess's Law, we can gain valuable insights into the enthalpy changes associated with these transitions, contributing to our understanding of the underlying chemical processes and providing essential data for further research and applications.

Hess's Law can be used to determine the enthalpy of formation of unstable intermediates like CO(g) and NO(g)

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. In other words, the enthalpy change of 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. This is because enthalpy is a state function, and so the enthalpy of a given reaction is constant.

To determine the enthalpy of formation of unstable intermediates, a table of standard enthalpies of formation is required. This table is compiled from empirical data, usually acquired using calorimetry. The key to solving this problem is to sum up all the product enthalpies of formation and then subtract the summed reactant enthalpies of formation. It is also important to ensure that the equation is balanced.

For example, to calculate the standard enthalpy of combustion for the reaction:

7⁄2O2(g) ---> 2CO2(g) + 3H2O(ℓ)

We can use the following values from the table of standard enthalpies of formation:

• ΔH°f for CO2(g) = −393.5 kJ/mol
• ΔH°f for H2O(ℓ) = −286 kJ/mol
• ΔH°f for O2(g) = 0 kJ/mol (as it is an element in its standard state)

Substituting these values into the equation and performing the necessary calculations will give us the standard enthalpy of combustion for the reaction.

By applying Hess's Law in this way, we can determine the enthalpy of formation for unstable intermediates like CO(g) and NO(g) by breaking down the reaction into individual steps and using the standard enthalpies of formation for each step.

Hess's Law can be used to calculate the enthalpy of a particular step in a chemical reaction

Hess's Law, also known as Hess's Law of Constant Heat Summation, states that the total enthalpy change of a reaction is the sum of all the changes, regardless of the number of steps or stages in the reaction. In other words, the net enthalpy and the number of steps in a reaction are independent of each other. This law is a manifestation of the fact that enthalpy is a state function, meaning that the enthalpy change 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.

For example, consider the reaction:

C_{(s)} + 2 H_2O_{(g)} -> CO_{2(g)} + 2 H_{2(g)>

This reaction can be thought of as occurring in two steps:

Step 1: C_{(s)} + \frac{1}{2} O_{2(g)} -> CO_{(g)>

Step 2: CO_{(g)} + \frac{1}{2} O_{2(g)} -> CO_{2(g)>

The enthalpy change for the overall reaction is -90.1 kJ. By applying Hess's Law, we can calculate the enthalpy change for each step. The enthalpy change for Step 1 is given as -393.5 kJ, and the enthalpy change for Step 2 is -283.0 kJ. Therefore, the enthalpy change for the overall reaction is the sum of the enthalpy changes of these two steps:

\[\Delta{H^°_{overall}} = \Delta{H^°_{Step 1}} + \Delta{H^°_{Step 2}}\]

\[\Delta{H^°_{overall}} = -393.5 kJ - 283.0 kJ\]

\[\Delta{H^°_{overall}} = -676.5 kJ\]

So, the enthalpy change for the overall reaction is -676.5 kJ, which is equal to the sum of the enthalpy changes of the two steps.

Hess's Law is a valuable tool in thermodynamics and physical chemistry, allowing us to calculate enthalpy changes for specific steps within a chemical reaction, even when direct measurement is challenging or impossible.

Hess's Law can be used to determine the enthalpy of reaction for two salts in an aqueous solution

Hess's Law, or Hess's Law of Constant Heat Summation, states that the total enthalpy change for a reaction is the sum of all enthalpy changes, regardless of the number of steps or stages in the reaction. In other words, the net enthalpy and the number of steps in a reaction are independent of each other. This law is a result of the fact that enthalpy is a state function, and so the change in enthalpy depends only on the final state of the products and the initial state of the reactants.

For example, let's say we want to determine the enthalpy of reaction for the conversion of graphite to diamond. This reaction cannot be examined directly in a common chemistry laboratory because, despite the low enthalpy, the reaction requires a very high activation energy to get started, which means very high temperatures and pressures. However, we can use Hess's Law to determine the enthalpy indirectly. We need two reactions that can be added together, and then we can add their respective enthalpies to get the enthalpy of the desired reaction.

The two reactions we need are:

C (s, graphite) + O2(g) ---> CO2(g)

C (s, diamond) + O2(g) ---> CO2(g)

We reverse the second equation so that the C (s, diamond) is on the product side, and then add the two equations together. The oxygen and carbon dioxide will cancel out, leaving us with:

C (s, graphite) ---> C (s, diamond)

The enthalpy of the desired reaction is then the sum of the enthalpies of the two original reactions.

This is just one example of how Hess's Law can be applied to determine the enthalpy of reaction for two salts in an aqueous solution.

Hess's Law can be used to calculate the enthalpy of atomisation

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. In other words, the enthalpy change is the same whether the reaction occurs in one step or several, provided that the initial and final states of the reactants and products are the same. This is because enthalpy is a state function, meaning that it only depends on the initial and final states of the system, and not on the path taken between them.

For example, let's consider the atomisation of water (H₂O) into hydrogen (H₂) and oxygen (O₂) gases. We could break this process down into two steps:

• H₂O(l) → H₂(g) + ½ O₂(g)
• H₂(g) + ½ O₂(g) → H₂(g) + O₂(g)

The first step involves the enthalpy change associated with the vaporisation of water, while the second step involves the enthalpy change associated with the combustion of hydrogen gas. By combining these two reactions, we get the overall atomisation reaction:

H₂O(l) → H₂(g) + O₂(g)

The enthalpy change for the atomisation reaction would then be the sum of the enthalpy changes for the two individual reactions.

It is important to note that the enthalpy changes for the individual reactions must be determined experimentally, and the accuracy of the overall enthalpy change calculation depends on the accuracy of these measurements. Additionally, the reactions must be carried out under the same conditions, particularly the same initial and final temperatures and pressures, to ensure that Hess's Law can be applied.

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