IB DP Chemistry: SL复习笔记5.2.1 Hess's Law

Hess's Law

  • In 1840, the Russian chemist Germain Hess formulated a law which went on to be known as Hess’s Law
  • This went on to form the basis of one of the laws of thermodynamics. The first law of thermodynamics relates to the Law of Conservation of Energy
  • It is sometimes expressed in the following form:

Energy cannot be created or destroyed, it can only change form

  • This means that in a closed system, the total amount of energy present is always constant
  • Hess’s law can be used to calculate the standard enthalpy change of a reaction from known standard enthalpy changes
  • Hess’s Law states that:

"The total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same."

  • This means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same

1.5-Chemical-Energetics-Hess-Cycles

The diagram above illustrates Hess’ Law: the enthalpy change of the direct route, going from reactants (A+B) to product (C) is equal to the enthalpy change of the indirect routes

  • Hess’ Law is used to calculate enthalpy changes which can’t be found experimentally using calorimetry, eg:

3C (s) + 4H(g) → C3H8(g)

  • [popover id="dGQl3oDiqUIQrR1A" label="ΔHf"] , ΔHf (propane) can’t be found experimentally as hydrogen and carbon don’t react under standard conditions

Calculating ΔHfrom ΔHf using Hess’s Law energy cycles

  • You can see the relationships on the following diagram:1.5-Chemical-Energetics-Direct-and-Indirect-Routes

The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route)

  • The products can be directly formed from the elements = ΔH2

OR

  • The products can be indirectly formed from the elements = ΔH1 + ΔHr
  • Equation

ΔH2 = ΔH1 + ΔHr

Therefore for energy to be conserved,

ΔH= ΔH2 – ΔH1

Exam Tip

You do not need to learn Hess's Law word for word as it is not a syllabus requirement, but you do need to understand the principle as it provides the foundation for all the problem solving in Chemical Energetics

 

 

 

 

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