ENTHALPY AND ENTROPY FREE

ENTHALPY AND ENTROPY FREE

In this question, we are asked to apply the formula from above to calculate Gibbs Free Energy change and then use the sign of ΔG° to find the spontaneity of the reaction. If at 25☌ ΔH° = 50.6 kJ/mol and ΔS° = -0.332 kJ/(molK), determine the value of ΔG° for the reaction and conclude whether it is thermodynamically favorable. If ΔG° is positive, the reaction is endergonic and therefore non-spontaneous and thermodynamically unfavorable.Ĭonsider the following reaction: 2H2 + N2 ⇌ N2H4. If ΔG° is negative, the reaction is exergonic and therefore spontaneous and thermodynamically favorable. The following rules can be applied when calculating ΔG°, but remember to logically understand the rules as well: We’ll see examples of reactions that are endothermic but spontaneous and vice versa because of this reason along with the equivalent statements regarding entropy. We see enthalpy and entropy combined in one equation meaning both factor into thermodynamic favorability. The formula for ΔG° can be seen as a combination of our two measures of spontaneity. From here, we can conclude whether a reaction is exergonic (energy releasing) or endergonic (energy absorbing) and therefore either spontaneous or nonspontaneous. Therefore, calculating ΔG° tells us the amount of energy either released from a reaction (if it’s negative) or absorbed (positive). In simple terms, Gibbs Free Energy calculates the amount of energy available in a system to do work. The equation for Gibbs Free Energy is given to you during the examination on the AP Chemistry reference sheet. Knowing if a reaction is exothermic or endothermic is the first piece of the puzzle. If ΔH° is positive, we know the opposite-the system is gaining heat from the surroundings and is therefore endothermic. If ΔH° is negative, we know our system is losing heat to the surroundings and is thus exothermic. There are two main measures that play into spontaneity:įirst, we have enthalpy change (ΔH°), the change in the heat of the system relative to the surroundings due to a reaction. By measuring a reaction’s spontaneity, we can draw predictions about whether the reaction is product-favored or reactant-favored. Spontaneous reactions are considered thermodynamically favorable and non-spontaneous reactions are considered thermodynamically unfavorable.Īs we’ll see in section 9.5, spontaneity has intimate connections with the equilibrium constant for the process in question. As we discussed earlier in this unit, a spontaneous process is one that occurs without external inputs, whereas a nonspontaneous reaction requires such external inputs in order to occur. Thermodynamic favorability is used to predict whether a reaction (or any process for that matter, but for our purposes, a chemical reaction) is either spontaneous or nonspontaneous. This section of unit nine is all about a concept known as thermodynamic favorability.

Enthalpy and entropy