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Entropy (A2 Only)

Chemistry: AQA A Level, topic 2.1.3

Entropy

Entropy is a measure of disorder. Entropy always increases during a reaction.

Entropy

Entropy

  • ΔH is not enough to determine whether or not a reaction happens.
  • When NaCl dissolves, the reaction is endothermic - but it still happens!
Stability

Stability

  • Substances are more stable when they have greater disorder.
  • This means that everything tends to increase its disorder.
  • This is why your phone cables get tangled in your pocket!
  • It’s also why NaCl dissolves in water - it's entropy increases.
Technical definition

Technical definition

  • Entropy is technically defined as the number of ways molecules can be arranged, and the number of ways their energy can be arranged between them.
  • Entropy is given the symbol S.
  • A big and positive value of S indicates a large disorder.

Factors Affecting Entropy

Entropy is affected by many factors, including temperature, physical state and number of moles.

Temperature

Temperature

  • An increase in temperature increases the speed particles move at. This increases disorder and so entropy increases.
Physical state

Physical state

  • The molecules in gases are free to move. This means that they have a higher entropy than solids and liquids.
  • In the same way, liquids have higher entropy than solids.
Number of moles

Number of moles

  • There are more ways you can arrange ten particles than two particles. So the more moles of substance, the greater the entropy.
Conditions for a reaction

Conditions for a reaction

  • This all means that a reaction is more likely to happen if:
  • The reaction increases the number of moles of substance.
  • The reaction produces gas and consumes solids.
  • The reaction is exothermic.

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.
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 Entropy (A2 Only)

  • Can ΔH alone determine whether or not a chemical reaction can happen?

  • Why is ΔH not enough to determine whether or not a reaction happens?

  • Why do gas molecules have a greater entropy than solids and liquids?

  • Entropy of Phases

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

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