EQUILIBRIUMMy Goals for this Lesson:

Explain the concept of dynamic equilibrium.I’m preparing to understand and to be able to explain the concept of dynamic equilibrium.

Introduction Fill in the blanks using the interactive component on the Introduction page.Imagine that there are two island communities connected to each other by a bridge. One island has a population of 100,000, and the other has a population of 10,000. How would the flow of traffic on the bridge affect the populations on each island?

If the bridge only allowed one-way traffic, how would that affect the populations on each island?

The population on the island from which the traffic was leaving would while the population on the other island would .

What if the bridge allows for two-way traffic, but there is only one lane heading east and three lanes heading west?

The island on the east side of the bridge would be more occupants than it would be , while the island on the west side of the bridge would be more occupants than it would be . This means that the population in the east would as the population in the west would .

How would the populations on each island be affected if the traffic flow on the bridge was exactly the same in both directions?

Even though the identities of the cars and people would be constantly changing as people moved across the bridge in either direction, the total population on each island would stay .

This illustrates a concept in chemistry that we call . As you learn about , it will be important to remember that the rates, or speeds, of opposite reactions are equal when a system is at . Just as the traffic did not stop on the bridge, reactions do not stop when they reach , but the equal rates provide us with steady amounts on either side of the reaction.

EquilibriumFill in this table with your own words using the interactive section from the Lesson.

When nitrogen dioxide gas (NO2) is sealed into an evacuated glass container at room temperature, you can observe the initial dark brown color begin to fade as the brown nitrogen dioxide is to colorless dinitrogen tetroxide (N2O4).

If only the reaction were to occur, we would expect the sample to eventually be completely as all of the NO2 is converted to N2O4. However, even over a long period of time, you will see that the gas inside the container becomes completely colorless. This is because the reaction (2NO2 → N2O4) does not go to .

The gas mixture of NO2 and N2O4 inside the container eventually reaches a constant color that does not change at a constant temperature. This constant color shows us that the concentrations of NO2 and N2O4 are no longer , which seems to indicate that the forward reaction has stopped before reaching completion.

If you could observe the reaction at the level, you would see that the reaction has not stopped. The reaction did not fully complete because the reaction (N2O4 → 2NO2) was also occurring. When the forward and the reverse reactions occur at the rate or speed, the concentration of the reactants and products have stopped changing.

At the beginning of this reaction, only the reaction was occurring because only nitrogen dioxide was present in the container. As the nitrogen dioxide particles collided together and reacted, its concentration as it was consumed in the reaction to form the product. Remember that as the concentration of reactants decrease during a reaction, the rate of the reaction also decreases.

At the same time, the concentration of the dinitrogen tetroxide as it was being formed by the forward reaction. As more and more dinitrogen tetroxide particles were produced, the collisions between these particles increased. Remember that an increase in concentration increases the probability of successful collisions, increasing the rate of the reaction. This means that the rate of the reverse reaction (N2O4 → 2NO2) is increasing as the rate of the forward reaction is decreasing.

Eventually, the rates of the forward and the reverse reactions will be . This does not mean that the concentrations of the reactants and the products will be equal with each other, but that each of the concentrations will remain constant once the rates of the forward and the reverse reactions are the same. Once the forward and the reverse reactions are occurring at the same rate, the system is said to be in a state of dynamic equilibrium.

Dynamic equilibrium is a state of in which the forward and the reverse reactions to occur, but the of the forward and the reverse reactions are equal. All reactions have the potential to reach equilibrium if they occur in a system.

Example - Treadmill We can compare an equilibrium system to a . If you have ever walked on a treadmill, you know that you are walking forward as the belt of the treadmill is moving in the direction.

If you walk or run than the rate of the treadmill’s belt, you will move on the treadmill.

If you are moving forward at a speed that is than the belt’s speed, you will find yourself moving (and possibly falling off the back of the treadmill).

If you are able to walk forward at the rate that the treadmill’s belt is moving backward, you will find yourself walking in place. This does not

mean that you or the belt has stopped moving; it just means that you are moving in directions at rates. This is true of a chemical system at equilibrium. When the forward and the reverse reactions are occurring at the rate, the concentrations of the reactants and the products will to remain constant. This does not mean that the reactions have stopped, but that the of the forward and the reverse reactions are happening at the speed.

Phase Equilibrium In your own words, tell how phase changes also undergo dynamic equilibrium. In your own words, tell how water sealed in an evacuated flask can demonstrate dynamic equilibrium.

GraphsUse the lesson to describe the parts of each graph and how they relate to dynamic equilibrium.

Equilibrium Constant Science is based largely on experimental investigations, and the description of equilibrium systems is no different. In 1864, two Norwegian chemists, Cato Guldberg and Peter Waage, used data and observations from numerous reaction systems to propose the law of mass action to describe the equilibrium condition.

Let’s examine the basic concepts of the law of mass action.

Consider the following general reversible reaction. In this example, w moles of A react with x moles of B to produce y moles of C and z moles of D in the forward reaction (w, x, y, and z are coefficients, A and B are reactants in the forward reaction, and C and D are products of the forward reaction).

wA + xB yC + zD

At equilibrium, the concentrations of each substance in the system can be plugged into the equation below. The law of mass action proposes that, for a reaction at a given temperature, this equation can be used to relate the equilibrium concentrations of the substances in the system to the reaction’s equilibrium constant (K).

K = [C] y x [D] z [A]w x [B]x

Remember that brackets are used to represent concentration in molarity (M).

The value of a system's equilibrium constant (K) is constant at a given temperature, regardless of the initial amounts of the reactants and the products introduced into the system. Although there is only one value for the equilibrium constant for a system at a given temperature, there are infinite combinations of equilibrium concentrations of the reactants and the products within the system.

Use the interactive section on Properties to complete this chart.Each set of equilibrium concentrations is called an . The specific equilibrium position reached by a system depends on the 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 and make about the behavior of a variety of equilibrium systems. We can use the values of K to 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 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.

A system with an equilibrium constant value greater than one.According to the law of mass action:If K > 1, that means [C]Y × [D]Z > [A]W × [B]X. This means that in a system where K is greater than one, you will expect the forward reaction to proceed farther to the right before the system reaches equilibrium, giving more products than reactants when the system reaches equilibrium.

A system with an equilibrium constant value less than one.A very low value for K (a decimal value less than one, 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.

According to the law of mass action: If K < 1, that means [C]Y × [D]Z ‹ [A]W × [B]X. This means that in a system where K is less than one, you will expect the forward reaction to not proceed very far to the right before the system reaches equilibrium, giving more reactants than products when the system reaches equilibrium.

It is important to note that the value of K indicates how far the forward reaction proceeds before reaching equilibrium. However, the value of K does not indicate the amount of time it will take for the system to reach equilibrium.

Be sure to do the “Let’s Review” section on the Lesson page.