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A reaction is spontaneous when
1. ΔG = -ve
2. ΔH = -ve
3. ΔS = +ve
4. ΔS = -ve

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Correct Answer - Option 1 : ΔG = -ve

The correct answer is ΔG = -ve.

  • A spontaneous reaction is a reaction that favors the formation of products at the conditions under which the reaction is occurring.
  • Example: A roaring bonfire is a spontaneous reaction.
    • A fire is exothermic, which means a decrease in the energy of the system as energy is released to the surroundings as heat.
    • The products of fire are composed mostly of gases such as carbon dioxide and water vapor, so the entropy of the system increases during most combustion reactions.
    • This combination of a decrease in energy and an increase in entropy means that combustion reactions occur spontaneously.
  • The free energy change (ΔG) of a reaction is a mathematical combination of the enthalpy change (ΔH) and the entropy change (ΔS).
  • The change in Gibbs free energy is equal to the change in enthalpy minus the mathematical product of the change in entropy multiplied by the Kelvin temperature.
    • Each thermodynamic quantity in the equation is for substances in their standard states, as indicated by the o superscripts.
  • A spontaneous reaction releases free energy, and so the sign of ΔG must be negative.
    • Since both ΔH and ΔS can be either positive or negative, depending on the characteristics of the particular reaction.

  • Summary of Gibbs free energy
Enthalpy change Entropy change Gibbs free energy Spontaneity
positive positive depends on T, maybe + or - yes, if the temperature is high enough
negative positive always negative always spontaneous
negative negative depends on T, maybe + or - yes, if the temperature is low enough
positive negative always positive never spontaneous

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