Fundamental Rule: If a change is applied to a system at equilibrium, the system will shift its position to oppose or counteract that change.
Predictive Power: This principle allows chemists to predict how changes in temperature, pressure, or concentration will affect the yield of products in industrial processes.
Position Shift: A 'shift to the right' indicates an increase in product formation, while a 'shift to the left' indicates an increase in reactant formation.
Equilibrium Constant: While the position shifts, the fundamental nature of the balance is restored according to the new conditions.
Temperature Adjustment: Increasing temperature favors the endothermic direction (energy-absorbing) to cool the system down. Decreasing temperature favors the exothermic direction (energy-releasing).
Pressure Manipulation: In reactions involving gases, increasing pressure shifts the equilibrium toward the side with fewer moles of gas molecules to reduce pressure. Decreasing pressure shifts it toward the side with more gaseous molecules.
Concentration Changes: Adding more reactant causes the system to shift to the right to consume the excess; removing product has the same effect.
Catalyst Application: A catalyst increases the speed at which equilibrium is reached by lowering activation energy for both directions equally, but it does not change the final position of equilibrium.
| Feature | Open System | Closed System |
|---|---|---|
| Matter Exchange | Allowed with surroundings | No matter enters or leaves |
| Equilibrium | Cannot be reached if species escape | Can reach dynamic equilibrium |
| Concentration | Usually changes until reactants deplete | Becomes constant at equilibrium |
Rate vs. Position: Rate refers to how fast the reactions occur, while position refers to the relative amounts of reactants and products present once balance is achieved.
Exothermic vs. Endothermic Directions: Every reversible reaction has one direction that releases heat and an opposite direction that absorbs it; identifying these is crucial for predicting temperature effects.
Two Essential Features: When asked to describe equilibrium, always state that: 1. The rates of forward and reverse reactions are equal. 2. The concentrations of reactants and products remain constant.
Gas Mole Counting: For pressure questions, always count the coefficients of the gaseous species only. Ignore solids and liquids when determining the direction of the shift.
Thermal Context: If the forward reaction is labeled as exothermic (negative ), remember that the reverse reaction is automatically endothermic.
Common Catalyst Trap: Examiners often try to trick students into saying a catalyst increases yield. It does not; it only saves time by reaching the same yield faster.
Static Misconception: Students often think a reaction 'stops' at equilibrium. It is actually a busy, continuous exchange between reactants and products.
Equality of Concentration: Equilibrium does not mean the concentration of reactants equals the concentration of products; it only means those concentrations no longer change.
Pressure and Non-Gases: Applying pressure changes to a reaction with no gaseous components will have no significant effect on the equilibrium position.
