#CALCULATING ABSOLUTE ENTROPY FREE#

#CALCULATING ABSOLUTE ENTROPY FREE#

The change in Gibbs free energy is also related to change in entropy for the universe: ΔG = -TΔSuniv (again through some algebra). Using the above equations and some algebra we can construct the formula: ΔG = ΔH – TΔSsyt, where ΔG is the change in free energy for the system. The opposite effect happens for an endothermic reaction.Īnd then finally probably the most useful formula is Gibbs free energy which measures how spontaneous a reaction is. So if a reaction is exothermic, a negative change in enthalpy, heat is given off by the reaction and is dispersed into the surroundings causing an increase in the surrounding’s entropy. It’s important to note the negative sign in the equation which means that the change in enthalpy for the system creates an opposite signed entropy change in the surroundings. And it only works at specific temperatures (different values of T cause different changes in entropy) since entropy is temperature dependent. This equation only works at constant pressure since enthalpy equals heat only at constant pressure. Since a change in entropy (and enthalpy) for a system creates an accompanying change in entropy for the surroundings we can use the formula: ΔSsur = -ΔHsyt/T, where ΔHsyt is the change in enthalpy for the system, and T is temperature in kelvins. The change in enthalpy for a reaction we can determine experimentally using calorimetry.

But we can relate the change in the surrounding’s entropy to another thermodynamic quantity we know well, the enthalpy change of the system. Measuring the change in entropy for a system is relatively easy, but measuring the change in entropy for the surroundings is less easy to do directly. Using this formula we can judge if a reaction is spontaneous or not based on whether its change in entropy, and the surrounding accompanying change in entropy, facilitates a positive or negative change in entropy for the universe (following the second law). Together we get a formula for the overall change of entropy based on the system and surroundings: ΔSuniv = ΔSsyt + ΔSsur, where ΔSsyt is the change in the entropy of the system and ΔSsur is the change in the entropy of the surroundings. And the entropy of the surroundings (everything but the system itself) also being influenced by the entropy of the system. So we can imagine a reaction as a system whose entropy we care about. Usually when we do chemistry though we’re concerned not so much directly with the entropy of the universe, but rather the entropy change which accompanies a chemical reaction. This is why gas particles spread out in a container instead of concentrate themselves into a small area. The law and the formula essentially state that the universe naturally prefers greater levels of entropy and allows any process which causes a positive increase, but prohibits any process which causes a negative increase.

A spontaneous process is one which happens naturally without the need of outside energy or work to help it along. So W is measuring how many energetically equivalent microstates a system can organize its energy into to get the same macrostate (or the system as we can observe it on a human scale).Īnother formula uses the second law of thermodynamics: ΔSuniv > 0, which put into words states that any spontaneous process increases the entropy of the universe (creates a positive change). A microstate is a particular way to order energy. Boltzmann’s formula uses a more mathematical approach to entropy by using W. The unit for entropy is the same as Boltzmann’s constant which means that entropy can also be understood as how many energy can be dispersed at a certain temperature since entropy is temperature dependent. One equation is Boltzmann’s equation: S = k*ln(W), where S is entropy (the usual variable for entropy), k is Boltzmann’s constant which is equal to the gas constant divided by Avogadro’s number which is approximately equal to 1.38 x 10^(-23) J/K, and W is the number of microstates which is a unitless quantity. There are several because entropy can be explained and used in a variety of ways. Since entropy is primarily dealing with energy, it’s intrinsically a thermodynamic property (there isn’t a non-thermodynamic entropy).Īs far as a formula for entropy, well there isn’t just one.

First it’s helpful to properly define entropy, which is a measurement of how dispersed matter and energy are in a certain region at a particular temperature.

Calculating absolute entropy