What is dynamic equilibrium, and how does a system respond to a stress?
Chemical equilibrium and Le Chatelier's principle: describe dynamic equilibrium in a reversible reaction and predict the shift when concentration, temperature or pressure changes.
A focused Virginia SOL Chemistry answer on equilibrium under CH.6: reversible reactions and dynamic equilibrium, and using Le Chatelier's principle to predict how an equilibrium shifts when concentration, temperature or pressure changes.
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What this topic is asking
Standard CH.6 finishes with chemical equilibrium. Virginia expects you to describe a reversible reaction reaching dynamic equilibrium, and to use Le Chatelier's principle to predict how the equilibrium shifts when the concentration, temperature or pressure changes. The reasoning is qualitative: you predict the direction of the shift, not a numerical equilibrium constant.
Reversible reactions and dynamic equilibrium
Equilibrium does not mean the reactants and products are equal in amount, and it does not mean the reactions have stopped. It means the rates are balanced: as fast as products form, they turn back into reactants, so the concentrations hold steady. This is why it is called "dynamic", in contrast to a reaction that has simply finished.
Le Chatelier's principle
The principle gives a simple way to predict the direction of a shift without calculation: identify the stress, then ask which way the system must move to counteract it. The system cannot fully cancel the change, but it shifts to reduce its effect.
Predicting shifts
Apply the principle to each kind of stress:
- Concentration. Adding a substance shifts the equilibrium away from it (to use it up); removing a substance shifts toward it (to replace it). Add a reactant and the system makes more product.
- Pressure (for gases). Increasing the pressure shifts the equilibrium toward the side with fewer moles of gas; decreasing the pressure shifts toward the side with more moles of gas. Count the gas molecules on each side.
- Temperature. Treat heat as a reactant (for an endothermic reaction) or a product (for an exothermic reaction). Raising the temperature shifts in the endothermic direction; lowering it shifts in the exothermic direction.
A catalyst speeds the forward and reverse reactions equally, so it helps equilibrium be reached faster but does not change the position of the equilibrium.
Try this
Q1. For , which way does the equilibrium shift if more hydrogen is added? [1 point]
- Cue. To the right (toward ammonia), to use up the added hydrogen.
Q2. Does adding a catalyst change the position of an equilibrium? [1 point]
- Cue. No; a catalyst speeds both directions equally, so equilibrium is reached faster but its position is unchanged.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksAt dynamic equilibrium, the rates of the forward and reverse reactions are (A) both zero (B) equal (C) the forward faster (D) the reverse fasterShow worked answer →
The answer is (B) equal.
At dynamic equilibrium the forward and reverse reactions are still occurring, but at the same rate, so the concentrations of reactants and products stay constant. The reactions have not stopped (that is why it is called dynamic); they balance each other. Option (A) describes a reaction that has stopped, which is not equilibrium.
The trap is thinking equilibrium means the reactions have stopped; they continue at equal and opposite rates.
SOL (tech-enhanced, fill in the blank)3 marksFor at equilibrium, predict the direction of the shift when: (a) more is added; (b) the pressure is increased; (c) is removed.Show worked answer →
A 3-point Le Chatelier item.
(a) Adding (1 point): the system shifts to the right (toward products) to use up the added nitrogen.
(b) Increasing pressure (1 point): the system shifts toward the side with fewer gas molecules; the right has molecules versus on the left, so it shifts right.
(c) Removing (1 point): the system shifts to the right to replace the removed product.
Markers reward applying Le Chatelier's principle: the equilibrium shifts to oppose each change, counting gas molecules for the pressure effect.
Related dot points
- Reaction rates and collision theory: explain reaction rate using collision theory, including effective collisions, orientation and the activation energy.
A focused Virginia SOL Chemistry answer on collision theory under CH.6: what reaction rate measures, why particles must collide with enough energy and the correct orientation, the role of activation energy, and the meaning of an effective collision.
- Factors affecting reaction rate: describe how concentration, temperature, surface area, a catalyst and the nature of the reactants change the rate of a reaction.
A focused Virginia SOL Chemistry answer on rate factors under CH.6: how concentration, temperature, surface area, catalysts and the nature of the reactants change reaction rate, each explained with collision theory.
- Endothermic and exothermic reactions: distinguish endothermic and exothermic processes by the direction of energy flow and the sign of the enthalpy change.
A focused Virginia SOL Chemistry answer on reaction energy under CH.6: the difference between endothermic and exothermic reactions, the direction of energy flow, the sign of the enthalpy change, and how temperature change signals each type.
- Potential energy diagrams and activation energy: interpret a potential energy diagram, identify the activation energy and the energy change, and explain the effect of a catalyst.
A focused Virginia SOL Chemistry answer on energy diagrams under CH.6: reading a potential energy diagram, identifying the activation energy, the energy of the products versus reactants, and how a catalyst lowers the activation energy.
- The gas laws: use Boyle's law, Charles's law, Gay-Lussac's law and the combined gas law to relate the pressure, volume and temperature of a gas.
A focused Virginia SOL Chemistry answer on the gas laws under CH.4: Boyle's law (pressure and volume), Charles's law (volume and temperature), Gay-Lussac's law (pressure and temperature), and the combined gas law, with worked calculations and the need for Kelvin temperature.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)