Chemical Equilibrium

What is equilibrium?Reversible reactions and chemical equilibrium

The dynamic nature of equilibrium

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Tug-of-War Analogy

• Imagine a tug-of-war between two teams.• Because the rope between them is not

moving, it might seem that neither team is pulling.

• In fact, both teams are pulling, but the forces exerted by the two teams are equal and opposite, so they are in complete balance.

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What is equilibrium?

• Often, chemical reactions reach a point of balance or equilibrium.

• During the water tank analogy activity, you found that a point of balance was reached in the transfer of water from each partner’s large graduated cylinder.

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Ammonia Example

• Consider the reaction for the formation of ammonia from nitrogen and hydrogen:

N2 (g) + 3 H2 (g) 2 NH3 (g)• This reaction occurs naturally in a closed container, but

goes faster if heated.• During the reaction, the reactants, H2 and N2, are

consumed, so their concentrations gradually decrease. The concentration of NH3, the product, will gradually increase.

• After a period of time, however, the concentrations of N2, H2 and NH3 no longer change.

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The concentrations of the reactants (H2 and N2) decrease at first, while the concentration of the produce (NH3) increases. Then, before the reactants are used up, all concentrations become constant.

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Reversible Reactions

• When a reaction results in an almost complete conversion of reactants to products, chemists say that the reaction goes to completion—but most reactions do not go to completion.

• The reactions appear to stop because they are reversible.– A reversible reaction is a chemical reaction that

can occur in both the forward and reverse directions.

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Reversible Equations• Forward: N2 (g) + 3 H2 (g) 2 NH3 (g)• Reverse: 2 NH3 (g) N2 (g) + 3 H2 (g) • Chemists combine these two equations into a single

equation that uses a double arrow to show that both reactions occur.

N2 (g) + 3 H2 (g) 2 NH3 (g)• The reactants in the forward reaction are on the left of the

arrows. The reactants in the reverse reaction are on the right of the arrows.– In the forward reaction, hydrogen and nitrogen combine to

form the product ammonia.– In the reverse reaction, ammonia decomposes into the products

hydrogen and nitrogen.

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Concentration Effects• How does the reversibility of this reaction affect the

production of ammonia?– In the beginning, no ammonia is present, therefore only the

forward reaction can occur.– As hydrogen and nitrogen combine to form ammonia, their

concentrations decrease. The rate of a reaction depends on the concentration of the reactants, so now the forward reaction is slowing down.

– As soon as ammonia is present, the reverse reaction can occur, slowly at first, but at an increasing rate as the concentration of ammonia increases.

– As the reaction proceeds, the rate of the forward reactions continues to increase until the two rates are equal.• At this point, ammonia is produced at the same rate that it is

decomposed, so the concentrations of N2, H2, and NH3, remain constant.

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Chemical Equilibrium Definition

• The word equilibrium means that opposing processes are in balance.

• Chemical equilibrium is a state in which the forward and reverse reactions balance each other because they take place at equal rates.

• Rateforwardrxn = Ratereverserxn

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BE CAREFUL!!!!

• At equilibrium, the concentrations of reactants and products are constant.

• HOWEVER, that DOESN’T mean that the concentrations of the reactants and products are EQUAL.

• In fact, it is not unusual for the equilibrium concentrations of a reactant and product to differ by a factor of one million or more!

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Connection to Physics• A push or pull on an object is a force.• When you push on a door or pull on a dog’s leash, you

exert a force (hopefully not a large one on your poor dog!).

• When two or more forces are exerted on the same object in the same direction, they add together.

• One force subtracts from the other if the forces are in opposite directions.

• Thus, in a tug-of-war, when two teams pull on a rope with equal force, the resulting force has a magnitude of zero and the rope does not move. – The system is said to be in equilibrium.

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Seesaws

• Similarly, people balanced on a seesaw represent a system in equilibrium.

• The equal-and-opposite forces on both ends of the seesaw are called balanced forces.

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If, instead, one force is greater in magnitude than the other, the combined force is greater than zero and is called an unbalanced force.

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An unbalanced force causes an object to accelerate, which is what has happened in the picture above.

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Another Analogy!• Like equal forces opposing each other, dynamic equilibrium is a

state of action, not inaction.• Consider this analogy: a glassed-in walkway connects two buildings.

– Suppose that all entrances and exits for the buildings, except the walkway, are closed for a day.

– And suppose that the same number of persons cross the walkway in each direction every hour.

– Given these circumstances, the number of persons in each building in each building remains constant even though people continue to cross between the two buildings.

– Note that the numbers of persons in the two buildings do not have to be equal.

– Equilibrium only requires that the number of persons crossing the walkway in one direction is equal to the number crossing in the opposite direction.

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Iodine Equilibrium• The dynamic nature of chemical equilibrium can be illustrated

by placing equal masses of iodine crystals in two interconnected flasks.

• The flask on the left contains iodine molecules made up entirely of the nonradioactive isotope I-127. The flask on the right contains iodine molecules made up of the radioactive isotope I-131.

• Radiation counters can show the difference in the levels of radioactivity within each flask.

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Iodine Cont’d• Each flask is a closed system.– No reactant or product can enter or leave.

• At 298 K and 1 atm, this equilibrium is established in both flasks:

I2 (s) I2 (g)• In the forward process, called sublimation, iodine

molecules change directly from the solid phase to the gas phase.

• In the reverse process, gaseous iodine molecules return to the solid phase.– A solid-vapor equilibrium is established in each flask.

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Some More Iodine Stuff

• When the stopcock in the tube connecting the two flasks is opened, iodine vapor can travel back and forth between the two flasks.

• After a period of time, the readings on the radiation counters indicate that the flask on the left contains as many radioactive I-131 molecules as the flask on the right in both the vapor and the solid phases.

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Iodine Conclusions

• The evidence suggests that iodine molecules constantly change from the solid phase to the gas phase according to the forward process, and that gaseous iodine molecules convert back to the solid phase according to the reverse process.

• The constant readings on both radiation detectors indicate that equilibrium has been established in the combined volume of the two flasks.

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HOMEWORK

1) Given the fact that the concentration of reactants and products are not changing, why is the word dynamic used to describe chemical equilibrium?

2) Explain how a person bailing out a row boat with a leak could represent a state of physical equilibrium.

3) A system, which contains only molecules as reactants and products, is at equilibrium. Describe what happens to the concentrations of the reactants and products and what happens to individual reactant and product molecules.

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