Chemical Equilibrium

Rachel Won

Period: 2

4/13/09

<http://www.fordhamprep.org/gcurran/sho/sho/images/pics/chem.jpg>

Overview

Vocabulary that relates to Equilibrium Concept of Equilibrium Scientists and their accomplishment Theory Sample Problems

Glossary Chemical equilibrium- a state of dynamic

balance in which the rate of formation of the products of a reaction from the reactants equals the rate of formation of the reactants from the products.

Equilibrium-constant expression- the expression that describes the relationship among the concentrations of the substances present in a system at equilibrium.

Glossary Homogeneous equilibrium- The

equilibrium established between reactants and product substances that are all in the same phase.

Heterogeneous equilibrium- The equilibrium established between substances in two or more different phases

Glossary Reaction quotient- The value that is

obtained when concentrations of reactants and products are inserted into the equilibrium expression.

Le Chatelier's Principle- Principle that states if a system at equilibrium is disturbed, the equilibrium will shift to minimize the disturbing influence

Concept of Equilibrium

Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.

(3)

Both the forward and reverse reactions occur as a system approaches equilibrium.

They are occurring at the same rate.

<http://www.history.com/encyclopedia/fwne/images/ChemicalReactionC3.gif>

Once equilibrium is established, the concentration of reactant and product do not change.

We indicate the reaction proceeds in both the forward and reverse direction

Example)

N2O4 (g) 2 NO2 (g)

The Equilibrium Constant

The Equilibrium condition can be reached from either direction.

Mass action expression-simple relationship among the concentrations of the reactants and products for any chemical system at equilibrium.

Forward reaction:N2O4 (g) 2 NO2 (g)

Rate law:Rate = kf [N2O4]

Reverse reaction:2 NO2 (g) N2O4 (g)

Rate law:

Rate = kr [NO2]2

aA + bB cC + dD

Kc = [C]c[D]d

[A]a[B]b

a,b,c and d are their coefficients in the balanced chemical equation.

The equilibrium condition is expressed by the equation above.

Sample Problem 1

Write the equilibrium expression for Kc

For the following reactions.

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

Answer: Kc = [HI]^2 / [H2][I2]

It has to be from of a quotient. The Numerator contains the concentrations on the product side and denominator contains the concentration on the reactant side

Equilibrium constants in terms of pressure

Kp =(PC)c (PD)d

(PA)a (PB)b

Kp=Kc are only the same when the same number of moles of gas appear on both sides of the balanced chemical equation.

The magnitude of Equilibrium constants

If K>>1 : Equilibrium lies to the right

If K <<1 : Equilibrium lies to the left

image

(3)

The direction of the chemical equation and K The Equilibrium- constant expression

for a reaction written in one direction is the reciprocal of the one for the reaction written in the reverse direction.

Heterogeneous Equilibria

If a pure solid or a pure liquid is involved in a heterogeneous Equilibrium, its concentration is not involved in the equilibrium- constant expression.

They are not appeared in the equilibrium constant expression, but they participate in the reaction for an equilibrium to be established.

Sample Problem

Write the equilibrium constant expression for Kc

3 Fe (s) + 4 H2O (g) Fe3O4 (s)+ 4H2 (g)

The answer:

Kc = [ H2]^4 / [ H2O]^4

Solid does not included in the expression.

Calculating equilibrium constants

The equilibrium- constant expression can be used to calculate value of the equilibrium constant

Sample Problem

Ex) Nitryl Chloride, NO2Cl, is in equilibrium with NO2 and Cl2

2NO2Cl(g) 2NO2(g)+Cl2(g)

[NO2Cl]=0.00106M, [NO2]=0.0108M and [Cl2]=0.00538M

Calculate Kc

Explanation

First, we calculate equilibrium constant expression

Kc = [NO2 ]^2[Cl2] / [NO2Cl]^2

Second, plug the given numbers

[0.0108]^2[0.00538] / [ 0.001061 M]

The answer is 0.558

When we don’t know the equilibrium concentration of all chemical species?!!!!

USE ICE BOXXXXX!!!!!!!!!!!!!!!!!!!!

Initial

Change

Equilibrium

Hear are the steps

1. Tabulate the initial and equilibrium concentration of all the species in the equilibrium equation.

2. Calculate the change in concentration of substance

3. Calculate the equilibrium concentration, using the initial concentrations and the changes

Sample Problem

Sulfur trioxide decompose at higher temperature in a sealed container :

2SO3(g) 2SO2(g)+O2(g)Identify the vessel is charged at 1000K with SO3 (g) at concentration of 6.09 X 10-3M. At equilibrium the SO3 concentration is 2.44 X 10-3M

Calculate Kc at 1000K

Explanation

Initial 6.09 X 10-3 0 0

Change -.00365 +.00365 + .001825

Equilibrium

2.44 X 10-3 + .00365 +. 00182

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

[+.00365]^2[+.00182] / [2.44 X 10-3]^2

The answer is 4.08 x 10-3

Reaction Quotient If the system at equilibrium, Q = K If Q and K are not at equilibrium Q< K, move toward equilibrium by more

products ( move to the right) Q > K the reaction will proceed to right

to left Q is found by substituting reactant and

product concentration into the equilibrium-constant expression.

Sample Problem

H2(g) + I2 2HI (g)

Predict how the reaction will proceed if we start with 2.0 x 10-2 mol of HI, 1.0 x 10 –2 mol of H2, and 3.0 x 10-2 mol of I2 in a 2.0 container.

Explanation

1. Find the concentration of Each substances

[HI] = 2.0 x 10-2 mol / 2.0 L = 1.0 x 10-2M

[H2]= 1.0 x 10-2mol / 2.0 L = 5.0 x 10-3M

[I2] = 3.0 x 10-2 mol / 2.0 L = 1.5 x 10-2M

Q = [HI]^2 / [H2][I2] = 1.3

Q< K so, it will proceed from left to right

Le Chatelier’s Principle

- French industrial chemistHe stated that if a system at equilibrium

will shift to minimize the disturbing influence.

If a reaction or product is added to a system at equilibrium, the equilibrium will shift to consume the added substance

When More of a Reactant Is Added to a System?

(3)

The Effect of Changes in Pressure

(3)

The Effect of Changes in Temperature

(3)

The enthalpy change for a reaction indicates how an increase in temperature affects the equilibrium

Endothermic reaction- increase in temperature, shifts the equilibrium to the right.

Exothermic reaction-Shifts the equilibrium to the left

Catalysts increase the rate of both the forward and reverse reactions.

(3)

Catalysts affect the speed at which equilibrium is reached but do not affect K

Equilibrium is achieved faster, but the equilibrium composition remains unaltered.

(3)

Work Cited Page

1.<Theodore, Brown L, and Bursten E Bruce. Chemistry. New Jersey: n.p., 2000>

2.<http://www.fordhamprep.org/gcurran/sho/sho/images/pics/chem.jpg>

3. <Chapter 15 Chemical Equilibrium .18 April.2009 <http://www.pjmcelligottcom.com/chapter15brown>.

4.<http://www.history.com/encyclopedia/fwne/images/ChemicalReactionC3.gif>