Entropy

Entropy and the Ocean

Entropy Definition and Significance

• Entropy is a state function represented by the symbol S.
• It measures the degree of randomness or disorder in a system.
• Systems tend towards a state of maximum entropy, as per the second law of thermodynamics.
• In chemical reactions this is evident in the transformation of reactants to products, establishing an equilibrium where entropy is at a maximum.

Entropy in Chemical Reactions

• ΔS is the change in entropy during a reaction. If ΔS > 0, the reaction increases the system’s disorder, while if ΔS < 0, the reaction decreases the system’s disorder.
• Entropy typically increases (positive ΔS) with the dissolution of a solid, the formation of gas, and generally, amount of substances in chemical reactions.
• Combustion reactions often result in a positive ΔS because one molecule forms several products, increasing disorder.

Entropy in Oceanic Chemistry

• Understanding entropy aids in recognising how solutes dissolve in seawater.
• The dissolution of salts in seawater is largely an entropy-driven process, where the structured solid becomes disordered ions in solution, significantly increasing entropy (positive ΔS).
• Many common salts, like sodium chloride and potassium chloride, have positive entropy values upon dissolution.
• Ocean water, being a mixture of numerous ionic entities, exhibits complex entropy-related behaviour. An understanding of these behaviours can contribute to insights about ocean currents and marine life processes.
• The concentration of salts in seawater also affects the entropy and thermodynamic behaviour of the ocean, influencing everything from the freezing point of the water to the density of sea ice.

Gibbs Free Energy and Entropy

• Gibbs free energy (ΔG) is a measure of the maximum reversible work that a thermodynamic system can perform.
• The relationship between entropy, enthalpy, and Gibbs free energy is expressed in the equation ΔG = ΔH - TΔS.
• Negative ΔG (ΔG < 0) implies a spontaneous reaction, often driven by entropy increase in dissolution processes in the ocean.
• Positive ΔG (ΔG > 0) implies a reaction is nonspontaneous, but entropy can still contribute if the energy input (temperature * entropy change) outweighs the enthalpy input.
• Understanding the interplay between entropy and Gibbs free energy is important in explaining how salts dissolve and whether the process is spontaneous or not.