Section 17-1

Section 17.1 A State of Dynamic Balance

• List the characteristics of chemical equilibrium.

• Write equilibrium expressions for systems that are at equilibrium.

• Calculate equilibrium constants from concentration data.

Section 17-1

What is equilibrium?

• Chemical reactions often reach a balancing point, or equilibrium.

• N2 (g) + 3H2 (g) 2NH3 (g) ∆G° = -33.1 kJ

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

Section 17-1

What is equilibrium? (cont.)

• A reversible reaction is a chemical reaction that can occur in both the forward and reverse directions, such as the formation of ammonia.

• Forward: N 2(g) + 3H2(g) → 2NH3(g)

• Reverse: N2 (g) 3H2 (g) 2NH3 (g)an office.

N2(g) + 3H2(g) ↔ 2NH3(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.

Section 17-1

What is equilibrium? (cont.)

• The reaction reaches equilibrium in figure d.

• Study the diagrams to answer the following questions. In a, how do you know that the reaction has not yet begun?

• Only reactants are present.

•

• In b, what evidence indicates that the reverse reaction has begun?

• More than two ammonia molecules are present, so they can react in the reverse direction

• Compare c with d. How do you know that equilibrium has been reached?

• The number of reactant and product molecules are the same in c and d, so equilibrium is established.

Section 17-1

What is equilibrium? (cont.)

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

• Equilibrium is a state of action, not inaction.

• Radioactive iodine molecules in the solid in the flask on the right are separated from nonradioactive iodine in the flask on the left. Note the readings on the radiation monitors. b. After the stopcock has been open for a time, the radiation monitors show that radioactive molecules are in both flasks. The particles must have moved back and forth between the flasks and between the solid and the gaseous phases.

• the constancy does not mean that the reactions have stopped, but that a state of equilibrium has been established

Section 17-1

Equilibrium Expressions (cont.)

• The law of chemical equilibrium states that at a given temperature, a chemical system might reach a state in which a particular ratio of reactant and product concentrations has a constant value.

• The Equilibrium Constant Expression

Section 17-1

Equilibrium Expressions (cont.)

• The equilibrium constant, Keq, is the numerical value of the ratio of product concentrations to reactant concentrations, with each concentration raised to the power equal to its coefficient in the balanced equation.

Keq > 1: Products are favored at equilibrium

Keq < 1: Reactants are favored at equilibrium

• The value of Keq is constant only at a specified temperature.

Equilibrium position:

Section 17-1

Equilibrium Expressions (cont.)

H2(g) +I2(g) ↔ 2HI(g)

• This reaction is a homogeneous equilibrium, which means that all the reactants and products are in the same physical state.

Section 17-1

Equilibrium Expressions (cont.)

• When the reactants and products are present in more than one physical state, the equilibrium is called a heterogeneous equilibrium.

• Ethanol in a closed flask is represented by C2H5OH(l) ↔ C2H5OH(g).

Section 17-1

Equilibrium Constants

• For a given reaction at a given temperature, Keq will always be the same regardless of the initial concentrations of reactants and products.

• Equilibrium characteristics:

• To attain equilibrium, a reaction mixture must be in a closed container, it must be at constant temperature, and all reactants and products must be present

Q1: Explain the meaning of a double arrow in chemical equations.

• The double arrow means that a reaction is at equilibrium and that the forward reaction and the reverse reaction are occurring at the same rate.

Q2:Explain how the size of the equilibrium constant relates to the amount of product formed at equilibrium.

• The larger the equilibrium constant the greater the amount of products formed at equilibrium.

• Q3:Compare homogeneous and heterogeneous equilibria.

• In a homogenous equilibrium, all the reactants and products are in the same physical state. In a het- erogeneous equilibrium, reactants and products are in different physical states.

• Q4:List three characteristics a reaction mixture must have if it is to attain a state of chemical equilibrium.

• To attain equilibrium, a reaction mixture must be in a closed container, it must be at constant temperature, and all reactants and products must be present

A. A

B. B

C. C

D. D

Section 17-1

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Section 17.1 Assessment

A reaction is in equilibrium when:

A. there are more products than reactants

B. the amount of products equals the reactants

C. the rate of the forward reaction is greater than the reverse reaction

D. the rate of the forward and reverse reactions are equal

Q5:Interpret Data The table below shows the value of the equilibrium constant for a reaction at three different temperatures. At which temperature is the concentration of the products the greatest? Explain your answer.

• 373 K; The products are in the numerator of the ratio, so the larger the Keq is, the greater the concentration of the products.

A. A

B. B

C. C

D. D

Section 17-1

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Section 17.1 Assessment

The value of the equilibrium constant is constant for a given ____.

A. temperature

B. pressure

C. volume

D. density

A. A

B. B

C. C

D. D

Chapter Assessment 1

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A change in ____ alters both the equilibrium position and the equilibrium constant.

A. pressure

B. temperature

C. volume

D. density

A. A

B. B

C. C

D. D

Chapter Assessment 2

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A(n) ____ equilibrium is when all reactant and products are in the same physical state.

A. endothermic

B. exothermic

C. heterogeneous

D. homogeneous

A. A

B. B

C. C

D. D

Chapter Assessment 3

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Which is NOT an example of a homogeneous equilibria?

A. H2(g) +I2(g) ↔ 2HI(g)

B. 2H2(g) +O2(g) ↔ 2H2OI(g)

C. HCl(aq) + NaOH(aq) ↔NaCl(aq) + H2O

D. C2H5OH(l) ↔ C2H5OH(g)

A. A

B. B

C. C

D. D

Chapter Assessment 5

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The state in which the forward and reverse chemical reactions take place at the same rate is called ____.

A. Le Châtelier’s Principle

B. Henry’s Law

C. Charles’s Law

D. chemical equilibrium

A. A

B. B

C. C

D. D

STP 1

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Which does NOT describe a system that has reached chemical equilibrium?

A. No new product is formed by the forward reactions.

B. The forward and reverse reactions occur at equal rates.

C. The concentration of products is equal to the concentration of reactants.

D. All the reactants have been used up.

A. A

B. B

C. C

D. D

STP 2

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A(n) ____ equilibrium is when all reactant and products are present in more than one physical state.

A. endothermic

B. exothermic

C. heterogeneous

D. homogeneous

End of section 17.1