Spontaneous Reaction: Videos & Practice Problems

Thermodynamics studies the transformation of heat and energy in chemical reactions, focusing on spontaneity. A spontaneous reaction occurs naturally without external energy, like a boulder rolling downhill, while a non-spontaneous reaction requires continuous energy input, such as a car needing fuel. Spontaneous reactions move towards equilibrium, and their rate can vary significantly. Key variables include free-energy change (ΔG), entropy change (ΔS), and equilibrium constant (K). Understanding these concepts is crucial for analyzing chemical processes and their feasibility in real-life scenarios.

Spontaneous Reaction Concept 1

Thermochemical processes are fundamentally rooted in thermodynamics, the branch of physical science that examines the transformation of heat and energy. A key concept in this field is the idea of spontaneity in chemical reactions, which can be classified as either spontaneous or non-spontaneous based on specific conditions. A spontaneous reaction occurs naturally without the need for an external energy source, while a non-spontaneous reaction requires continuous energy input to proceed.

To understand spontaneity, we often refer to variables such as \(\Delta G\) (Gibbs free energy), \(\Delta S\) (entropy), and the equilibrium constant \(K\). A spontaneous reaction is characterized by a negative \(\Delta G\), indicating that the process can occur without additional energy. For example, consider a boulder rolling down a hill; it moves due to its own momentum, exemplifying a spontaneous reaction. Conversely, a car requires fuel and a battery to operate, making its function a non-spontaneous process.

It is crucial to differentiate between spontaneity and kinetics. While spontaneity refers to whether a reaction can occur without external energy, kinetics deals with the speed of the reaction. A spontaneous reaction may take a short time or an exceedingly long time to complete, but its classification as spontaneous is independent of the duration.

In thermodynamics, the concept of equilibrium is vital. A spontaneous reaction tends to move towards equilibrium, where the system reaches a state of balance. For instance, if a reaction is spontaneous in one direction, it will be non-spontaneous in the reverse direction. This relationship highlights the dynamic nature of chemical processes.

Furthermore, while it is possible to initiate a non-spontaneous reaction by supplying energy, once that energy source is removed, the reaction ceases. This underscores the idea that non-spontaneous reactions are unnatural; they cannot sustain themselves without continuous energy input. Therefore, it is inaccurate to claim that one can create a non-spontaneous reaction that continues indefinitely without energy.

In summary, understanding the principles of thermodynamics and the nature of spontaneous versus non-spontaneous reactions is essential for grasping how energy transformations occur in chemical processes. By applying these concepts, one can better analyze real-life scenarios and determine the spontaneity of various reactions.

Which of the following statements is/are true?
a) The rusting of iron by oxygen is a non-spontaneous reaction.
b) The addition of a catalyst to a reaction increases spontaneity.
c) The movement of heat from a cold object to a hot object is a non-spontaneous reaction.
d) The diffusion of perfume molecules from one side of a room to the other is a non-spontaneous reaction.
e) None of the above. 

• A. C
• B. A,C
• C. D
• D. B,C

Answer: (A)

Here’s what students ask on this topic:

What is a spontaneous reaction in thermodynamics?

A spontaneous reaction in thermodynamics is a process that occurs naturally without the need for external energy input. This means the reaction can proceed on its own once it has started. For example, a boulder rolling down a hill is a spontaneous process because it doesn't require any additional energy to continue moving. In chemical terms, spontaneity is determined by the Gibbs free energy change (ΔG). If ΔG is negative, the reaction is spontaneous. Spontaneous reactions move towards equilibrium and can occur at varying rates, from very fast to extremely slow.

How do ΔG, ΔS, and K determine if a reaction is spontaneous?

The spontaneity of a reaction is primarily determined by the Gibbs free energy change (ΔG). If ΔG is negative, the reaction is spontaneous. ΔG is calculated using the equation:

$\mathrm{\Delta G}=\mathrm{\Delta H}-T\mathrm{\Delta S}$

where ΔH is the enthalpy change, T is the temperature in Kelvin, and ΔS is the entropy change. A positive ΔS (increase in disorder) and a negative ΔH (exothermic reaction) favor spontaneity. The equilibrium constant (K) also plays a role; if K is greater than 1, the reaction tends to be spontaneous under standard conditions.

What is the difference between spontaneous and non-spontaneous reactions?

Spontaneous reactions occur naturally without the need for continuous external energy input. They move towards equilibrium and are driven by a negative Gibbs free energy change (ΔG). An example is a boulder rolling downhill. Non-spontaneous reactions, on the other hand, require continuous energy input to proceed. They do not occur naturally and have a positive ΔG. An example is a car running, which needs a constant supply of fuel and battery power to operate.

Can a spontaneous reaction be slow?

Yes, a spontaneous reaction can be slow. Spontaneity is determined by thermodynamics, specifically the Gibbs free energy change (ΔG), and not by the reaction rate. A spontaneous reaction has a negative ΔG, meaning it can occur without external energy input. However, the rate at which it occurs is governed by kinetics, not thermodynamics. Therefore, a spontaneous reaction can take anywhere from a few seconds to millions of years to complete.

Why do spontaneous reactions move towards equilibrium?

Spontaneous reactions move towards equilibrium because they aim to reach a state of maximum stability and minimum free energy. At equilibrium, the forward and reverse reaction rates are equal, and the system's Gibbs free energy (ΔG) is at its lowest possible value. This state is thermodynamically favorable, as it represents a balance where no net change occurs over time. Thus, spontaneous reactions naturally progress towards this balanced state to achieve equilibrium.