Entropy: how far? (summary)

Entropy: how far?

The word “entropy” refers to disorder. The greater the entropy S, the greater the disorder. Here’s a list of the equations and concepts you need to know for this unit

Here’s a good link I found that explains this topic. http://www.chem.tamu.edu/class/majors/tutorialnotefiles/entropy.htm

·         S_solid < S_liquid < S_gas

·         The entropy of a substance increases as its complexity increase, i.e. the larger the atom/molecule the greater its entropy

·         Units of entropy change (ΔS) is JK^-1mol^-1

·         Units of enthalpy change (ΔH) is kJmol-¹

·         ΔS° _(total) = ΔS° _(system) + ΔS° _{(surroundings)}

·         ΔS° _(total) = ΔS° _(system)  + (-ΔH /T)

·         ΔS° _(system) = ∑S_products - ∑S_reactants

·         A reaction is thermodynamically spontaneous only if ΔS° _(total) is positive

·         In reactions: if a solid is formed from a gas or liquid then ΔS° _(system) of the reaction is –ve. If a gas is formed from a solid or liquid then ΔS° _(system) of the reaction is +ve

If products and reactants are in the same state, but the number of molecules formed on the products side is greater than the number of molecules on the reactants side then ΔS° _(system) of the reaction is +ve. That means more disorder is created.

·         Exothermic reactions where ΔH is –ve result in a +ve ΔS° _{(surroundings)}

              Endothermic reactions where ΔH is +ve result in a –ve ΔS° _{(surroundings)}
 
Exothermic reactions release energy to the surroundings which results in an increase in the movement (kinetic energy) of the surrounding molecules, leading to an increase in entropy. Endothermic reactions use up energy from the surroundings which results in a decrease in the entropy of the surroundings

ΔS° (system)	ΔS° (surroundings)	Feasible
+ve	+ve	Feasible at all temperatures
-ve	+ve	Feasible if ΔH/T > ΔS (system)
+ve	-ve	Feasible if ΔS (system) > ΔH/T
-ve	-ve	Not feasible at any temperature (thermodynamically & kinetically stable)

 When reaction is exothermic, at lower temperature the value of ΔH/T is more +ve hence ΔS _(total) is more +ve, reaction is more feasible. At higher temperatures, reaction is not feasible.

When reaction is endothermic, at higher temperature the value of ΔH/T is less negative, therefore the value of ΔS _(total) is more +ve.

·         Temperature in K = C° + 273

·         For exothermic reactions: when the surroundings are hot, the entropy increase of surroundings is small, because molecules already have high entropy, and vice versa.

·         If ΔS _(total) is –ve, the reverse reaction is thermodynamically spontaneous at all temperatures

·         Even if ΔS _(total) of a reaction is +ve, the reaction may be too slow (reactants are kinetically stable) at low temperatures.

·         Kinetically stable means that reactants require high activation energy for the reaction to start. This high energy barrier could be overcome by applying more heat or using a catalyst.

·         Thermodynamics give no information about reaction rate. A thermodynamically feasible reaction might have such a high activation energy that it does not proceed at room temperature.

·         Spontaneous endothermic reactions that take place at room temperature normally require small activation energy. Therefore, reactants are thermodynamically & kinetically unstable.

·         A +ve ΔS _(total) means that reactants are thermodynamically unstable relative to the products

·         A –ve ΔS _(total) means that the reactants are thermodynamically stable relative to the products

·         A small activation energy means that the reactants are kinetically unstable relative to the products

·         A large activation energy means that the reactants are kinetically stable relative to the products.