What is dynamic equilibrium, and how does Le Chatelier's principle predict the effect of a stress on a system?
Equilibrium and Le Chatelier's principle: describe dynamic equilibrium and predict the shift in a system when concentration, temperature or pressure is changed.
A focused Regents Chemistry answer on dynamic equilibrium and Le Chatelier's principle: equal forward and reverse rates, and how a change in concentration, temperature or pressure shifts the equilibrium to relieve the stress.
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What this topic is asking
The Core Curriculum asks you to describe dynamic equilibrium and to apply Le Chatelier's principle: to predict how a system at equilibrium shifts when you change the concentration, temperature or pressure. The Regents keeps this qualitative (no equilibrium-constant calculations), and it is a reliable Part B-2 question.
Dynamic equilibrium
The word "dynamic" is important: at equilibrium, reactant is still becoming product and product is still becoming reactant, but at matching rates, so nothing appears to change. Equilibrium is reached only in a closed system, and the concentrations at equilibrium are constant but not necessarily equal.
Le Chatelier's principle
This single principle handles every Regents equilibrium question. The skill is to identify the stress, then state which direction relieves it. A shift "toward the products" means the forward reaction is temporarily favored; "toward the reactants" means the reverse reaction is favored.
The three stresses
- Concentration
- Adding a reactant or product shifts the equilibrium away from the added substance (the system consumes the excess); removing a substance shifts toward it (the system replaces it). Adding more reactant therefore pushes the reaction toward products.
- Temperature
- Treat heat as a reactant or product. For an exothermic forward reaction, heat is a product, so adding heat shifts toward reactants and cooling shifts toward products. For an endothermic forward reaction, the reverse holds.
- Pressure (gases only)
- Increasing the pressure on a gaseous equilibrium shifts it toward the side with fewer moles of gas; decreasing the pressure shifts it toward the side with more gas molecules. Count the gas-phase coefficients on each side. Pressure changes have no effect if both sides have equal numbers of gas molecules.
Try this
Q1. State what happens to the rates of the forward and reverse reactions at equilibrium. [1 point]
- Cue. They are equal (dynamic equilibrium).
Q2. For a gaseous equilibrium with moles of gas on the left and on the right, state the direction of shift when pressure is increased. [1 point]
- Cue. Toward the right (the side with fewer gas molecules).
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Part B-2 style)3 marksConsider the equilibrium , which is exothermic in the forward direction. State the direction of the shift (toward products, toward reactants, or no shift) when each change is made: (a) more is added; (b) the temperature is increased; (c) the pressure is increased.Show worked answer →
A 3-point constructed-response item applying Le Chatelier's principle.
(a) Add (1 point): adding a reactant shifts the equilibrium toward the products (to use up the added ).
(b) Increase temperature (1 point): the forward reaction is exothermic, so heat is a product; adding heat shifts the equilibrium toward the reactants.
(c) Increase pressure (1 point): higher pressure shifts toward the side with fewer gas molecules; there are gas molecules on the left and on the right, so the shift is toward the products.
Markers reward each correct direction with reasoning that the system shifts to relieve the applied stress.
Regents (Part A style)1 marksAt equilibrium, the rate of the forward reaction is (1) greater than the reverse rate (2) less than the reverse rate (3) equal to the reverse rate (4) zeroShow worked answer →
A 1-point Part A item on the meaning of equilibrium. The answer is (3) equal to the reverse rate.
Dynamic equilibrium is reached when the forward and reverse reactions occur at the same rate. The reactions do not stop (it is dynamic), but because the rates are equal, the concentrations of reactants and products stay constant. A rate of zero would mean the reaction had stopped, which is not what equilibrium means.
Markers reward recognizing that equilibrium is equal forward and reverse rates, not a halt.
Related dot points
- Reaction rates and collision theory: use collision theory to explain how concentration, temperature, surface area and a catalyst affect the rate of a reaction.
A focused Regents Chemistry answer on reaction rates and collision theory: why effective collisions need enough energy and the right orientation, and how concentration, temperature, surface area, the nature of the reactants and a catalyst change the rate.
- Potential energy diagrams: interpret potential energy diagrams to identify activation energy, the activated complex and the heat of reaction, and show how a catalyst changes the diagram.
A focused Regents Chemistry answer on potential energy diagrams: reading the activation energy, the activated complex and the heat of reaction (delta-H), distinguishing exothermic from endothermic reactions, and how a catalyst lowers the activation energy without changing delta-H.
- Acids, bases and the pH scale: identify Arrhenius acids and bases, interpret the pH scale, and relate a change in pH to a change in hydrogen ion concentration.
A focused Regents Chemistry answer on Arrhenius acids and bases, the pH scale, and how each pH unit means a tenfold change in hydrogen ion concentration, using Table K and Table L of the Reference Tables.
- Solutions and solubility curves: classify solutions as unsaturated, saturated or supersaturated, and use the Table G solubility curves to determine how much solute dissolves at a given temperature.
A focused Regents Chemistry answer on solutions and the Table G solubility curves: solute and solvent, saturated, unsaturated and supersaturated solutions, the factors that affect solubility, and how to read grams of solute per 100 g of water from the curve.
- Neutralization and salts: write neutralization reactions of an acid with a base to form a salt and water, and identify the salt produced.
A focused Regents Chemistry answer on neutralization: how an acid and a base react to form a salt and water, how to predict the salt from the acid's anion and the base's cation, and the role of the hydrogen and hydroxide ions.
Sources & how we know this
- Physical Setting/Chemistry Core Curriculum — New York State Education Department (2002)
- Reference Tables for Physical Setting/Chemistry, 2011 Edition — New York State Education Department (2011)