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If ΔH is negative and ΔS is positive, what can be said about ΔG?

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Question: If ΔH is negative and ΔS is positive, what can be said about ΔG?

Options:

  1. ΔG is always positive.
  2. ΔG is always negative.
  3. ΔG can be positive or negative depending on temperature.
  4. ΔG is zero.

Correct Answer: ΔG is always negative.

Solution: 

If ΔH is negative and ΔS is positive, ΔG will always be negative, indicating that the reaction is spontaneous at all temperatures.

If ΔH is negative and ΔS is positive, what can be said about ΔG?

Practice Questions

Q1
If ΔH is negative and ΔS is positive, what can be said about ΔG?
  1. ΔG is always positive.
  2. ΔG is always negative.
  3. ΔG can be positive or negative depending on temperature.
  4. ΔG is zero.

Questions & Step-by-Step Solutions

If ΔH is negative and ΔS is positive, what can be said about ΔG?
Correct Answer: ΔG is negative
  • Step 1: Understand what ΔH means. ΔH is the change in enthalpy (heat content) of a reaction. If ΔH is negative, it means the reaction releases heat.
  • Step 2: Understand what ΔS means. ΔS is the change in entropy (disorder) of a reaction. If ΔS is positive, it means the disorder of the system increases.
  • Step 3: Know what ΔG means. ΔG is the change in Gibbs free energy, which helps determine if a reaction is spontaneous (can happen on its own).
  • Step 4: Use the Gibbs free energy equation: ΔG = ΔH - TΔS, where T is the temperature in Kelvin.
  • Step 5: Since ΔH is negative and ΔS is positive, the term -TΔS will also be negative (because T is always positive).
  • Step 6: When you combine a negative ΔH and a negative -TΔS, ΔG will be negative.
  • Step 7: A negative ΔG means the reaction is spontaneous, which means it can occur without needing extra energy.
  • Step 8: Therefore, if ΔH is negative and ΔS is positive, ΔG will always be negative at all temperatures.
  • Gibbs Free Energy – ΔG is the change in Gibbs free energy, which determines the spontaneity of a reaction based on enthalpy (ΔH) and entropy (ΔS).
  • Thermodynamic Relationships – The relationship ΔG = ΔH - TΔS shows how changes in enthalpy and entropy affect the spontaneity of a reaction.
  • Spontaneity of Reactions – A negative ΔG indicates that a reaction is spontaneous, while a positive ΔG indicates non-spontaneity.
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