IB DP Chemistry - R2.3.1 Dynamic equilibrium - Study Notes - New Syllabus - 2026, 2027 & 2028
IB DP Chemistry – R2.3.1 Dynamic equilibrium – Study Notes – New Syllabus
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Reactivity 2.3.1 – Dynamic Equilibrium and Its Characteristics
Reactivity 2.3.1 – Dynamic Equilibrium and Its Characteristics
- Equilibrium is a fundamental concept in chemistry where a system stabilizes after undergoing reversible changes. It can occur in both physical and chemical systems under specific conditions.
- A state of dynamic equilibrium is reached in a closed system when the rate of the forward reaction equals the rate of the reverse reaction.
Key Characteristics of a System at Equilibrium (Physical or Chemical):
- Dynamic in Nature: Even though the system appears unchanging at the macroscopic level, the forward and reverse processes are continuously occurring at the molecular level.
- Equal Rates: The rate of the forward reaction equals the rate of the backward reaction, resulting in no net change in the concentrations of reactants and products.
- Constant Macroscopic Properties: Observable properties like pressure, temperature, concentration, color, and volume remain constant over time once equilibrium is achieved.
- Requires a Closed System: A closed system is essential to prevent the loss or gain of matter. Only energy (like heat) may be exchanged with the surroundings.
- Reversibility: The process must be reversible, usually represented by a double arrow (\( \rightleftharpoons \)).
- Position of Equilibrium: The ratio of products to reactants is constant at equilibrium but may favor either side, depending on conditions such as temperature, pressure, and concentration.
At this point (i.e., at equilibrium):
- The concentrations of reactants and products remain constant over time (not necessarily equal, but unchanging).
- Both the forward and reverse reactions continue to occur — the system is dynamic, not static.
- No macroscopic changes are observed, even though particles are still reacting on the molecular level.
Types of Equilibrium:
- Physical Equilibrium: Involves changes in physical state or phase with the same chemical substance.
- Chemical Equilibrium: Involves reversible chemical reactions where reactants form products and vice versa.
| Aspect | Physical Equilibrium | Chemical Equilibrium |
|---|---|---|
| Process Type | Physical change (e.g., phase change) | Chemical reaction involving bond breaking/forming |
| Example | \( \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_2\text{O}(g) \) | \( \text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) \) |
| Substance Identity | Same substance in different physical states | Different chemical species as reactants and products |
| Bond Changes | No chemical bonds are broken or formed | Involves making and breaking of bonds |
Example – Chemical Equilibrium: In the Haber process:
\( \text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) \)
When this reaction is carried out in a sealed container at constant temperature, the concentrations of nitrogen, hydrogen, and ammonia will eventually become constant. The system is still dynamic — ammonia is constantly being formed and decomposed at equal rates.
Example – Physical Equilibrium: In a closed container with liquid water and water vapor:
\( \text{H}_2\text{O}(l) \rightleftharpoons \text{H}_2\text{O}(g) \)
Evaporation and condensation continue to happen at the same rate, and the amount of liquid and vapor stays constant. This is a physical dynamic equilibrium.
Example
A sealed container initially contains only hydrogen iodide gas, \( \text{HI}(g) \). Over time, it partially decomposes according to the reaction:
\( 2\text{HI}(g) \rightleftharpoons \text{H}_2(g) + \text{I}_2(g) \)
After a few hours, measurements show that the concentrations of all species remain constant.Why?
▶️Answer/Explanation
This system has reached dynamic equilibrium. Although the concentrations no longer change, the forward and reverse reactions are still occurring at equal rates. The fact that products have formed indicates the forward reaction occurred, and their constant concentration implies the reverse reaction is also occurring at the same rate. This balance of opposing processes is the hallmark of chemical equilibrium.
Example
A closed container contains solid calcium carbonate, \( \text{CaCO}_3(s) \), which decomposes according to the equation:
\( \text{CaCO}_3(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_2(g) \)
Eventually, the pressure of the carbon dioxide gas in the container remains steady.Why?
▶️Answer/Explanation
This is a heterogeneous equilibrium involving both solids and gases. The constant pressure of \( \text{CO}_2 \) gas indicates that equilibrium has been reached. The reaction is dynamic—\( \text{CaCO}_3 \) continues to decompose while \( \text{CaO} \) and \( \text{CO}_2 \) react to reform it at equal rates. Importantly, the concentrations (or amounts) of pure solids like \( \text{CaCO}_3 \) and \( \text{CaO} \) are not included in the equilibrium expression; only the gaseous \( \text{CO}_2 \) determines the equilibrium condition in this case.
Example
A container holds a mixture of nitrogen dioxide and dinitrogen tetroxide gases at equilibrium:
\( \text{N}_2\text{O}_4(g) \rightleftharpoons 2\text{NO}_2(g) \)
The brown color of \( \text{NO}_2 \) becomes darker when the container is heated.Why?
▶️Answer/Explanation
Heating the system causes the position of equilibrium to shift. Since \( \text{NO}_2 \) is brown and the color intensifies, more \( \text{NO}_2 \) is formed. This indicates that the forward reaction (endothermic) is favored at higher temperatures. According to Le Châtelier’s Principle, the system adjusts to absorb the added heat by shifting toward the side that consumes energy—in this case, the formation of \( \text{NO}_2 \). This demonstrates how equilibrium responds to temperature changes.