the forward and reverse reactions are happening at the same rate
  • At equilibrium, the rate of the forward reaction is ____ the rate of the reverse reaction.
  • Once a system is at equilibrium, the concentrations of all reactants and products ____ when there is no change in conditions such as temperature and pressure.
  • Treadmill Comparison
  • A system is said to be in a state of dynamic equilibrium when ____.
Each set of equilibrium concentrations is called an equilibrium position. The specific equilibrium position reached by a system depends on the initial concentrations of each substance and can be predicted based on the value of the equilibrium constant (K).Equilibrium constants and the law of mass action can be used to describe and make predictions about the behavior of a variety of equilibrium systems. We can use the values of K to compare the progress of different reactions.Because the law of mass action provides an equation where the concentration of products is in the numerator and the concentration of reactants is in the denominator, it follows that the magnitude of the value of the equilibrium constant is related to the position of the equilibrium.As you can see from the law of mass action's equation, the greater the value of K, the farther to the right the system will proceed before reaching equilibrium.K = [C]^y times [D]^z / [A] ^w times [B]^xA system with an equilibrium constant with a value GREATER than 1:This means that the system will consist of more products than reactants at equilibrium.This type of equilibrium system lies farther to the right, meaning that the forward reaction proceeded close to completion before equilibrium was established.A system with an equilibrium constant with a value LESS than 1:A very low value for K (a decimal value less than 1, because K cannot be negative) means that the equilibrium system lies to the left.The forward reaction will not proceed very far before equilibrium is established, and the system will have more reactants than products at equilibrium.Remember that brackets are used to represent concentration, in molarity (M). Notice that only solutions and gases have concentrations, so they are the only phases represented in the equilibrium constant expression. Pure liquids and solids are not included, and their addition or removal does not usually affect the equilibrium position.
  • Equilibrium Constant
  • Graphs
  • 2NO2 (g) ⇄ N2O4 (g)
  • equilibrium constant (k)
When the forward and reverse reactions are occurring at the same rate, the concentrations of the reactants and products will appear to remain constant. This does not mean that the reactions have stopped, but that the changes of the forward and reverse reactions are happening at the same speed.
  • H2O (I) → H2O (g)
  • Treadmill Comparison
  • Equilibrium Constant
  • Phase Equilibrium
Graphs offer visual representations of data or concepts. There are two types of graphs commonly used to represent the concepts of equilibrium.1) A graph that compares the rates of the forward and the reverse reactions over time helps us to see what really is equal when a system reaches equilibrium. Notice that the rate of the forward reaction decreases over time, as the reactants are consumed and less reactant particles are available to collide and produce products. On the other hand, as the products are produced and their concentrations increase, the rate of the reverse reaction also increases. Eventually, in a closed system, the rate of the forward reaction and the rate of the reverse reaction will become equal. At this point the system has reached equilibrium. Both the forward and the reverse reactions will continue to occur, but at equal rates with no net change to the amounts of reactants and products present in the system.2) Another graph describes the change in concentration of reactants and products over the course of a reaction. Remember that the products of the forward reaction, as they are labeled here, are also the reactants of the reverse reaction.Remember that as a forward reaction occurs, the reactant particles collide together to produce the products. As this happens, the concentration of the reactants decreases over time. Remember that as concentration of the reactants decreases, so does the rate of the forward reaction. As the reactants are being consumed, the products are being produced. The increase in concentration of the products results in an increase in rate of the reverse reaction. Eventually, the concentrations of the reactants and the products become constant, but not necessarily equal to each other. We know that, at this point, the system has reached a dynamic equilibrium where the rate of the forward reaction is equal to the rate of the reverse reaction.
  • Equilibrium Constant
  • 2NO2 (g) ⇄ N2O4 (g)
  • equilibrium constant (k)
  • Graphs
phase equilibrium
  • When solid iodine is placed in a sealed container, some purple vapor will begin to appear above the solid. After a while, the amount of purple vapor remains constant and the color does not change. This is an example of ____.
  • If the equilibrium constant of a system is greater than 1, there will be ____ products than reactants when the system reaches equilibrium.
  • At equilibrium, the rate of the forward reaction is ____ the rate of the reverse reaction.
  • Phase Equilibrium
When liquid water is placed in an evacuated, sealed flask, some of the faster-moving molecules will start to evaporate.As water vapor begins to form above the liquid water, some of the slower-moving vapor molecules will start to condense.The evaporation process will begin to slow down as there is less liquid water present, and the condensation process will begin to speed up as there is more vapor present.Eventually, the rate of evaporation will equal the rate of condensation.At this point, the amount of liquid and vapor inside the sealed flask will remain constant, though not necessarily equal to each other, even though both processes will continue to occur.
  • H7O (I) → H2O (g)
  • H2O (I) → H1O (g)
  • H2O (I) → H2O (g)
  • H3O (I) → H2O (g)
Dynamic equilibrium also occurs when physical changes take place. When water is sealed in a flask, equilibrium will eventually be reached between the evaporation and the condensation processes. The faster-moving molecules in the liquid will escape as vapor, while the slower-moving vapor molecules will condense back into a liquid.
  • Phase Equilibrium
  • Treadmill Comparison
  • Equilibrium Constant
  • H2O (I) → H2O (g)
Many reactions do proceed close to completion, but there are also many reactions that stop far short of completion because a reverse reaction takes place. The products formed in a chemical reaction may react to re-form the original reactants. Reactions in which the products can react to re-form the reactants are called reversible reactions. Theoretically, all reactions are reversible, but some reverse reactions occur more easily than others.A double arrow can be used to represent a reversible reaction. Most reactions are reversible, but the double arrow is often only shown when discussing both the forward and the reverse reactions.
  • H2O (I) → H2O (g)
  • Phase Equilibrium
  • 2NO2 (g) ⇄ N2O4 (g)
  • To begin...
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