Feasible Reaction: The Chemistry of What Can Happen
The Driving Forces of Nature
Think of a ball resting on a hillside. If you nudge it, it will naturally roll down to the bottom. This happens because the ball is moving from a state of higher potential energy to a state of lower potential energy. Chemical systems behave in a remarkably similar way. They have a natural tendency to move toward states that are more stable, which generally means having lower energy and being more disordered.
Two key concepts help us understand this tendency:
1. Enthalpy (H): The Heat Story
Enthalpy is essentially a measure of the heat content of a system. In a chemical reaction, the change in enthalpy ($\Delta H$) tells us if heat is released or absorbed.
- $\Delta H < 0$: The reaction releases heat to its surroundings. This is called an exothermic reaction. It feels hot. Examples include burning wood or the reaction in a hand warmer. - $\Delta H > 0$: The reaction absorbs heat from its surroundings. This is an endothermic reaction. It feels cold. Examples include dissolving some salts in water or photosynthesis.
Nature often favors exothermic processes because they lead to a more stable, lower-energy state for the products.
2. Entropy (S): The Disorder Story
Entropy is a measure of randomness or disorder in a system. A neat room has low entropy; a messy room has high entropy. The natural trend of the universe is toward greater entropy.
The change in entropy ($\Delta S$) in a reaction indicates whether the disorder increases or decreases.
- $\Delta S > 0$: The products are more disordered than the reactants (e.g., a solid turning into a gas). - $\Delta S < 0$: The products are more ordered than the reactants (e.g., a gas turning into a solid).
Reactions that increase entropy are also naturally favored.
Gibbs Free Energy: The Deciding Judge
But what happens when these two drivers conflict? What if a reaction is exothermic (good!) but also creates more order (bad!)? Or vice versa? This is where Josiah Willard Gibbs, a brilliant American scientist, made a crucial contribution. He combined enthalpy and entropy into a single, powerful quantity called Gibbs Free Energy (G).
Gibbs Free Energy can be thought of as the "available energy" in a system to do useful work. The change in Gibbs Free Energy during a reaction, $\Delta G$, is the ultimate judge of feasibility.
$\Delta G = \Delta H - T \Delta S$
Where:
• $\Delta G$ = Change in Gibbs Free Energy (in Joules or kilojoules, kJ)
• $\Delta H$ = Change in Enthalpy (kJ)
• $T$ = Absolute Temperature in Kelvin (K)
• $\Delta S$ = Change in Entropy (kJ/K)
The sign of $\Delta G$ gives us the verdict:
| Value of ΔG | Feasibility | Spontaneity | Simple Explanation |
|---|---|---|---|
| $\Delta G < 0$ (Negative) | Feasible | Spontaneous | The reaction can happen on its own, like a ball rolling downhill. |
| $\Delta G > 0$ (Positive) | Not Feasible | Non-spontaneous | The reaction will not happen on its own. It's like a ball trying to roll uphill without a push. |
| $\Delta G = 0$ (Zero) | At Equilibrium | Neither | The reaction has no tendency to go forward or backward. It's like a ball sitting on a flat surface. |
Let's see how temperature ($T$) acts as a controller in the equation $\Delta G = \Delta H - T \Delta S$.
Temperature: The Reaction's Thermostat
Temperature is not just about things being hot or cold; it determines which of the two driving forces—enthalpy or entropy—wins the battle for feasibility. By looking at the signs of $\Delta H$ and $\Delta S$, we can predict how temperature affects a reaction's feasibility.
| ΔH | ΔS | Effect of High Temperature | Example Reaction Type |
|---|---|---|---|
| Negative (Exothermic) | Positive (More disorder) | Always feasible at any temperature. $\Delta G$ is always negative. | Burning of fuels (combustion). |
| Positive (Endothermic) | Negative (More order) | Never feasible at any temperature. $\Delta G$ is always positive. | Turning a gas into a solid crystal at high heat (generally impossible on its own). |
| Negative (Exothermic) | Negative (More order) | Feasible only at low temperatures. The $-T\Delta S$ term becomes large and positive at high T, making $\Delta G$ positive. | Water freezing into ice. It's exothermic and creates order, so it happens when it's cold. |
| Positive (Endothermic) | Positive (More disorder) | Feasible only at high temperatures. The $-T\Delta S$ term is large and negative at high T, overcoming the positive $\Delta H$. | Water boiling into steam. It requires heat input (endothermic) and creates great disorder. |
Feasible Reactions in Action: From Rust to Rockets
Let's apply the concept of $\Delta G$ to real-world scenarios to see how it explains what we observe.
Example 1: The Formation of Rust
The rusting of iron is a classic spontaneous reaction. Iron reacts with oxygen and water from the air:
$4Fe(s) + 3O_2(g) + 6H_2O(l) \rightarrow 4Fe(OH)_3(s)$ (simplified)
This reaction is exothermic ($\Delta H < 0$). While solid rust might seem more ordered, the overall process involves gas molecules ($O_2$) being consumed to form part of a solid, which often results in a net increase in the disorder of the system if you consider the entire surroundings. For rusting, $\Delta G$ is negative at room temperature. That's why your bicycle left in the rain will rust on its own over time—it's a feasible reaction. We don't have to add energy; it happens spontaneously.
Example 2: Baking a Cake
Baking involves many reactions, but a key one is the decomposition of baking soda (sodium bicarbonate) when heated:
$2NaHCO_3(s) \xrightarrow{\text{heat}} Na_2CO_3(s) + H_2O(g) + CO_2(g)$
This reaction is endothermic ($\Delta H > 0$—it needs heat from the oven). Crucially, it produces two gases from a solid, creating a huge increase in entropy ($\Delta S >> 0$). At room temperature, $\Delta G$ is positive—the baking soda sits quietly in its box. But inside a hot oven, the $-T\Delta S$ term becomes very large and negative, making the overall $\Delta G$ negative. The reaction becomes feasible, the $CO_2$ gas forms, and your cake rises!
Example 3: A Battery Powering a Flashlight
Inside a common alkaline battery, a chemical reaction occurs between zinc and manganese dioxide. This reaction is carefully designed to have a negative $\Delta G$. The "downhill" energy drop of this feasible reaction doesn't just turn into heat; it's harnessed to push electrons through a circuit, creating electric current to light the bulb. When the battery is "dead," the reactants have been mostly used up, and the system is near equilibrium ($\Delta G \approx 0$).
Important Questions
Q1: If a reaction is not feasible ($\Delta G > 0$), does that mean it can never happen?
Q2: Can a reaction with $\Delta H > 0$ (endothermic) ever be feasible?
Q3: How do scientists calculate or find the values for $\Delta H$, $\Delta S$, and $\Delta G$?
The concept of a feasible reaction, governed by a negative change in Gibbs Free Energy ($\Delta G < 0$), is a cornerstone of chemistry. It provides a clear, mathematical criterion to predict whether a chemical change is possible on its own. By balancing the competing influences of energy release ($\Delta H$) and disorder creation ($\Delta S$), with temperature ($T$) as the crucial switch, the Gibbs equation elegantly explains phenomena from the ice melting in your drink to the complex reactions powering life itself. Understanding feasibility helps chemists design new materials, create efficient fuels, and even understand biological processes. It separates the possible from the impossible, guiding innovation and deepening our comprehension of the natural world.
Footnote
[1] Third Law of Thermodynamics: A fundamental law stating that the entropy of a perfect crystal at absolute zero temperature (0 Kelvin) is exactly zero. This provides a reference point for measuring absolute entropy values.
[2] Standard Enthalpy of Formation ($\Delta H_f^o$): The enthalpy change when one mole of a compound is formed from its elements in their standard states (most stable form at 1 bar pressure and a specified temperature, usually 298.15 K).
Spontaneous: A process that, once started, can proceed on its own without continued external influence. It is driven by a decrease in the system's Gibbs free energy.
Kinetics: The branch of chemistry that deals with the rates (speeds) of chemical reactions and the mechanisms by which they occur. It is separate from thermodynamics, which deals with feasibility.