CHMB16H3 Chapter Notes - Chapter 6: Enthalpy, Gas Constant, Sodium Hydroxide

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30 Oct 2011

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The equilibrium constant: a reaction is favored whenever k > 1, when you evaluate an equilibrium constant, If the direction of a reaction is reversed, the new value of k is simply the reciprocal of the original value of k; k". If two reactions are added, the new k is the product of the two individual values; k3 = If n reactions are added, the overall equilibrium constant is the product of n individual equilibrium constants. Entropy (s): a measure of the substance"s disorder: s is positive, products are more disordered than reactants, s is negative, products are less disordered than reactants. o. J/k mol) and t is temperature: the more negative the g , the larger the equilibrium constant. If q > k, the reaction must go to the left to decrease the numerator and increase the denominator until q = k. If a reaction is at equilibrium and products are added, the reaction goes to the left.

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ENT ives To determine equilibrium concentrations for species in solution . Examine temperature effects on reaction equilibrium Investigate the shift in equilibrium from different stresses on a system kills . Use colorimeter to determine unknown solution concentrations ced .Preparing temperature baths to change reaction conditions tionEquilibrium is defined as a state in which the concentrations of both the reactants and products have no tendency to change with time. We can restate this by saying that the rate of the forward reaction is equal to the rate of the reverse reaction. Recall that with rate we are simply defining how fast a reaction is proceeding, a speed at which the reaction proceeds. Take note that when writing an equilibrium expression, pure liquids (denoted by l or 2) and pure solids (denoted by s) are omitted. Typically we express the equilibrium by K or Kp where c and p denote either concentration or partial pressure respectively. We refer to this value of K, or Kp as an equilibrium constant For any reaction, Kc can be written as the product of the concentration of products, raised to their stoichiometric coefficients, divided by the product of the concentration of the reactants raised to their stoichiometric coefficients. See Equation (5-1) below for an example: aA(aq) + bB(aq-cC(aq) + dD(aq) (5-1) A] [B]b As mentioned before, K, or K, are equilibrium constants meaning that they will have the same value unless temperature is changed. This brings in two important ideas 1) We can perturb the equilibrium concentrations of a reaction, say [A] for instance which will cause the other concentrations to shift so that K for the reaction remains constant. 2) If we change temperature, we will either increase or decrease our value of K depending on the specific reaction we are examining. Determining K, and Equilibrium Concentrations During lecture we often discuss the idea of using ICE tables as a method for organizing and calculating concentrations of reactions as they proceed towards equilibrium. In a typical ICE table you need to know initial concentrations and the magnitude of K to determine the equilibrium concentrations. Alternatively, if you know the equilibrium concentrations for a reaction you can calculate K. For this experiment we will be examining a complex ion formation, when iron(III) ion

Â± Calculating Equilibrium Constants
The equilibrium constant, K, of a reaction at a particular temperature is determined by the concentrations or pressures of the reactants and products at equilibrium.

For a gaseous reaction with the general form

aA+bBâcC+dD

the Kc and Kp expressions are given by

Kc=[C]c[D]d[A]a[B]b

Kp=(PC)c(PD)d(PA)a(PB)b

The subscript c or p indicates whether K is expressed in terms of concentrations or pressures. Equilibrium-constant expressions do not include a term for any pure solids or liquids that may be involved since their composition does not change throughout the reaction. The standard state of a pure substance is the pure substance itself, and although the quantity may change the sample remain pure. The concentration is effectively equal to 1, and will not impact the magnitude of K.
Part A
Phosgene (carbonyl chloride), COCl2, is an extremely toxic gas that is used in manufacturing certain dyes and plastics. Phosgene can be produced by reacting carbon monoxide and chlorine gas at high temperatures:

CO(g)+Cl2(g)âCOCl2(g)

Carbon monoxide and chlorine gas are allowed to react in a sealed vessel at 456 âC . At equilibrium, the concentrations were measured and the following results obtained:

Gas Partial Pressure
(atm)
CO 0.900
Cl2 1.18
COCl2 0.250

What is the equilibrium constant, Kp, of this reaction?
Express your answer numerically.

Kp =
0.235

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Deriving concentrations from data
In Part A, you were given the equilibrium pressures, which could be plugged directly into the formula for K. In Part B however, you will be given initial concentrations and only one equilibrium concentration. You must use this data to find all three equilibrium concentrations before you can apply the formula for K.
Part B
The following reaction was performed in a sealed vessel at 774 âC :

H2(g)+I2(g)â2HI(g)

Initially, only H2 and I2 were present at concentrations of [H2]=3.10M and[I2]=2.10M. The equilibrium concentration of I2 is 0.0600 M . What is the equilibrium constant, Kc, for the reaction at this temperature?
Express your answer numerically.

Kc =

CHMB16H3 Chapter Notes - Chapter 6: Enthalpy, Gas Constant, Sodium Hydroxide

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