Understanding why some chemical reactions occur independently while others require continuous intervention is fundamental. This inherent drive is known as spontaneity, governed by Gibbs Free Energy (ΔG). ΔG quantifies the maximum non-expansion work extractable from a system at constant temperature and pressure, indicating whether a process will proceed without external energy.
Understanding Chemical Spontaneity
When chemists refer to a “spontaneous” reaction, they do not imply it happens instantly or rapidly. Instead, spontaneity describes a reaction that is energetically favorable and, once initiated, will proceed without continuous energy input. Think of a ball rolling downhill; it moves spontaneously due to gravity, but its speed depends on the slope and surface friction.
The distinction between spontaneity and reaction rate is important. Thermodynamics, dealing with spontaneity and energy changes, tells us if a reaction can occur. Chemical kinetics describes how fast it will proceed. For example, diamonds spontaneously convert to graphite over geological timescales, a process imperceptible at room temperature.
The Driving Forces Behind Spontaneity
Two thermodynamic factors influence chemical spontaneity: enthalpy change (ΔH) and entropy change (ΔS). Enthalpy change refers to heat absorbed or released during a reaction. Exothermic reactions (ΔH < 0) release heat, favoring spontaneity by moving to a lower energy state. Conversely, endothermic reactions (ΔH > 0) absorb heat, opposing spontaneity.
Entropy change measures a system’s disorder or randomness. Systems tend towards higher disorder; thus, reactions increasing entropy (ΔS > 0) favor spontaneity. For instance, a solid dissolving into a liquid increases disorder, resulting in a positive entropy change.
Temperature (T) is the third factor influencing spontaneity, particularly through its interaction with entropy. It acts as a multiplier for the entropy term in the Gibbs Free Energy equation: ΔG = ΔH – TΔS. At higher temperatures, the TΔS component becomes more significant, giving entropy a greater influence on the reaction’s spontaneity.
Predicting Spontaneous Reactions
The sign of Gibbs Free Energy change (ΔG) directly predicts a chemical reaction’s spontaneity under constant temperature and pressure. A negative ΔG indicates a spontaneous reaction, meaning it will proceed without continuous external energy input. A positive ΔG signifies a non-spontaneous reaction, which requires continuous energy input to occur. If ΔG is zero, the system is at equilibrium, with no net change occurring.
Four scenarios for spontaneity based on the signs of ΔH and ΔS. When ΔH is negative (exothermic) and ΔS is positive (increasing disorder), the reaction is always spontaneous, regardless of temperature. Both factors favor spontaneity, resulting in a negative ΔG.
Conversely, if ΔH is positive (endothermic) and ΔS is negative (decreasing disorder), the reaction is never spontaneous. Both factors oppose spontaneity, leading to a positive ΔG.
When both ΔH and ΔS are negative, the reaction is spontaneous only at lower temperatures. Negative enthalpy favors spontaneity, but negative entropy opposes it. At low temperatures, |ΔH| term dominates |TΔS| term, keeping ΔG negative.
If both ΔH and ΔS are positive, the reaction becomes spontaneous only at higher temperatures. Positive enthalpy opposes spontaneity, but positive entropy favors it. At sufficiently high temperatures, the positive TΔS term can overcome the positive ΔH, leading to a negative ΔG.
Everyday Examples of Spontaneous Processes
Everyday phenomena illustrate chemical spontaneity. Combustion of fuels, like burning wood or natural gas, is a common example of an always spontaneous reaction. These processes are exothermic (ΔH < 0), releasing heat, and increase disorder by converting solids or liquids into gases (ΔS > 0), ensuring a negative ΔG.
Melting ice at room temperature is an example of a process spontaneous at higher temperatures. Melting is endothermic, absorbing heat (ΔH > 0). However, the transition from ordered solid ice to disordered liquid water results in an increase in entropy (ΔS > 0). Above 0°C, the TΔS term outweighs positive ΔH, making ΔG negative and melting spontaneous.
Photosynthesis in plants offers an example of a non-spontaneous process that is made to occur by external energy input. This reaction converts carbon dioxide and water into glucose and oxygen, which involves a decrease in entropy (ΔS < 0) and is highly endothermic (ΔH > 0). Consequently, photosynthesis requires a continuous input of light energy from the sun to proceed, making it a non-spontaneous reaction driven by an external energy source.