If there are 2 moles of reactant forming 1 mole of product, the number of states possible decreases and entropy decreases, likewise, if 1 mole of reactant forms 2 moles of product, the entropy increases.Įntropy can be calculated using the equation Example of increasing entropy: Ice melting in a warm room is a common example of increasing entropy. If a substance goes from a liquid to a solid, the molecules in the system occupy a lower number of states (think about the decrease of molecule activity in water when it freezes to ice) and therefore the entropy decreases if a substance goes from a solid to a liquid, the entropy increases. When considering whether reactions have an increase or decrease in entropy, two factors are important: The number of moles created and states of the reactants versus the products. Liquids can be considered the second most disordered, and solids can be considered the least disordered. Because molecules in gases are able to change both volume and shape, they can be considered the most disordered. We can think of the degrees of the number of different states the molecules in each can occupy. Solids, liquids, and gasses all have different degrees of disorder. If an ideal gas undergoes a change from P 1, v 1, T 1 to P 2, v 2, T 2 the change in entropy can be calculated by devising a reversible path connecting the two given states.

The concept of disorder can best be described in terms of the states of matter. ds (du/T) + (Pdv/T) ds (dh/T) (vdP/T) Change of state for an ideal gas. The unit of entropy is Joules per Kelvin or J/K.Įntropy is the measure of the disorder of a system and can is the energy of a system over its temperature, represented as J/K.

The Second Law of Thermodynamics states that the entropy (or disorder) of a system is constantly increasing.

Change in entropy equation