11 IB Topic 7: Equilibrium 7.1: Dynamic equilibrium 7.1.1Outline the characteristics of chemical and...

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IB Topic 7: Equilibrium7.1: Dynamic equilibrium

7.1.1 Outline the characteristics of chemical and physical systems in a state of equilibrium.

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7.1.1 Outline the characteristics of chemical and physical systems in a state

of equilibrium.

The reactions we have studied so far have gone to completion. In other words the reaction proceeds until

oneor more of the reactants runs out.

Mg + 2HCl MgCl2 + H2

There are reactions that are reversible. In other words, the

reactions occur simultaneously in both directions

2SO2(g) + O2(g) 2SO3(g)

2SO3(g) 2SO2(g) + O2(g)


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of equilibrium.

Reversible Reactions:Reactions occurring simultaneously in both

directions

2SO2(g) + O2(g) 2SO3(g)

Reaction 1: Sulfur dioxide reacts with oxygen to produce sulfur trioxide. SO2(g) & O2(g) are reactants, SO3(g) is the product.

Reaction 2: Sulfur trioxide decomposes to sulfur dioxide & oxygen. SO3(g) is the reactant and SO2(g) & O2(g) are products.



of equilibrium.

Chemical Equilibrium• A state in which the

rate of the forward reaction equals the rate of the reverse reaction.

• Once equilibrium is reached, the concentrations of the reactants and the concentrations of the products do not change.

• The reaction continues but no change in concentrations

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of equilibrium.

2HI H2 + I2

• Reaction 1 (Graphs 1 & 2) Starting with a concentration of 2.0 HI and 0 H2 or I2. HI begins to decompose, forming H2 & I2. As H2 & I2 form, they begin to react forming HI. Eventually the rates become equal so the amount of HI reacted = the amount of HI produced. The reaction continues but no change in concentrations.

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of equilibrium.

2HI H2 + I2

• Reaction 2 (Graphs 3 & 4)

Explain what is happening.

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of equilibrium.

Physical System at Equilibrium

Liquid water evaporates to form water vapor. At a given temperature in a closed system, water will evaporate until the vapor reaches a certain pressure. When that occurs, equilibrium is reached. Water still evaporates but at the same rate as water condensing.

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IB Topic 7: Equilibrium7.2: The position of equilibrium

7.2.1 Deduce the equilibrium constant expression (Kc) from the equation for a homogeneous reaction.

7.2.2 Deduce the extent of a reaction from the magnitude of the equilibrium constant.

7.2.3 Apply Le Chatelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium and on the value of the equilibrium constant.

7.2.4 State and explain the effect of a catalyst on an equilibrium reaction.

7.2.5 Apply the concepts of kinetics and equilibrium to industrial processes.

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7.2.1Deduce the equilibrium constant expression (Kc) from the

equation for a homogeneous reaction.

Equilibrium Constant (Kc)

[ ] means concentration expressed in mol dm-3

When a system reaches equilibrium, the [reactants] stays the same and the [products] stays the same. There is a mathematical relationship between the [rcts] and [prod].

aA + bB cC + dD

Kc = [C]c x [D]d

[A]a x [B]b

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Write the equilibrium constant expression for the following:

Contact Process (manufacture of sulfuric acid)2SO2(g) + O2(g) 2SO3(g)

Kc = [SO3]2

[SO2]2 x [O2]

A homogeneous reaction is one in which all the reactants and products are in the same phase. We can write an equilibrium expression if the substances are all gases, all liquids or all in aqueous solution.

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Write the equilibrium constant expression for the following:

Haber Process (manufacture of ammonia)3H2(g) + N2(g) 2NH3(g)

The dissociation of hydrogen iodide2HI(g) H2(g) + I2(g)

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7.2.2Deduce the extent of a reaction from the magnitude of the

equilibrium constant.The equilibrium constant is a measure of the amount of products at equilibrium compared with the amount of

reactants.

a)More products than reactants at equilibrium. The reaction goes almost to completion.

Kc >>1

H2(g) + I2(g) 2HI(g) Kc = 794

ProductsReactants

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reactants.

b)More reactants than products at equilibrium. The reaction hardly proceeds.

Kc <<1

N2(g) + O2(g) 2NO(g) Kc = 1 x 10-30

ProductsReactants

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reactants.

c) Reactants and products present in somewhat equal amounts.Kc ≈1

C2H6O + C2H4O2 C4H8O2 + H2O Kc = 4

ProductsReactants

7.2.3 Apply Le Chatelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of

equilibrium and on the value of the equilibrium constant.

Henri-Louis Le Chatelier (1850-1936)

French industrial chemist

If a system at equilibrium is disturbed by a change in temperature, pressure, or the concentration of one of the components, the

system will shift its equilibrium position so as to counteract the effect of

the disturbance.

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7.2.3 Apply Le Chatelier’s principle to predict the qualitative effects of changes of temperature,

pressure and concentration on the position of equilibrium and on the value of the equilibrium

constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

Effects of Concentration Change

• If a chemical system is at equilibrium and we add a substance (either a reactant or a product) the reaction will shift to reestablish equilibrium by consuming part of the added substance. Removal of a substance will result in a shift that forms more of the substance.

• The value of Kc does not change (think paper clips)

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7.2.3 Apply Le Chatelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of

equilibrium and on the value of the equilibrium constant.

Added H2

• Reaction will shift to use up some of the added H2.

• Forward reaction temporarily speeds up.

• N2 used up as it reacts with some of the extra H2.

• More NH3 is being produced.

• Eventually a new equilibrium is reached.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92.0

kJ

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constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

What effect will removing NH3 have on the equilibrium?

• System will shift to make more NH3 so it will temporarily speed up to the right.

• Some N2 & H2 will react to produce more NH3.

• At the new equilibrium there will be less N2, less H2, and less NH3 than the original equilibrium.

• The value of Kc remains the same.

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constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

What effect will adding NH3 have on the equilibrium?

What effect will removing N2 have on the equilibrium?

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constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

Effects of Pressure/Volume Change

• If a chemical system is at equilibrium and we increase the pressure (reduce the volume), the reaction will shift toward the side having the fewest moles of gas. Decreasing the pressure (increasing the volume) causes a shift in the direction that produces more gas molecules.

• Only affects systems containing gas molecules.• The value of Kc does not change.

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constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

What effect does increasing the pressure have on the equilibrium?

• Reaction will shift toward the side with the fewest gas molecules.

• The left side has 4 gas molecules (1N2 & 3H2). The right side has 2 gas molecules. Reaction will shift to the right.

• N2 & H2 will react and more NH3 will be produced.

• Kc does not change

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constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

What effect does decreasing the pressure have on the equilibrium?

If a reaction has equal numbers of gas molecules on the left and on the right, changing the pressure has no effect.

2HI(g) H2(g) + I2(g)

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constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

Effects of Temperature Change

• Increasing temperature causes the equilibrium position to shift in the direction that absorbs heat (endothermic).

• Decreasing temperature causes the equilibrium position to shift in the direction that produces heat (exothermic).

• The value of Kc will change with a change in temp. If the reaction shifts right, the value of Kc increases. If the reaction shifts left, the Kc value decreases.

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constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

What effect does increasing the temperature have on the equilibrium?

• Increasing the temperature causes the reaction to shift to use up some of the added heat (endothermic rx).

• The reaction as written is exothermic so the endothermic rx is from right to left. The rx will shift left.

• [N2] increases, [H2] increases, [NH3] decreases.

• Kc value will decrease

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constant.

N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

What effect does decreasing the temperature have on the equilibrium?

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constant.

N2O4(g) 2NO2(g) ΔHo = 58.0 kJ

Both gases are present in a flask at equilibrium. N2O4 is a colorless gas while NO2 is brown.

1)What color will the contents of the flask be if the pressure is increased? Explain.

2)State and explain three (3) ways the amount of NO2 production can be increased.

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7.2.4 State and explain the effect of a catalyst on an equilibrium reaction.

1) A catalyst lowers the activation energy barrier for both the forward and the reverse reactions.

2) Therefore a catalyst increase the rates of both reactions by the same factor.

3) A catalyst increases the rate at which equilibrium is achieved, but does not change the final composition of the substances.

4) The Kc value is not affected by the presence of a catalyst.

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N2(g) + 3H2(g) 2NH3(g) ΔHo = -92 kJ

The Haber process to manufacture ammonia

• Ammonia is an important starting point for the production of fertilizers, nitric acid, explosives and polymers (nylon).

• Under what conditions can will an industrial chemist run this reaction to increase the yield of ammonia?

• An optimum temperature must be found• Read pg. 133-134

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2SO2(g) + O2(g) 2SO3(g) ΔHo = -192 kJ

The contact process to manufacture sulfuric acid

SO3(g) + H2O(l) H2SO4(l)

• Sulfuric acid is used in many chemical processes• Under what conditions can an industrial chemist run this reaction to

increase the yield of sulfur trioxide?• An optimum temperature must be found.• Read pg. 134

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11 IB Topic 7: Equilibrium 7.1: Dynamic equilibrium 7.1.1Outline the characteristics of chemical and...

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