Hess's law

Examples of Hess's law in the following topics:

• Hess's Law

  • Hess's Law sums the changes in enthalpy for a series of intermediate reaction steps to find the overall change in enthalpy for a reaction.
  • Hess's law is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician.
  • Hess's law derives directly from the law of conservation of energy, as well as its expression in the first law of thermodynamics.
  • Keep in mind that when reversing reactions using Hess's law, the sign of ΔH will change.
  • By Hess's law, we can sum the ΔH values for these intermediate reactions to get our final value, $\Delta H^\circ_{rxn}$.

• Change in Enthalpy

  • Hess's law addresses how to calculate the enthalpy for the overall reaction.
  • The law states that the enthalpy change for a reaction is the same whether it occurs in one or many steps.
  • Hess's law states that the standard enthalpy change of the overall reaction is the sum of the enthalpy change of all the intermediate reactions that make up the overall reaction.
  • By remembering and employing Hess's Law, the change in enthalpy for the overall reaction can be determined by adding up the enthalpies of the intermediate reactions.

• Thermochemical Equations

  • This becomes important once we begin working with Hess's law.

• The Uncertainty Principle

  • A more formal inequality relating the standard deviation of position (${ \sigma }_{ x }$) and the standard deviation of momentum (${ \sigma }_{ \rho }$) was derived by Earle Hesse Kennard later that year (and independently by Hermann Weyl in 1928):

• The Law of Multiple Proportions

  • The law of multiple proportions states that elements combine in small whole number ratios to form compounds.
  • The law of multiple proportions, also known as Dalton's law, was proposed by the English chemist and meteorologist John Dalton in his 1804 work, A New System of Chemical Philosophy.
  • The law, which was based on Dalton's observations of the reactions of atmospheric gases, states that when elements form compounds, the proportions of the elements in those chemical compounds can be expressed in small whole number ratios.
  • Dalton's law of multiple proportions is part of the basis for modern atomic theory, along with Joseph Proust's law of definite composition (which states that compounds are formed by defined mass ratios of reacting elements) and the law of conservation of mass that was proposed by Antoine Lavoisier.
  • These laws paved the way for our current understanding of atomic structure and composition, including concepts like molecular or chemical formulas.

• The Law of Definite Composition

  • The law of definite composition states that chemical compounds are composed of a fixed ratio of elements as determined by mass.
  • In 1806, Proust summarized his observations in what is now called Proust's Law.
  • There are some exceptions to the law of definite composition.
  • In addition, the law of definite composition does not account for isotopic mixtures.
  • This video examines the law of definite proportions and the law of multiple proportions.

• The Law of Conservation of Mass

  • The law of conservation of mass states that mass in an isolated system is neither created nor destroyed.
  • However, Antoine Lavoisier described the law of conservation of mass (or the principle of mass/matter conservation) as a fundamental principle of physics in 1789.
  • This law was later amended by Einstein in the law of conservation of mass-energy, which describes the fact that the total mass and energy in a system remain constant.
  • An additional useful application of this law is the determination of the masses of gaseous reactants and products.
  • A portrait of Antoine Lavoisier, the scientist credited with the discovery of the law of conservation of mass.

• Overall Reaction Rate Laws

  • Step two is the slow, rate-determining step, so it might seem reasonable to assume that the rate law for this step should be the overall rate law for the reaction.
  • The overall rate law cannot contain any such intermediates, because the rate law is determined by experiment only, and such intermediates are not observable.
  • How to determine the rate law for a mechanism with a fast initial step.
  • Remember, the overall rate law must be determined by experiment.
  • Therefore, the rate law must contain no reaction intermediates.

• The Integrated Rate Law

  • Recall that the rate law for a first-order reaction is given by:
  • Recall that the rate law for a second-order reaction is given by:
  • The final version of this integrated rate law is given by:
  • In this case, we can say that [A]=[B], and the rate law simplifies to:
  • This is the standard form for second-order rate law, and the integrated rate law will be the same as above.

• Charles' and Gay-Lussac's Law: Temperature and Volume

  • Charles' and Gay-Lussac's Law states that at constant pressure, temperature and volume are directly proportional.
  • Charles' Law describes the relationship between the volume and temperature of a gas.
  • The law was first published by Joseph Louis Gay-Lussac in 1802, but he referenced unpublished work by Jacques Charles from around 1787.
  • This extrapolation of Charles' Law was the first evidence of the significance of this temperature.
  • Discusses the relationship between volume and temperature of a gas, and explains how to solve problems using Charles' Law.