hess's law SCH4U1 - Hess's law A representation of Hess law(where H represents enthalpy Hess law is a relationship in physical chemistry named after

# hess's law SCH4U1 - Hess's law A representation of Hess...

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Hess's law A representation of Hess' law (where H represents enthalpy) Hess' law is a relationship in physical chemistry named after Germain Hess, a Swiss-born Russian chemist and physician. This law states that if a reaction takes place in several steps then its standard reaction enthalpy is the sum of the standard enthalpies of the intermediate reactions into which the overall reaction may be divided at the same temperature. The law is an expression of the principle of conservation of energy, also expressed in the first law of thermodynamics, and that the enthalpy of a chemical process is independent of the path taken from the initial to the final state. Hess' law can be used to determine the overall energy required for a chemical reaction, when it can be divided into synthetic steps that are individually easier to characterize. This affords the compilation of standard enthalpies of formation, that may be used as a basis to design complex syntheses. Definition Hess' law states that the energy change for any chemical or physical process is independent of the pathway or number of steps required to complete the process provided that the final and initial reaction conditions are the same. In other words, an energy change is path independent, only the initial and final states being of importance. This path independence is true for all state functions. Hess' law allows the enthalpy change (ΔH) for a reaction to be calculated even when it cannot be measured directly. This is accomplished by performing basic algebraic operations based on the chemical equation of reactions using previously determined values for the enthalpies of formation. Addition of chemical equations may lead to a net equation. If enthalpy change is known for each equation, the result will be the enthalpy change for the net equation. If the net enthalpy change is negative (ΔHnet < 0), the reaction is

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