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Absolute Entropy & Entropy Change

Chemistry: IB Diploma Higher Level, topic 4.3.2

Entropy Calculations

You need to be able to calculate entropy changes.

Standard entropies

Standard entropies

  • Over time, chemists have developed a table of entropies of hundreds of molecules and elements. These entropies are all measured under standard conditions:
  • 1atm pressure.
  • 298K temperature.
  • To calculate the entropy change, you must find the difference between the standard entropies of the products and the reactants.

Calculating entropy changes

  • The standard entropy change (ΔS°) for a reaction may be computed using standard entropies as shown below:
  • Δ𝑆° = ∑𝜈𝑆°(products) − ∑𝜈𝑆°(reactants)
  • Where ν represents stoichiometric coefficients in the balanced equation representing the process.
Example - the Haber process

Example - the Haber process

  • The equation for the Haber process is:
  • N2(g) + 3H2(g) ⇆ 2NH3(g)
  • If the entropy of nitrogen and ammonia gas is 192JK-1mol-1 and the entropy of hydrogen gas is 131JK-1mol-1, what is the entropy change?
  • Entropy change = (2 x 192) - (3 x 131) − 192 = −201JK-1mol-1
  • This fits well with the observation that the Haber process doesn’t produce much ammonia!

Test yourself on Absolute Entropy & Entropy Change

  • What is a key feature of entropy values that is not true for enthalpy values?

  • The Haber process is: N2(g) + 3H2(g) ⇆ 2NH3(g)
    We found that the entropy change is equal to −201JK-1mol-1.
    Why does this value suggest that not much ammonia is produced?

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