Entropy Changes (Edexcel International A Level Chemistry)

Changes of State

• The entropy of a substance changes during a change in state
• The entropy increases when a substance melts (change from solid to liquid)
  • Increasing the temperature of a solid causes the particles to vibrate more
  • The regularly arranged lattice of particles changes into an irregular arrangement of particles
  • These particles are still close to each other but can now rotate and slide over each other in the liquid
  • As a result, there is an increase in disorder

• The entropy increases when a substance boils (change from liquid to gas)
  • The particles in a gas can now freely move around and are far apart from each other
  • The entropy increases significantly as the particles become very disordered

• Similarly, the entropy decreases when a substance condenses (change from gas to liquid) or freezes (change from liquid to solid)
  • The particles are brought together and get arranged in a more regular arrangement
  • The ability of the particles to move decreases as the particles become more ordered
  • There are fewer ways of arranging the energy so the entropy decreases

The entropy of a substance increases when the temperature is raised as particles become more disordered

Production of a gas

• During the thermal decomposition of calcium carbonate (CaCO₃) the entropy of the system increases:

CaCO₃ (s) → CaO (s) + CO₂ (g)

• In this decomposition reaction, a gas molecule (CO₂) is formed
• The CO₂ gas molecule is more disordered than the solid reactant (CaCO₃), as it is constantly moving around
• As a result, the system has become more disordered and there is an increase in entropy

• Another example is the reaction of ethanoic acid with ammonium carbonate

(NH₄)₂CO₃ (s) + 2CH₃COOH (aq) → 2CH₃COO^- (aq) + 2NH₄⁺ (aq) + CO₂ (g) + H₂O (l) 

• There is a slight fall in temperature during the reaction indicating the process is endothermic
  • Energy is taken in from the surroundings
• The particles are well-ordered in the solid, and the disorder increases because a solution and, especially, a gas is formed so entropy increases during the reaction

Dissolving a solid

• When ammonium nitrate is dissolved in water, the temperature of the solution decreases, therefore the process is endothermic
  • Energy is taken in from the surroundings

NH₄NO₃ (s) + aq → NH₄⁺ (aq) + NO₃^- (aq)    ΔH = 25.7 kJ mol^-1

• The level of disorder increases as the particles are no longer in a fixed position and are now free to move

• However, it is not always this simple!
• When an ionic solid dissolves
  • Bonds between the particles are broken increasing the disorder and taking in energy 
  • Bonds between the solvent and particles are made reducing the disorder and releasing energy
• Therefore it is difficult to predict whether the process will be endothermic or exothermic 

Endothermic reactions between two solids 

• When barium hydroxide and ammonium chloride are mixed, they form a paste and the temperature drops significantly 
• Therefore this is an endothermic process
  • Energy is taken in from the surroundings

2NH₄Cl (s) + Ba(OH)₂.8H₂O (s)  → BaCl₂.2H₂O (s) + 2NH₃ (g) + H₂O (l)

• The gas produced is ammonia and this can be detected by the smell and it will turn damp red litmus paper blue
• When two solids are mixed and react together, the entropy change will depend on the physical state of the compounds made and not just on the energy changes in the reaction
• Two solids will of course have a very low entropy due to the high order 
• If a liquid or gas is produced the entropy will have increased due to the increased disorder of the particles