Entropy Change

• The spontaneity of a chemical reaction is determined by the entropy change.
• Entropy change refers to the measure of the dispersal of energy and particles.
• It is denoted by the symbol ‘ΔS’ with the units ‘J K^-1 mol^-1’.
• A positive change in entropy (ΔS) indicates a system has become more disorderly, or has seen an increase in randomness.
• Conversely, a negative change in entropy suggests a decrease in randomness or an increase in order.
• The second law of thermodynamics suggests that for any given process occurring in a closed system, the entropy should always increase.
• When a substance is transformed from a solid to a liquid and eventually to a gas, there is an increase in entropy.
• The entropy of a pure perfect crystal is zero at absolute zero according to the third Law of Thermodynamics.
• Entropy change can either occur in the system where the reaction is happening or in the surroundings where energy is transferred.
• Calculations involving entropy change often employ the formula ΔS(total) = ΔS(system) + ΔS(surroundings).
• The entropy change for the surroundings (ΔS(surroundings)) can be calculated using the equation ΔS = q(rev) / T.
• To find the entropy change of the system, the entropy values of the products are subtracted from the reactants.
• A reaction is ‘feasible’ as long as the total entropy change (ΔS total) is positive.
• However, even if total entropy is positive, reactions may not occur without the application of energy due to the energy barrier.
• Gibbs Free Energy (∆G) takes into account not only the entropy change but also the enthalpy change.
• Gibbs Free Energy helps to predict the feasibility of a reaction at non-standard conditions. If ∆G is negative, a reaction may occur spontaneously.
• Note that a reaction can still potentially occur non-spontaneously if ∆G is positive, but it will require added energy inputs.

These core points cover the concept of entropy change, offering a baseline of understanding upon which the specifics of various reactions can be studied. As with other chemistry topics, practise helps solidify comprehension of entropy change.