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MCAT Content / Energy Changes In Chemical Reactions / Spontaneous Reactions And Dg

Spontaneous reactions and ΔG°

Topic: Energy Changes In Chemical Reactions

Gibbs Free Energy (G) is a single property that combines that entropy calculations for both a system and its surroundings and is measured in Joules; It is incorrect to assume that exothermic reactions are spontaneous.

The Gibbs free energy of a reaction dictates where a reaction is spontaneous (-ΔG) or nonspontaneous (+ΔG). This is due to the components of the free energy equation (ΔG = ΔH -TΔS) that add up to be negative or positive.

While an exothermic reaction (-ΔH) may suggest spontaneous reaction, a high temperature (T) or large entropy (ΔS) may negate the negative enthalpy and result in a positive ΔG value. Likewise, an endothermic reaction may be spontaneous from low temperature and small entropy change.

The designation of ΔG0 indicates the thermodynamic favorability of the reaction at equilibrium (with a positive value being unfavorable). This is a property of the reaction.

Practice Questions

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MCAT Official Prep (AAMC)

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Key Points

• Gibbs Free energy can be calculated from the equation ΔG = ΔH -TΔS and is in Joules.

• Exothermic reactions can be nonspontaneous

• Endothermic reactions can be spontaneous

Key Terms

spontaneous reaction: reaction that occurs without external activity

non-spontaneous reaction: reaction that requires the input of additional action to proceed

Gibbs free energy: the difference between the internal energy of a system and the product of its entropy and absolute temperature

exothermic: net release of heat.

endothermic: net absorbance of heat

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