Reversibility of Chemical Reactions

What Reversibility Means in Chemistry

In chemistry, a reaction is called reversible if it can proceed in both directions under given conditions:

• Forward reaction: Reactants form products
  $$
  \text{Reactants} \;\rightleftharpoons\; \text{Products}
  $$
• Reverse reaction: Products can react to form the original reactants again.

The double arrow $\,\rightleftharpoons\,$ is the usual symbol for a reversible reaction. A reaction written with a single arrow $\,\rightarrow\,$ is usually treated as practically irreversible under the conditions considered.

Reversibility is about the possibility of going back, not about whether this actually happens at a noticeable rate in a given experiment.

Forward and Reverse Reactions

For any reversible reaction such as
$$
\text{A} \rightleftharpoons \text{B}
$$

we distinguish:

• Forward reaction: $ \text{A} \rightarrow \text{B} $
  with a certain rate that depends on the concentration of A (and other conditions).
• Reverse reaction: $ \text{B} \rightarrow \text{A} $
  with its own rate, depending on the concentration of B (and the same conditions).

Key points specific to reversibility:

• Both reactions can be present at the same time.
• Which direction is dominant depends on conditions (concentrations, temperature, pressure, etc.).
• At equilibrium (treated in a separate chapter), the rates of forward and reverse reaction are equal. Here, we are just concerned with the fact that both directions are possible and can compete.

Microscopic Versus Macroscopic View

On the microscopic level (individual particles):

• Molecules continuously react and re-form.
• A molecule that has become a product can collide and turn back into a reactant.
• There is a constant back-and-forth between reactants and products.

On the macroscopic level (what we see in the lab):

• We often observe only the overall change in amounts of reactants and products.
• When the visible changes stop, the system may still contain ongoing microscopic forward and reverse reactions.
• This “no visible change despite ongoing reactions” is closely tied to the concept of dynamic equilibrium (covered elsewhere). Reversibility is the basis for this behavior.

Criteria and Indicators of Reversibility

A reaction is considered reversible when:

• Both directions are chemically possible:
• The products are not so stable that they will never react back.
• The reverse process is not blocked by the formation of a separate phase that isolates products from reactants (for example, an insoluble solid that settles, or a gas that escapes and is not allowed back).
• The reverse reaction rate is not negligibly small under the conditions:
• Some reactions are “in principle” reversible but effectively don’t go back within useful time scales. These are often treated as irreversible.

Experimental indicators that a reaction is reversible:

• Starting from reactants alone, a certain mixture of reactants and products forms and then remains constant in composition.
• Starting from products alone, under the same conditions, the system evolves to the same final composition.
• Changing conditions (such as concentration, pressure, temperature) can shift the composition back and forth (discussion of shifts belongs to later chapters, but the ability to be shifted is a sign of reversibility).

Reversible vs. Practically Irreversible Reactions

In theory, many chemical reactions are reversible. In practice, some behave as if they only run in one direction.

Typical situations where a reaction behaves practically irreversibly:

• Gas escapes from the system
  If a product is a gas and it is allowed to leave the reaction vessel, it cannot easily recombine with the remaining components to form reactants again. In an open container, this makes the reverse reaction effectively impossible.
• Formation of a precipitate
  Formation of an insoluble solid (precipitate) removes ions from solution. If the solid is not redissolved, the reverse process (getting back the original ions in solution) is effectively prevented.
• Very large energy change
  If the reverse reaction would require a very large input of energy under the given conditions, it may occur so slowly that it can be ignored.
• Chemical destruction of products
  If products quickly undergo further irreversible steps (e.g., decomposition, side reactions), they are removed from the system and cannot revert to the original reactants.

Conversely, reactions are strongly reversible when:

• Reactants and products stay in contact (same phase or well-mixed phases).
• Energy barriers for both directions are reasonable at the working temperature.
• No component is continuously removed from the system.

Notation and Representation of Reversibility

The way a reaction is written often encodes assumptions about reversibility:

• Single arrow →
• Indicates the focus is on the forward direction.
• Used when the reverse reaction is negligible, or not relevant to the context.
• Double arrow ⇌ or ↔
• Indicates the reaction can proceed in both directions.
• Implies that we must consider the interaction of forward and reverse reaction and that a balance (equilibrium) can be established.

In equations, reversibility is independent of stoichiometry. A general reversible reaction can be written as
$$
a\,\text{A} + b\,\text{B} \rightleftharpoons c\,\text{C} + d\,\text{D}
$$
The stoichiometric coefficients $a, b, c, d$ are used in the balance of amounts (and later in the equilibrium expression), but the reversibility is simply indicated by the double arrow.

Role of Reversibility in Establishing Equilibrium

Although the details of equilibrium are treated in other chapters, reversibility is the necessary precondition for a chemical equilibrium to exist:

• If a reaction were truly irreversible, there would be no reverse reaction to balance the forward reaction.
• For a dynamic equilibrium, both forward and reverse reactions must be:
• Possible,
• Active under the given conditions,
• Capable of reaching a state where their rates become equal.

Therefore:

• Reversibility is about possibility: both directions can occur.
• Equilibrium is about outcome: under certain conditions, both directions proceed at equal rates, giving constant macroscopic composition.

Without reversibility, there is no chemical equilibrium to analyze.

Dependence of Reversibility on Conditions

Whether a reaction behaves as reversible or practically irreversible is often condition-dependent:

• Temperature:
• Raising or lowering temperature can change how fast each direction proceeds.
• At one temperature, the reverse reaction might be negligible; at another, it may become appreciable.
• Concentrations:
• High product concentrations can enhance the reverse reaction, because more product molecules are available to collide and react back.
• Low product concentrations (for example, because a product is removed) make the reverse reaction less important.
• Pressure (for gases):
• For reactions involving gases, total and partial pressures influence both directions.
• If products are allowed to expand into a large volume and become highly diluted, the likelihood of the reverse process can drop.
• Phase and mixing:
• If all components stay well-mixed in the same or contacting phases, both directions remain possible.
• If phases separate in a way that reactants and products almost never meet, one direction can become practically suppressed.

In summary, the same chemical system may be treated as reversible or practically irreversible depending on how the experiment or process is designed.

Conceptual Importance of Reversibility

Reversibility is central to how chemists think about reactions:

• It forces us to consider both directions when predicting the final composition of a system.
• It provides the foundation for:
• The law of mass action and equilibrium constants,
• The concept of dynamic equilibrium,
• The analysis of how changes in conditions affect chemical systems.

For beginners, the essential idea is:

• A reversible reaction is not a one-way road from reactants to products.
• It is a two-way street, where reactants and products can transform into one another, and the observed state of the system results from the competition between forward and reverse processes.