Question

Explain the difference between a heterogeneous and a homogeneous equilibrium. Give an example of each.

Short answer

Homogeneous equilibrium involves reactants/products in the same phase. Heterogeneous equilibrium involves different phases.

Step-by-step solution

- Define Equilibrium

Equilibrium in chemistry refers to the state in which both reactants and products are present in concentrations which have no further tendency to change with time. This occurs when the forward and reverse reactions occur at the same rate.

- Define Homogeneous Equilibrium

A homogeneous equilibrium occurs when all the reactants and products are in the same phase. Typically, this phase is either all gases or all solutions. An example of a homogeneous equilibrium is: \[ \text{N}_{2}(g) + 3\text{H}_{2}(g) \rightleftharpoons 2\text{NH}_{3}(g) \]

- Define Heterogeneous Equilibrium

A heterogeneous equilibrium occurs when the reactants and products are in different phases. For example, in the equilibrium between calcium carbonate and its decomposition products: \[ \text{CaCO}_{3}(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_{2}(g) \] the system involves both solid and gas phases.

- Compare Homogeneous and Heterogeneous Equilibrium

The main difference between homogeneous and heterogeneous equilibrium is the phases of the substances involved. In a homogeneous equilibrium, all substances are in the same phase, whereas in a heterogeneous equilibrium, substances are in different phases.

- Provide Examples

For homogeneous equilibrium: \[ \text{N}_{2}(g) + 3\text{H}_{2}(g) \rightleftharpoons 2\text{NH}_{3}(g) \] For heterogeneous equilibrium: \[ \text{CaCO}_{3}(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_{2}(g) \]

Key concepts

Heterogeneous Equilibrium

In chemistry, a heterogeneous equilibrium happens when reactants and products of a reaction are in different phases. Imagine a situation where a solid reacts to form a gas. This clear distinction in physical states makes it easy to identify a heterogeneous equilibrium.
A common example of such equilibrium is the decomposition of calcium carbonate \(\text{CaCO}_{3}(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_2(g)\). In this reaction:

• \(\text{CaCO}_3\) and \(\text{CaO}\) are solids


• \(\text{CO}_2\) is a gas


• The different phases (solid and gas) define the reaction as heterogeneous.

Heterogeneous equilibria are significant because the concentrations of pure solids or liquids are constant, simplifying calculations. Concentrations of solids and liquids are not included in the equilibrium expression, which typically only involves the gaseous or aqueous reactants and products.

Homogeneous Equilibrium

A homogeneous equilibrium occurs when all the reactants and products are in the same phase. This is most often seen in reactions involving only gases or only aqueous solutions. In such reactions, all participating chemical species are uniformly distributed.
A typical example of homogeneous equilibrium is the synthesis of ammonia: \[ \text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) \] In this reaction:

• All reactants \(\text{N}_2\) and \(\text{H}_2\), as well as the product \(\text{NH}_3\), are gases.


• This uniformity in phase is what defines it as a homogeneous equilibrium.

One interesting aspect of homogeneous equilibria is their dependence on temperature and pressure, making them critical in industrial processes like the Haber process for ammonia production.

Phase of Substances

The phase of a substance refers to its physical state: solid, liquid, gas, or plasma. In the context of chemical equilibrium:

• Solid: Defined by a definite shape and volume with particles closely packed in a regular pattern.


• Liquid: Has a definite volume but takes the shape of its container with particles close together but not regularly arranged.


• Gas: No definite shape or volume, with particles far apart and moving freely.

Understanding the phase of substances is crucial in determining the type of equilibrium (heterogeneous or homogeneous).
In heterogeneous equilibria, the different phases help identify which species' concentrations remain constant and are thus excluded from equilibrium expressions. For homogeneous equilibria, dealing with substances in a single phase allows for more straightforward calculations involving equilibrium constants. Recognizing the phase behavior influences everything from the form of the equilibrium expression to the understanding of reaction kinetics and mechanisms.