AP Chemistry : Endergonic and Exergonic Reactions
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Example Questions
Example Question #1 : Endergonic And Exergonic Reactions
Which of these conditions will result in a spontaneous reaction?
All exothermic reactions
Exothermic reactions with reactants going from liquid to gaseous state
Endothermic reactions with reactants going from liquid to gaseous state
Exothermic reactions with reactants going from gaseous to liquid state
Endothermic reactions with reactants going from gaseous to liquid state
Exothermic reactions with reactants going from liquid to gaseous state
To determine whether a reaction will occur spontaneously, one needs to look at whether the Gibbs free energy will decrease. In order for Gibbs free energy to decrease, the combination of the heat of the reaction and the change in entropy must give a negative Gibbs free energy. The equation is:
If a reaction is exothermic, the heat of the reaction is negative. If the reactants go from liquid to gaseous state, the entropy increases (is positive). When the enthalpy is negative and the entropy is positive, the equation must always give a negative solution. This leads to a negative Gibbs free energy, so the reaction will be spontaneous.
Example Question #2 : Endergonic And Exergonic Reactions
Which of the following reaction conditions cannot result in a spontaneous reaction?
Exothermic reactions in which reactants go from gaseous to liquid state
Exothermic reactions in which reactants go from liquid to gaseous state
Endothermic reactions in which reactants go from gaseous to liquid state
Exothermic reactions in which reactants go from liquid to solid state
Endothermic reactions in which reactants go from liquid to gaseous state
Endothermic reactions in which reactants go from gaseous to liquid state
To determine whether a reaction will occur spontaneously, one needs to look at whether the Gibbs free energy will decrease. In order for Gibbs free energy to decrease, the combination of the heat of the reaction and the change in entropy must give a negative Gibbs free energy. The equation is:
If a reaction is endothermic, the heat of the reaction is positive. If the reactants go from gaseous to liquid state, the entropy decreases (is negative). When the enthalpy is positive and the entropy is negative, the equation must always give a positive solution. This leads to a positive Gibbs free energy, so the reaction can never be spontaneous.
Example Question #3 : Endergonic And Exergonic Reactions
Which of the following values must be negative in order for a reaction to be spontaneous?
In order to determine if a reaction is spontaneous, one needs to look at 
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