A21.2 Entropy

What is a feasible reaction and how can we determine if it’s feasible or not?

A reaction that can occur is called a feasible reaction. The change in enthalpy cannot be the only factor that determines whether a reaction is feasible or not. Entropy is also involved in this.

What is entropy?

Entropy is a measure of disorder or randomness. Entropy is given the symbol S. An increase in the disorder of a system has a positive entropy value and is measured in Jk-1 mol-1.

Entropy changes of state?

At zero kelvin all substances should have 0 entropy. This is because at zero kelvin substances should not be moving and be perfectly ordered. As temperature increases the substance becomes more disordered as the particles gain energy and move around. The vertical sections on the graph show changes in state where there is a large increase in entropy at a constant temperature. The vertical sections for boiling is larger than melting. As during boiling the substance turning into a gas. Gases have a higher entropy value as they are more disordered.

Entropy equation?

S= s products- s reactants units=Jk-1 mol-1

The number of moles has to be included. A negative value= more ordered

Free energy?

In feasible reactions free energy must be less than zero. The units for free energy are KJ mol-1. When finding free energy you must remember to convert entropy to kilojoules by dividing by 1000. G=H+TS.

How do you find temperature of a feasible reaction?

T=H/S

Predicting feasibility?

Exothermic+ positive entropy= feasible at any temperature

Endothermic+ negative entropy= never feasible.

The other options depend on the temperature