Master Spontaneous vs. Nonspontaneous with Picmonic for College

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Hydrogen Bonds

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Gibbs Free Energy

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Spontaneous vs. Nonspontaneous

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Gibbs free energy measures whether a reaction is spontaneous or not with the equation ΔG = ΔH - TΔS. Thus, based on whether ΔH or ΔS are positive or negative, the reaction may be spontaneous or nonspontaneous at low or high temperatures. It is useful to remember at what temperatures the reaction is spontaneous as written with varying enthalpy and entropy values. If ΔH and ΔS are both negative, the reaction is only spontaneous at low temperatures. This is because at low temperatures the entropy term will be a smaller positive number, making it more likely that the negative enthalpy can drive the spontaneity of the reaction. The opposite is true if ΔH and ΔS are both positive. In this case, a high temperature is needed to make the entropy term a large negative number, because the enthalpy change is positive. If ΔH is negative and ΔS is positive, the reaction is spontaneous at all temperatures because the change in Gibbs free energy is always negative. Contrastingly, if ΔH is positive and ΔS is negative, the reaction is nonspontaneous at all temperatures as written. This is because the change in Gibbs free energy is always positive.

12 KEY FACTS

Based on the equation for Gibbs Free Energy, there are certain reactions that are spontaneous at only low temperatures.

If ΔS, entropy, is negative, and ΔH, enthalpy, is negative, the reaction is spontaneous at low temperatures. Based on the equation ΔG = ΔH - TΔS, we want Gibbs free energy to be negative. This requires a low temperature because a negative change in entropy makes that term positive. If the temperature is high, Gibbs free energy is more likely to be positive.

Based on the equation for Gibbs Free Energy, there are certain reactions that are spontaneous at only high temperatures.

If ΔS, entropy, is positive, and ΔH, enthalpy, is positive, the reaction is spontaneous at high temperatures. Based on the equation ΔG = ΔH - TΔS, we want Gibbs free energy to be negative. This requires a high temperature because a positive change in enthalpy makes that term in the equation positive. If the temperature is high. In order for Gibbs free energy to be negative, the entropy term must be greater in magnitude than the enthalpy term, requiring a high temperature.

Some reactions are spontaneous at all temperatures depending on the values of enthalpy and entropy.

If ΔS, entropy, is positive, and ΔH, enthalpy, is negative, the reaction is spontaneous at all temperatures. This is because based on the equation ΔG = ΔH - TΔS, Gibbs free energy would always be negative. Thus, the reaction would always be spontaneous.

Some reactions will never be spontaneous as written based on the changes in entropy and enthalpy associated with them.

If ΔS, entropy, is negative, and ΔH, enthalpy, is positive, the reaction isnon spontaneous at all temperatures. This is because based on the equation ΔG = ΔH - TΔS, Gibbs free energy would always be positive and can never be negative. Thus, the reaction would always be nonspontaneous.

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