Problem 59 - Chemistry: An Atoms-Focused Approach

Question

Are exothermic reactions spontaneous only at low temperature? Explain your answer.

Step-by-Step Solution

Answer

Explain why or why not. Answer: No, exothermic reactions are not always spontaneous at low temperatures. The spontaneity of an exothermic reaction depends on the balance between enthalpy change (ΔH) and entropy change (ΔS), as well as temperature. A reaction is spontaneous when its Gibbs Free Energy (ΔG) is negative. An exothermic reaction can be spontaneous at both low and high temperatures if the combined effects of enthalpy (ΔH) and entropy (ΔS) are favorable.

Step 1: Understanding exothermic reactions

An exothermic reaction is a type of chemical reaction that releases energy in the form of heat. In these reactions, the total energy of the products is lower than that of the reactants, resulting in a negative enthalpy change.

Step 2: Understanding reaction spontaneity

A spontaneous reaction is one that occurs without any external influence, such as a catalyst or added energy. Spontaneous reactions typically occur when the products are at a lower energy state than the reactants. The Gibbs Free Energy (ΔG) is used to determine the spontaneity of a reaction and is described by the following equation: ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy of the system. A reaction is spontaneous when ΔG is negative.

Step 3: Examining the relation between Gibbs Free Energy and temperature

By analyzing the equation ΔG = ΔH - TΔS, we can understand the effect of temperature on the spontaneity of a reaction. In an exothermic reaction, ΔH is negative, so the term ΔH - TΔS will be negative if the second term, -TΔS, is smaller in magnitude than ΔH or positive if it's larger in magnitude. When T is low, the term -TΔS is smaller, making the Gibbs Free Energy more likely to be negative, suggesting the exothermic reaction is likely to be spontaneous at low temperatures.

Step 4: Considering the effect of entropy on reaction spontaneity

However, the entropy change (ΔS) also plays a significant role in determining spontaneity. In a reaction with a positive ΔS, the entropy of the system increases, favoring spontaneity. In contrast, a negative ΔS denotes a decrease in entropy and usually opposes spontaneity. Therefore, the spontaneity of an exothermic reaction at different temperatures depends on the interplay between ΔH and ΔS.

Step 5: Final conclusion

It's not accurate to say that exothermic reactions are spontaneous only at low temperatures. The spontaneity of exothermic reactions depends on the balance between enthalpy change (ΔH) and entropy change (ΔS), as well as temperature. An exothermic reaction can be spontaneous at both low and high temperatures if the combined effects of enthalpy and entropy are favorable.