Static and Dynamic Equilibrium

Inquiry question: What happens when chemical reactions do not go through to completion?

A static equilibrium in a chemical reaction is where no more reactants are converted to products.  We might say the reaction has stopped.  Also, no products are converted to reactants.

For example, when we burn a magnesium strip, all of the magnesium reacts with oxygen in the air and produces magnesium oxide, that is,

2Mg + O2 → 2MgO. 

When no more reactants are converted to products, the reaction has stopped and we say that there is static equilibrium.

An analogy (or model) for this is in the physical world when we place a book on a table.  The weight of the book (force downwards due to gravity) is balanced by the normal force of the table on the book.  This is a static equilibrium, since the net force on the book is zero, so it does not accelerate.

Another model of static equilibrium is shown in the diagram below.

Factors that Affect Equilibrium

Inquiry question: What factors affect equilibrium and how?

Many chemical processes are irreversible, which means that when products are formed, they do not revert back into reactants. This could be due to many reasons:

• It is an open system, so products are no longer in the system.

• Products are highly stable, so they do not react with each other .

• The forward reaction has a negative enthalpy change and a positive entropy change.

Consider the combustion of methane in air (oxygen).  It has the chemical equation:

CH4(g) + 2O2(g) ⇋ CO2(g) + 2H2­O(g) 

This is a reaction with 2 gases reacting to form 2 other gases.

Calculating the Equilibrium Constant (Keq)

Inquiry question: How can the position of equilibrium be described and what does the equilibrium constant represent?

While Chemical Kinetics is concerned with how fast reactions proceed (rates of reaction), Chemical Equilibrium is concerned with how far reactions proceed.

In different chemical equilibria the concentration of reactants and products varies considerably.  Chemical equilibrium is the state reached when the concentrations of reactants and products remain constant over time in a reversible reaction. Reactant and product concentrations stabilise because the rate of the forward reaction equals the rate of the reverse reaction at equilibrium. A homogeneous equilibrium has all reactant and product species in the same phase e.g. all gaseous or all in the same solution.

The concentration of a species X (mol dm^-3) involved in the expression for Kc is represented by [X].

Equilibrium Law: If the concentrations of all the substances present at dynamic equilibrium are raised to the power of the number of moles that they appear in the chemical equation, then the product of the concentrations of the products divided by the product of the concentrations of the reactants equals the equilibrium constant, Keq.  See diagram below. Source: https://secondaryscience4all.wordpress.com/a2-chemistry/a2-physical/1-10-equilibrium-constant-kc-for-homogeneous-systems/.

Solution Equilibria

Inquiry question: How does solubility relate to chemical equilibrium?

Dissolution is the process in which solutes dissolve in water and form a solution. In the case of an ionic solute, such as nickel (II) chloride crystals, the surface of the crystal undergoes collisions with water molecules. Some collisions lead to ions breaking free of the crystal lattice. Positive nickel (II) ions become solvated (hydrated) by the water solvent as the negative ends of the water molecule are attracted to the nickel (II) ions. This is an ion-dipole attraction. The chloride ions are solvated by water molecules because the positive end of the water dipole is attracted to the negative chloride ions.

If more solute is added to the water, a point is reached when no more will dissolve and a saturated solution forms. The saturated solution is an example of a heterogeneous equilibrium. This is a dynamic equilibrium in which ions dissociate from the crystal at the same rate as ions associate together and form a crystal.

Dissolution equilibria are temperature dependent. Below is a graph that compares the solubility of three ionic compunds: ammonium chloride, potassium chloride and sodium chloride as a function of temperature. Both salts increase in solubility with temperature.  Also, there are graphs of exothermic and endothermic solution formation.

Sources: https://www.semanticscholar.org/paper/Temperature-invariance-of-NaCl-solubility-in-water%3A-Bharmoria-Gupta/c49c8f3b75d7b2c7f7a6f06124e247a8f114f263, https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_General_Chemistry%3A_Principles_Patterns_and_Applications_%28Averill%29/13%3A_Solutions/13.01%3A_Factors_Affecting_Solution_Formation.