Question

Which statement is true? a. A spontaneous reaction is always a fiast reaction. b. A spontaneous reaction is always a slow reaction. c. The spontaneity of a reaction is not necessarily related to the speed of a reaction.

Short answer

The correct statement is c. The spontaneity of a reaction is not necessarily related to the speed of the reaction.

Step-by-step solution

Identifying the Correct Statement

To solve this exercise, we must understand that a spontaneous reaction is one that occurs without the need for continuous external energy once it has started. This means the spontaneity of a reaction is about thermodynamics, not kinetics; it refers to whether a reaction is thermodynamically favourable, not how quickly the reaction proceeds.

Clarification of Misconceptions

The terms 'fast' and 'slow' refer to the rate of reaction, which is a kinetic concept and does not determine whether a reaction is spontaneous or not. This eliminates options a and b as correct because they incorrectly link spontaneity directly to reaction speed.

Selection of the Correct Answer

Therefore, the correct statement is c, which correctly identifies that spontaneity of a reaction is not linked to how fast or slow the reaction is. This means even a spontaneous reaction might proceed slowly if its activation energy is high or if it lacks a suitable catalyst.

Key concepts

Understanding Spontaneity in Thermodynamics

In thermodynamics, spontaneity is a key concept referring to the likelihood of a process or reaction occurring without continuous input of energy from an external source. Unlike what some might assume, spontaneous does not necessarily mean fast. A spontaneous reaction can occur at any speed—quickly, slowly, or at a moderate pace. For instance, the rusting of iron is a spontaneous process, yet it does not happen rapidly.

The spontaneity of a reaction is determined by its Gibbs free energy change (ΔG), where a negative ΔG indicates a spontaneous process, and a positive ΔG suggests a non-spontaneous one. This is underpinned by two fundamental components: enthalpy (ΔH), which represents heat changes, and entropy (ΔS), which measures the disorder or randomness in the system. The equation ΔG = ΔH - TΔS (where T is the temperature in Kelvin) encapsulates this relationship. It's crucial to have a firm understanding of these concepts to grasp why certain reactions occur without intervention, while others do not.

Reaction Kinetics and Rate

Reaction kinetics, on the other hand, is the study of the rates of chemical processes and the factors affecting these rates. It is focused on the 'how quickly' aspect of reactions rather than the 'if' aspect that is covered by thermodynamics. Several factors can influence the kinetic behavior of a reaction, including temperature, concentration of reactants, surface area, and the presence of a catalyst.

Kinetics is crucial in many practical applications, such as in the pharmaceutical industry where the rate at which a drug is metabolized can determine its dosage and effectiveness. While thermodynamics tells us if a reaction can happen, kinetics tells us how fast it will happen under certain conditions. Therefore, understanding both thermodynamics and kinetics is imperative, as a reaction that is thermodynamically favorable might not proceed at a practical rate without the right kinetic conditions.

The Role of Activation Energy

Activation energy is the minimum energy that reactant molecules must possess to undergo a chemical reaction. It can be visualized as a barrier that reactants need to overcome for a reaction to proceed. This concept is intrinsically linked to both thermodynamics and kinetics. Even if a reaction is spontaneous (favorable from a thermodynamic standpoint), it might not occur unless the molecules have enough energy to reach or surpass the activation energy threshold.

A common way to reduce the activation energy is through the use of a catalyst, which provides an alternative pathway for the reaction with a lower barrier, hence increasing the reaction rate without altering the thermodynamics. To put it simply, the activation energy determines how many molecules have enough energy to react at any given temperature, influencing the speed at which the reaction proceeds. Thus, a spontaneous reaction with high activation energy may be slow, explaining why a spontaneous reaction isn’t always fast as one might intuitively think.