For each of the following statements, indicate whether it is true or false. Why?
a) A spontaneous reaction always releases heat.
b) A spontaneous reaction is always a fast reaction.
c) The entropy of a system always increases for a spontaneous change.
d) The entropy of a system and its surrounding always increases for a spontaneous change.
e) The energy of a system always increases for a spontaneous change.
The correct answer and explanation is :
Here are the answers and explanations for each of the statements:
a) A spontaneous reaction always releases heat.
False.
A spontaneous reaction does not always release heat. The spontaneity of a reaction is determined by the change in the Gibbs free energy (ΔG), which is a function of enthalpy (ΔH) and entropy (ΔS). For a reaction to be spontaneous, ΔG must be negative (ΔG = ΔH – TΔS). While exothermic reactions (reactions that release heat) have a negative ΔH and are often spontaneous, endothermic reactions (those that absorb heat) can also be spontaneous if the entropy increase (ΔS) is sufficiently large to offset the heat absorption. Therefore, spontaneity is not directly tied to heat release.
b) A spontaneous reaction is always a fast reaction.
False.
Spontaneity refers to whether a reaction can occur without external input, but it does not indicate the speed of the reaction. A reaction can be spontaneous but occur very slowly, such as the rusting of iron. The rate of reaction depends on factors like activation energy, catalysts, and temperature, not spontaneity. Therefore, a spontaneous reaction is not always fast.
c) The entropy of a system always increases for a spontaneous change.
False.
While the second law of thermodynamics states that the total entropy of the universe must increase for a spontaneous process, the entropy of the system itself may not always increase. In some cases, the system’s entropy may decrease, but this is compensated by a larger increase in the entropy of the surroundings, ensuring that the total entropy of the universe increases. For example, the freezing of water decreases the entropy of the system but increases the entropy of the surroundings.
d) The entropy of a system and its surroundings always increases for a spontaneous change.
True.
This statement is true because, according to the second law of thermodynamics, for a process to be spontaneous, the total entropy change (ΔS_total) must be positive. This includes both the entropy change of the system (ΔS_system) and the surroundings (ΔS_surroundings). Even if the entropy of the system decreases, the increase in the entropy of the surroundings will more than compensate for it, leading to an overall increase in total entropy.
e) The energy of a system always increases for a spontaneous change.
False.
The energy of a system does not always increase for a spontaneous change. Spontaneous processes tend to move towards a state of lower potential energy. For instance, in exothermic reactions, energy is released from the system to the surroundings. This release of energy is associated with a decrease in the internal energy of the system, not an increase. The key factor in spontaneity is the Gibbs free energy (ΔG), not just energy (ΔE), which accounts for both enthalpy and entropy.