Question

Which of the following pairs of a chemical reaction is certain to result in a spontaneous reaction?

• A. Exothermic and decreasing disorder
• B. Endothermic and increasing disorder
• C. Exothermic and increasing disorder
• D. Endothermic and decreasing disorder

Answer 1

The Correct Option is C

To determine which pair of chemical reaction characteristics is certain to result in a spontaneous reaction, we need to consider two key thermodynamic concepts: enthalpy change (\Delta H) and entropy change (\Delta S).

1. Enthalpy Change (\Delta H): Spontaneous reactions are often exothermic, which means they release heat, thus \Delta H is negative.
2. Entropy Change (\Delta S): Spontaneous reactions also tend to involve an increase in disorder or randomness, indicated by a positive change in entropy.

For a reaction to be spontaneous, the change in Gibbs free energy (\Delta G) should be negative, which is calculated by the equation:

\Delta G = \Delta H - T\Delta S

Where:

• \Delta G is the change in Gibbs free energy
• T is the temperature in Kelvin
• \Delta S is the change in entropy

Now, let's evaluate each option:

1. Exothermic and decreasing disorder: With a negative \Delta H but negative \Delta S as well, the reaction may not be spontaneous if the negative temperature times negative entropy results in a reduction in Gibbs free energy.
2. Endothermic and increasing disorder: Although the \Delta S is positive, an endothermic reaction implies \Delta H is positive, which might not favor spontaneity unless T\Delta S is greater than \Delta H.
3. Exothermic and increasing disorder: This is ideal for spontaneity, as both negative \Delta H and positive \Delta S contribute to a negative \Delta G, ensuring the reaction is spontaneous.
4. Endothermic and decreasing disorder: Both positive \Delta H and negative \Delta S lead to a positive \Delta G, making the reaction non-spontaneous.

Therefore, the correct answer is Exothermic and increasing disorder, as this combination ensures a negative \Delta G and a spontaneous reaction.