Edexcel A Level Chemistry:复习笔记1.8.4 Hess Cycles

• 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

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, e.g.:

3C (s) + 4H2 (g) → C3H8(g)

• ΔfH (propane) can’t be found experimentally as hydrogen and carbon don’t react under standard conditions

Calculating ΔrH from ΔfH using Hess’s Law energy cycles

• You can see the relationships on the following diagram:

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,
ΔHr = ΔH2 – ΔH1

Exam Tip

• Hess cycles can be used to calculate various enthalpy changes as long as sufficient information about the other sides of the cycle is known

Worked Example

Calculating the enthalpy change of reaction
Calculate the ΔHf for the following reaction:
2NaHCO3 (s)  → Na2CO3 (s) + CO2 (g) + O(I)
The table below shows the standard enthalpy of formations (ΔHfꝋ) relevant to this reaction:

• • ΔrH = ΔH2 - ΔH1
    • It is minus ΔH1 because you have to go in the opposite direction of the arrow
  • ΔH2 is ΔfH [Na2CO3 (s)] + ΔfH [CO2 (g)] + ΔfH [H2O (l)]
  • ΔH1 is 2 x ΔfH [NaHCO3 (s)]
  • ΔrH = (ΔfH [Na2CO3 (s)] + ΔfH [CO2 (g)] + ΔfH [H2O (l)]) - (2ΔfH [NaHCO3 (s)])
  • ΔrH = ((-1130.7) + (-393.5) + (-285.8)) - (2 x (-950.8))
  • ΔrH = +91.6 kJ mol-1

Exam Tip