2.1.3

Entropy (A2 Only)

Test yourself

Entropy

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

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

• 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

• 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

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

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

• 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

• 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

• 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

• 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!