High School Science - Equilibrium

Chemical Equilibrium

Objective Students will understand that chemical equilibrium is a dynamic process at the molecular level. Students will demonstrate their understanding of this concept by comparing container trade with chemical reactions and solving equilibrium problems involving the equilibrium constant (K).

California Content Standard (Chemistry) 9. Chemical equilibrium is a dynamic process at the molecular level. As a basis for understanding this concept: b. Students know equilibrium is established when forward and reverse reaction rates are equal. c. Students know how to write and calculate an equilibrium constant expression for a reaction.

Introduction The Port of Long Beach is one of the world's busiest seaports, a leading gateway for trade between the United States and Asia. East Asian trade accounts for more than 90% of the shipments through the port. In today's global economy, the effort involved in transporting goods between manufacturers, retailers and consumers across the planet is remarkable in its scale and sophistication. The enormous quantities and economic value of cargo being transported through seaports, such as the Port of Long Beach, demands a flexible, efficient and well-organized supply chain. Today's system is made possible through the use of metal cargo containers, which revolutionized the maritime industry soon after they were introduced in the late 1950s. Cargo containers act as individual storage units that can be switched quickly between ships, trucks and trains. Cargo containers hold just about any consumer item -- shoes, computers, auto parts or frozen seafood.

In this lesson, students will analyze data regarding container trade in TEU’s and compare that to chemical equilibrium. *One TEU is a Twenty-foot Equivalent Container.

Anticipatory Set Can you think of situations in science, economics, and politics that have a balancing of competing influences? • Examples include: – Science • Homeostasis—the ability of an open system to regulate its internal environment. • Mechanical equilibrium—the state in which the sum of the forces is zero.

– Economics • Equilibrium price—the price at which supply equals demand.

Lesson Input • When you hear the word Static, what comes to mind? – List a few words that you associate with static. • When you hear the word Dynamic, what comes to mind? – List a few words that you associate with dynamic. • What do you think Static means? • What do you think Dynamic means? • A simplistic definition of Static is unchanging. • A simplistic definition of Dynamic is in motion,

Container Trade in TEU’s Take a look at the following slide depicting container trade through the port of Long Beach and consider the three questions that follow.

Container Trade in TEU’s

1. Are the empty containers inbound or outbound? How do you know? 2. What is static about container trade with East Asia? 3. What is dynamic about container trade with East Asia?

Equilibrium If you look at the data you can see that container trade between East Asia and the Port of Long Beach has reached equilibrium. – Equilibrium does not imply that the number of containers in East Asian ports are equal to the number of containers in Long Beach. If the number of containers in each port are not equal, then what is equal in this equilibrium? Let’s take another look at the data and find out.

Data analysis for the month of January • Inbound – There were approximately 261,000 loaded inbound TEU’s for the month of January. • Outbound – There were approximately 122,000 loaded outbound TEU’s and approximately 138,000 empty TEU’s for the month of January.

261,000

͌ 122,000 + 138,000

Dynamic and Static • Equilibrium is a dynamic process. Containers continually move between ports. However, the number of containers at each port remains relatively static due to the importing and exporting of the same number of containers on a monthly basis. Take a look at the following four slides that show the movement of containers between East Asia and the port of Long Beach.

Oversees container transport

Chemical Equilibrium • Chemical equilibrium, like the port analogy, is a highly dynamic process at the molecular level. Chemical equilibrium only appears to be static due to the unchanging concentrations of reactants and products. Dynamic: the chemical reaction continues in the forward and reverse directions. Nitrogen and hydrogen react to form ammonia and ammonia breaks into hydrogen and nitrogen at the same rate. N2 + H2  NH3 Static: at equilibrium the concentrations of reactants and products remain unchanged

The Equilibrium Expression The equilibrium condition follows this general description for a chemical reaction where lower case letters are the coefficients used to balance the equation. K = [C]c[D]d [A]a[B]b

aA + bB  cC + dD Brackets [ ] indicate concentration in mol/L

The Equilibrium Constant At a constant temperature, the equilibrium constant (K) does not change. Consider the following data for a set of equilibrium positions for the reaction between hydrogen and nitrogen to form ammonia.

Experimental results at a constant temperature N2(g)

+

H2 (g)



NH3(g)

During an experiment at a constant temperature, the initial concentrations of [N2], [H2], and [NH3] were 1.00M, 1.00M and 0 respectively. At equilibrium the concentrations were found to be [N2] = 0.875 M [H2] = 0.598 M [NH3] = 0.105 M What is the value of the equilibrium constant (K)?

Solving for the equilibrium constant (K) N2(g)

+

3H2 (g)

0.875 M 0.598 M 2. Write an equilibrium expression



2NH3(g) 0.105 M

K = [NH3]2 [N2][H2]3 5. Use the concentrations at equilibrium to find K. K = (0.105)2 (0.875)(0.598)3 K = 0.0589

Application of Equilibrium constant K What is the concentration of ammonia when the equilibrium concentrations of N2 and H2 are 0.231 M and 0.629 M respectively? N2(g) 0.231 M

+

3H2 (g)



2NH3(g)

0.629 M

*Remember that K was found to be 0.0589 for this reaction at the temperature given.

?

Solving this problem N2(g) 0.231 M

+

3H2 (g)  0.629 M

2NH3(g) ?

Algebraically rearrange this equation to solve for [NH3] and plug in the data to find the molar concentration of ammonia. K = [NH3]2 [N2][H2]3 [NH3]

=

K [N2][H2]3

[NH3]

=

[NH3]

= .00339 M

(0.0589)(0.231)(0.629)3

Guided Practice problems Write equilibrium expressions for these reactions. 2SO2(g) + O2(g)  2SO3(g) K=

[SO3]2 [SO2]2[O2]

4NH3(g) + 7O2(g)  4NO2(g) + 6H2O(g) K = [NO2]4[H2O]6 [NH3]4[O2]7

Practice problems Calculate the value for the equilibrium constant K for the reaction below when the equilibrium concentrations are given below. 1.50M 1.25M 3.50M 2SO2(g) + O2(g)  2SO3(g) K = 4.36

Practice problem cont. What is the equilibrium concentration of O2 when the concentrations of SO2 and SO3 are 2.27 M and 3.21 M respectively?

[O2] = 0.459 M

Closing Activity Ask students: What is static about chemical equilibrium? What makes chemical equilibrium a highly dynamic process? Have students write a 1 or 2 paragraph response to the following prompt. • Explain the similarities and differences between container trade and chemical equilibrium.