CHEM 122 Lecture 13: Chapter 13 (in-class notes)

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aquamarineox534

8 Mar 2017

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Chapter 13: chemical equilibrium: the extent of chemical reactions. Equilibrium is a state in which there are no observable changes in reactant or product concentration after a certain amount of time. Aa (aq) + bb (aq) cc (aq) + dd (aq) Aa (g) + bb (g) cc (g) + dd (g) Aa (g) + bb (aq) cc (aq) + dd (g) c. Solids and liquids are not included in the equilibrium expression. Kc and kp are related via the ideal gas law. Change in n = moles of gas (prod. 3: calculate equilibrium concentrations from initial concentrations. , reaction favors products, equilibrium lies to the right. , reaction favors reactants, equilibrium lies to the left. Consider a system that may not yet be at equilibrium. Aa + bb cc + dd c. If qc = kc, system is at equilibrium. 3: calculate equilibrium concentrations from initial concentrations when given kc.

Related Questions

Â± Relating Different Forms of the Equilibrium Constant

For chemical reactions where all reactants and products are in the gas phase the amount of each gas in the vessel can be expressed either as partial pressures or as concentrations. As such the equilibrium constant for a gas phase reaction can also be expressed in terms of concentrations or pressures. For the general reaction,

aA(g)+bB(g)âcC(g)+dD(g)

Kp=(PC)c(PD)d(PA)a(PB)b and Kc=[C]c[D]d[A]a[B]b.

It is possible to interconvert between Kp and Kc using

Kp=Kc(RT)În

where R=0.08314 L bar molâ1 Kâ1 and În is the difference in stoichiometric coefficients between gaseous products and gaseous reactants.

If we assume that the gaseous reactant and products are âidealâ and the partial pressures of the reactants and products were expressed in bar, then the thermodynamic equilibrium constant, K is the magnitude (or numerical value) of Kp.

Part A

For the reaction

2CH4(g)âC2H2(g)+3H2(g)

Kc=0.160 mol2 Lâ2 at 1621 âC. What is Kp for the reaction at this temperature?

Enter your answer numerically.

Part B

For the reaction

N2(g)+3H2(g)â2NH3(g)

Kp=3.80Ã10â3 barâ2 at 265 âC. What is Kc for the reaction at this temperature?

Enter your answer numerically.

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 =