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More Than 475 Free MCAT Questions with Detailed Answers!

## General Chemistry - Entropy

Entropy is the measure of disorder in a system. When left alone, systems tend to go from conditions of greater order (low entropy) to conditions of greater disorder (higher entropy). Entropy means disorder. Entropy has the symbol S, and the change in entropy after a reaction is ΔS.

You may have to figure out whether a reaction has positive or negative ΔS. If you do, think about whether the products are more or less random. If the products are more random, the ΔS is positive. If the products are less random, then the ΔS is negative. Remember that liquids are more random than solids, and gases are more random than liquids.

Enthalpy
The enthalpy of a substance is the amount of energy that comes from its bonds. The symbol for enthalpy is H, and different substances have different amounts of enthalpy. The amount of enthalpy in the substance changes when that substance reacts since bonds are broken and formed during the reaction.

The change in enthalpy from a reaction, ΔH, is often called the heat of reaction. It is equal to the difference between the enthalpy of the products of the reaction and the enthalpy of the reactants.

Rule number 16:
Change in enthalpy equals enthalpy of the product minus the enthalpy of the reactant, or ΔH = HP - HR.

If the products of the reaction have less enthalpy than the reactants, then the reaction is called exothermic, which means that heat has been released by the reaction and the ΔH is negative. If the products of the reaction have more enthalpy than the reactants, then the reaction is called endothermic, which means that heat has been absorbed by the reaction and the ΔH is positive.

Generally, all things tend to move toward two distinct states: high entropy and low enthalpy. We can use Gibbs free energy to find out how spontaneous a reaction is. The symbol for Gibbs free energy is G.

Rule number 17.
ΔG = ΔH - TΔS. T is the temperature in Kelvin.

If the ΔG obtained from this reaction is negative, then the reaction is spontaneous. If the ΔG obtained from this reaction is positive, then the reaction is nonspontaneous. If the ΔG is zero, then the reaction is at equilibrium.