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Chapter 16 Notes
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Chapter 16: Chemical Equilibrium

 

16-1 The Concept of Equilibrium

 

-         Equilibrium- exists when two opposing processes occur at the same rate

o       Example of condensation / vaporization vapor pressure equilibrium in a closed container

o       Saturation is another example of equilibrium solute particle leave the solid equals the rate at which the solute particles leave the liquid

 

Reversible Reactions

-         Not all reactions go to completion- in other words not all of one of the reactants needs to be used up

-         Some reactions do not go to completion because these reactions are said to be “reversible”- the products undergo a reaction that reforms the reactants.

-         Theoretically all reactions are reversible- we may just not have the means available.

-         Reactions which are reversible are written with a double half arrow.

 

Chemical Equilibria

-         Different reactions occur at different rates (speeds)

o       Rusting takes many years, while dynamite explosion takes a fraction of a second

o       The rate depends on:

         Concentration

         Temperature

         Pressure (for gases)

o       Rates tend to be faster when concentration is increased

-         In reactions- when the reaction begins, the concentration of reactants decreases and the concentration of products increases.

o       Since most reactions are reversible- there is a rate at which the forward reaction occurs and a rate at which the reverse reaction occurs

o       Since the concentration changes as the reaction occurs- the rate of the forward and reverse reaction changes as well

o       At some point- the rate of the forward reaction will equal the rate of the reverse reaction- known as chemical equilibrium

-         Chemical equilibrium is the state in which the concentrations of reactans and products remain constant with time because the rate at which they are formed in each reaction equals the rate at which they are consumed in the opposite reaction.

o       Any chemical reaction that takes place in a closed container will reach equilibrium

-         Brackets [  ] are used to denote concentration of substances

-         Reaching equlibrium does not mean that the reaction has stopped- just that the same number of reactants are turning to product as product is turning to reactant.

 

16-2 The Law of Chemical Equilibrium

-         A reversible reaction achieves a state of equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction.

-         The rate of the reaction depends on the concentration

 

The Equilibrium Constant

-         1864 Cato Guldberg and Peter Waage formulated a general expression describing the equilibrium condition

o       Law of mass action- the relative concentrations of reactants and products at equilibrium is a constant- the equilibrium constant

 

aA  + bB cC + dD

 

a, b, c, d are the coefficients for substance A, B, C, D

 

o       The equilibrium expression for this equation is:

 

[C]c[D]d

Keq =             _______

 

            [A]a[B]b

 

-         Keq is the equilibrium constant

o       It is the ratio of products concentrations (raised to powers indicated by the coefficients) to the reactant concentrations (raised to powers indicated by the coefficients).

o       Products are in the numerator, reactants are in the denominator

-         The value is constant for a given reaction regardless of the initial concentration- at a given temperature

-         Law of Chemical Equilibrium- every reversible reaction proceeds to an equilibrium stat that has a specific ratio of the concentration of reactants and products

-         Keq is an experimentally determined value

o       Keq is determined by inserting the equilibrium concentrations for the substances into the equilibrium expression (not the initial concentrations)

o       Concentration is given as molarity.

o       Equilibrium positions- concentrations of reactants and products at equilibrium

         Depends on the initial concentration- will reach a constant ratio

         Does not effect the equilibrium constant

-         The equilibrium constant does not describe the rate of reaction, but it does tell you information about a mixture at a set of conditions.

o       Measures the extent to which a reaction goes to completion

o       If Keq >>1 – the concentration of products at equilibrium is much greater than the concentration of reactants at equilibrium

o       If Keq <<1 – the concentration of reactants at equilibrium is much greater than the concentration of products at equilibrium

o       If Keq 1 – considerable concentrations of products and reactants are present.

 

Homogeneous and Heterogeneous Equilibria

-         Homogeneous Equilibria- equilibrium condition where all reactants and products are in the same state

-         Heterogeneous Equilibria- equilibrium conditions where substances are in different physical states.

o       Concentration of solid substances are given by the density divided by the molar mass

o       Density of solids and liquids changes little with changes in temperature, so the concentration varies little during the course of the reaction

o       For this reason, the concentrations of solids and liquids are left out of the equilibrium expression

 

The Reaction Quotient

-         When reactants are mixed, it is tough sometimes to tell if the reaction has reached equilibrium

o       If not at equilibrium, it will be useful to know in which direction the concentration will shift to reach equilibrium

o       Q- the reaction quotient- is used to determine if a reaction is at equlibrium.

         Calculate the same as the equilibrium constant, but non-equilibrium values are inserted into the equation

         The Keq known from previous experiments

o       Once the reaction quotient is determined, it can be compared to the Keq to see if the reaction has reached equilibrium.

         If Q is less than Keq – the denominator is too large, and the numerator is too small not enough products, so the reaction will proceed to the right

         If Q is greater than Keq – the denominator is too small, and the numerator is too large too much product, so the reaction will proceed to the left

         If Q = Keq the reaction is at equilibrium, no shift will occur.

 

16-3 Le Chatelier’s Principle

 

-         Chemical reactions at equilibrium can be disturbed and shifted away from equilibrium- by changing one of the following: pressure, temperature, or concentrations

-         These changes can be predicted- as determined by Henri Louis Le Chatelier

o       If a change in conditions is imposed on a system at equilibrium, the equilibrium position will shift in the direction that tends to reduce that change in conditions.

o       A reaction will shift in the forward or reverse direction to “undo” or compensate for the altering factor

 

Changes in Concentration

-         If more of a particular substance is added to a reaction at equilibrium, the concentration of that substance will increase. 

o       The reaction will then consume some of the added substance to return to the proper ratio of equilibrium.

-         If some substance is removed, the concentration of that substance will decrease.

o       The reaction will then return to equilibrium by producing more of that substance

-         The only value that changes is the equilibrium position not the equilibrium constant

 

Changes in Pressure

-         For some gases- changes in pressure will change the equilibrium position

-         If the total pressure of a system is increased, the system will shift to reduce that pressure by shifting to produce fewer molecules

o       Pressure can be increased by decreasing the volume- would cause the reaction to proceed toward the direction with fewer molecules

 

Effects of Changing Temperature

-         The equilibrium constant is dependent on temperature.

o       Some reactions proceed toward completion if heat is added

o       Some reaction proceed toward completion if heat is removed

-         Exothermic reactions with Le Chatelier’s principle

o       If heat is added, the reaction reestablishes equilibrium by consuming that additional heat through the reverse reaction (endothermic)

         Heat is a component of the reaction.

o       Unlike changing the pressure or concentration, changing the temperature will change the equilibrium constant

 

The Haber Process

-         Usable nitrogen products are not very abundant- since the nitrogen – nitrogen triple bond is tough to break.

-         Used to use nitrogen containing compounds as the source of nitrogen

o       Fritz Haber- wanted to find useable nitrogen to make explosives

         Looked at the reaction between nitrogen and hydrogen to make ammonia

         Tends to reach equilibrium before much ammonia was produced

o       Found that under the right temperature and pressure, large quantities of ammonia could be produced

         Found if you remove product- more ammonia is continually made

         If you increase pressure- more ammonia is made (2 molecules on the right vs. 4 molecules on the left)

         Exothermic reaction- so in theory decrease temperature would increase ammonia production- but in this case, the reaction slowed considerably actually increased temperature which lowered product level, so increased pressure even greater.

o       Today the process is used for explosive production, fertilizers, and household cleaners.

-         Plants also fix nitrogen to usable forms- called nitrogen fixation