2.1.3

Entropy (A2 Only)

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Entropy

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

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Entropy

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

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Temperature

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

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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.
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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!

Jump to other topics

1Physical Chemistry

1.1Atomic Structure

1.2Amount of Substance

1.3Bonding

1.4Energetics

1.5Kinetics

1.6Equilibria

1.7Redox

2Physical Chemistry 2 (A2 Only)

3Inorganic Chemistry

4Inorganic Chemistry 2 (A2 Only)

5Organic Chemistry 1

6Organic Chemistry 2 (A2 Only)

6.1Optical Isomerism (A2 Only)

6.2Aldehydes & Ketones (A2 Only)

6.3Carboxylic Acids & Esters (A2 Only)

6.4Aromatic Chemistry (A2 Only)

6.5Amines (A2 Only)

6.6Polymers (A2 Only)

6.7Biological Organic (A2 Only)

6.8Organic Synthesis (A2 Only)

6.9NMR Spectroscopy (A2 Only)

6.10Chromatography (A2 Only)

6.11A-A* (AO3/4) - Organic 2

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