Chemical Reactions and Equilibrium

Posted on Oct 13, 2024 in Chemistry

The Orientation of Shocks

Molecules must collide in a particular direction for a reaction to occur.

The Theory of the Transition State

Halfway between the reactants and products, an activated complex occurs. This consists of the reagents, which form a complex called an agglomerated activated complex or transition state. This complex is always higher in energy than the reactants and products, so it is very unstable and its life is very short. This theory assumes that the reactants collide with each other and, before giving rise to products, form an intermediate aggregate complex called a transition state or activated complex.

Influence of Concentration on the Reaction

If we increase the concentration of reactants, the number of molecules per unit volume increases, and according to collision theory, there will be a higher probability of effective collisions. The reaction rate is proportional to the concentration of reagents.

Influence of Temperature on Reaction Rate

With increasing temperature, the number of molecules that have the necessary activation energy to react increases significantly, while the overall energy doesn’t grow as much. Therefore, heating a reaction usually accelerates it.

Influence of the Physical State of the Reactants

If the reagents are solid, their conditions are very important. Thus, when reacting hydrochloric acid with iron, if it is block-shaped, it will react slowly. If the iron is in powder or filings form, the reaction is very energetic, since the surface of iron exposed to the acid is much greater, and there are far more likely effective collisions.

If the reagents are both in the gas phase (homogeneous reaction), the probability of contact between molecules is high, and so is the possibility of a favorable reaction.
If the reagents are both in the liquid phase (homogeneous reaction), the possibility is also large, and the reaction is favorable.
If there are two reagents in the solid phase (homogeneous reaction), the contact between its molecules will be small. In general, these cases will have slow reaction speeds.
In the case of reactions where the reactants are in different physical states (heterogeneous reactions), the reaction speed depends on the texture of each phase.

Influence of the Activation Energy

The activation energy indicates the minimum level of energy that molecules must possess to have the possibility of colliding effectively with other molecules.

Influence of Reagents

All reagents have different activation energies. If the activation energy is small, the reaction will run easily.

Influence of Catalysts

Catalysts are substances that change the speed of a reaction by lowering the activation energy. They are involved in the reaction, forming weak links with the reactive compounds. The reagents, acquiring a different structure, react easily, and the complex immediately breaks down. The catalyst is recovered and theoretically not consumed. Catalysts are involved in small quantities. This process is called catalysis and the reactions are catalyzed.

Types of Catalysts:

Enzymes or biocatalysts: Enzymes are highly specific, acting on specific reagents and under certain conditions of temperature, acidity, concentration, etc.
Chemical Catalysts:
Contact Catalysts: Used in reactions between gases and liquids. These catalysts have a very large solid contact surface (they are porous). Since the reagents and catalyst are in different phases in the process, it is called heterogeneous catalysis.
Transporter Catalysts: These are substances that are in the same phase as the reagents and are involved in forming intermediate reaction products with lower activation energy. At the end of the reaction, the catalyst is regenerated. Because the catalyst is in the same state of aggregation as the reagents, we say that this is a homogeneous catalysis.
Negative catalysts or inhibitors: These decrease the speed of reaction.
Autocatalysts: Many reactions produce products that act as catalysts in the same reaction.

There are poisons or catalyst poisons that completely nullify their activity.

Reaction Mechanisms

Most chemical reactions do not occur as indicated in the stoichiometric equation but proceed through a sequence of simpler reactions, known as elementary reactions. The sum of these elementary reactions gives the overall reaction.

Chemical Equilibrium

If you have a reaction where reactants (R) are transformed into products (P), there will come a time when, despite having reactants, the amount of products does not increase. At this point, the reaction is in equilibrium conditions. What happens is that some of the products transform back into reactants until an equilibrium state is reached where the speeds of the direct and reverse reactions are equal.

In the initial conditions, we have a certain amount of reactant. After some time, we observe that the reaction has begun, and the reagents become products. The resulting products decompose, giving back the reactants. A time arrives when the speed of the forward reaction equals the speed of the backward reaction, and equilibrium is reached. In this state, even if much time elapses, the equilibrium conditions do not change unless external conditions modify the system. The number of particles of reagents and products remains constant, but these particles are constantly changing. At equilibrium, the macroscopic properties remain unchanged over time. At the microscopic level (particles), reactions are still occurring, which is why it is said that chemical equilibrium is a dynamic equilibrium.

At a certain temperature, the equilibrium conditions are the same regardless of the reaction from which we have achieved equilibrium.

Equilibrium Constants

We can say that the reaction is in equilibrium when the same quantities of product are decomposing as are being formed, i.e., when the speed of the direct reaction is equal to the speed of the reverse reaction.

The Law of Mass Action (LMA) states that the equilibrium constant is a constant value that comes from the ratio between the equilibrium concentrations of products raised to their stoichiometric coefficients and the equilibrium concentrations of the reactants raised to their respective stoichiometric coefficients, for a given temperature.

For solving equilibrium exercises, a general method should be followed:

Initially: Identify the given data and what is being asked.
Steps: Convert data to molarity (concentration), both for reagents and products. Write the chemical equation corresponding to the reaction. Isolate the unknown using the LMA.
Finally: Conclusion and final result with the appropriate units.

Interpretation of the Equilibrium Constant

The numerical value of the equilibrium constant indicates the tendency of the reagents to become products.

If the value of the equilibrium constant is less than 1, it implies that the equilibrium concentration of reactants is greater than the concentration of products.
If the value of the equilibrium constant is greater than 1, it means that the equilibrium concentration of products is greater than the concentration of reactants. Therefore, the favored reaction is the formation of products.

Currently, several factors are known, such as pressure, temperature, and concentration, that change the equilibrium.

Le Chatelier’s Principle

This principle is the generalization of all the effects of the factors that modify the equilibrium. It was announced by the French scientist H.L. Le Chatelier and simultaneously by the German K.F. Braun. If a system is in equilibrium and receives an external perturbation that modifies any of the factors that determine the equilibrium, the system will evolve by counteracting the effect of the perturbation, i.e., it will shift to compensate for the change.

Effect of Concentration

When the concentration of a substance in a system at equilibrium is increased, the system evolves in the direction that tends to reduce this substance. In the case of a decrease in concentration, the equilibrium shifts in the direction of increasing the concentration of the species.

Effect of Pressure and Volume

This only affects the gaseous components of reactions. The pressure that gases exert on the container depends on the number of collisions of these particles against the walls of the container. The number of collisions increases if there are more particles (molecules) present.

Effect of Temperature

When the temperature of a system in equilibrium is lowered, it evolves towards the exothermic direction because the reaction releases energy. If, however, the temperature increases, the equilibrium shifts to the endothermic direction, thus counteracting the external effect.

Catalysts do not alter the equilibrium; rather, they cause equilibrium to be reached faster by increasing the reaction rate and decreasing the activation energy.

Heterogeneous Equilibrium

Homogeneous equilibrium: The substances are in the same phase.
Heterogeneous equilibria: These are equilibria where all species involved in the reaction are not in the same phase and include reactions between solids and gases, between solids and solutions (liquids), or between solutions and pure liquids or gases. In heterogeneous equilibria, the equilibrium constant expression does not include solids or pure liquids.

Precipitation Reactions

When a reaction produces a solid, we say that a precipitate is formed. Precipitation reactions are those in which ions from different solutions interact to form at least one solid product (insoluble).

Solubility and Solubility Product

Solubility is the maximum amount of a substance that dissolves in a given amount (mass or volume) of solvent at a certain temperature.

Soluble substances: In the dissolution, the solid completely dissociates into its ions.
Slightly soluble substances: The ions and the solid form are in equilibrium.

Solubility can be expressed in several ways. The most common are: moles of solute/liter of solution, g solute/liter solution, g solute/100 ml of solution, grams of solute per 100 grams of solvent.

Most dissolutions of solid substances are endothermic processes that increase entropy. Therefore, for each temperature, there is a solubility equilibrium. It is also apparent that solubility generally increases with temperature.

Solubility Product

The product of two constants is another constant that we call the solubility constant or solubility product. The solubility product of a saturated solution of a compound is the product of the concentrations of the ions that constitute the compound, each raised to its stoichiometric coefficient.

Relationship Between Solubility and Solubility Product

The solubility product value indicates the maximum concentration at which the compound can be dissolved. Therefore, it fixes the value of the product of the concentrations of its ions at equilibrium. The relationship between a very soluble substance in solution and its respective ions is the solubility.

We can identify three different situations:

The product of the concentrations of ions in solution is less than the value of the solubility product. This means that equilibrium has not been reached, and the solution is not yet saturated, so no precipitation occurs. If there were any solid present, it would dissolve.
Equilibrium situation.
The product concentration is greater than the solubility product. This means we are in a supersaturated situation. In this case, precipitation occurs to reach equilibrium.

Effect of Common Ions

If we have a slightly soluble salt in equilibrium with its ions, and we add a common ion, more precipitate will form because the equilibrium will shift.

Effect of Saline Ion

If, to a saturated solution of an insoluble salt, we add a solution of a compound such that one of its ions forms an insoluble substance (a precipitate) with an ion of the soluble salt, the effect will be to decrease the concentration of the ion in question, and therefore the equilibrium will shift, dissolving more of the compound.

This effect is due to Le Chatelier’s principle because the system has to counteract the disturbance. Thus, a decrease in the concentration of some ion causes the equilibrium to shift towards its formation, and the insoluble precipitate gradually dissolves to reach the value of its solubility product.

Fractionated Precipitation

In many industrial processes or chemical analyses, we have solutions containing several ions. In some cases, certain ions may interfere with the process. One method used for the separation of certain ions in solution is based on the precipitation of the unwanted ions, called selective or fractional precipitation.

The Constant Value from Other Equilibrium Constants

If a reaction is written as an initial reaction multiplied by a number n, the equilibrium constant is the initial reaction’s constant raised to the power of n.
If a chemical reaction is written in reverse of an initial reaction, the equilibrium constant is the inverse of the initial reaction’s constant.
When a reaction is the sum of two or more reactions, the equilibrium constant is the product of the individual equilibrium constants.

Equilibrium Constants and Pressures

At a given temperature, any chemical reaction involving gases has an equilibrium constant expressed in terms of the partial pressures of each species involved in the reaction.

Degree of Dissociation

The degree of dissociation is defined as the ratio of the number of moles dissociated to the initial number of moles. The degree of dissociation provides a clear idea about the evolution of the reaction.

Factors that Modify the Equilibrium

The increase or decrease of the reaction products in an equilibrium is crucial when optimizing a chemical process. In any industrial process involving an equilibrium reaction, it is important that the transformation occurs with an acceptable yield for the process to be viable. However, there are other cases where it is of interest that the reaction does not proceed (e.g., some reactions that cause pollution).