Equilibrium

A chemical equilibrium is when there is a chemical reaction and its back reaction reacts at the same rate. There is a balance between the forward reaction and the back reaction. It usually takes place in a closed container to prevent other substances to enter or escape. You know there is an equilibrium when there isn't a tendency for the quantities of reactants or products to change.

The chemical equations can be written forward and backwards, because both reactions are happening simultaneously.

For example:

H 2 + I 2 → 2 HI "synthesis of hydrogen iodide"

2 HI → H 2 + I 2 "dissociation of hydrogen iodide"

La Chatelier's Principle:

If the conditions of a system at equilibrium are changed, the system moves in such a way as to oppose the effect of that change.

In the chemical equation, each compound or element has a corresponding variable.

A + B C + D

To find the constant, "k", you plug in the values of the variable in this formula:

Kc = [C] x [D] / [A] x [B]

The values you plug in are the concentrations of each one. When the equation is balanced, if there are more than one of that compound, you need to put that value of the concentration to that power. For example, if there are 3 H's, then the concentration of H will be to the 3rd power.

La Chatelier's Principle helps determine which way the equilibrium will shift given special circumstances.

If there is a change in pressure, it will only affect the equilibriums that contains gases. If there are an uneven distribution in the number of moles on the different sides of the equation, then there will be a shift in the equilibrium. It shifts to the side with less gas molecules.

A change in concentration is also a stress on the equilibrium, and will cause it to shift. If there in an increase in the concentration of "A", the equilibrium will shift to the right. This is because A and B are used up faster, and will produce more product. There will be a lower concentration of "B" to reduce the stress due to the higher concentration of "A". Even though changes in concentrations cause the equilibrium to shift, it has no effect on the constant, "k".

Reversible reactions are exothermic in one direction and endothermic in the other. The addition of heat shifts the equilibrium so that heat is absorbed. This favors the endothermic reaction but the removal of heat favors the exothermic reaction. Since the rates of the opposing reactions of unequal, it affects the equilibrium constant, "k".

In order to solve for equilibrium constants, or to do any equilibrium calculations, you need to know how to set up an equilibrium constant expression. To do this you must start with the balanced equations, with all states of matter labeled:
 * Setting up Equilibrium constant expressions/Equilibrium Calculations:**

aA + bB ↔ cC + dD

in which the lowercase letters are the mole ratios, and the uppercase are the concentrations (measured in molarity) or pressure of the Molecule in that place of the reaction.

From this expression, to form an equilibrium constant expression, you change aA + bB ↔  cC + dD into

=
Equilibrium constant = K c =[C]^c [D]^d / [A]^a [B]^b ======

CH 4 + 2 O 2 ↔ CO 2 + 2H 2 O
Becomes, Kc = [CO 2 ][H 2 O] ² / [CH 4 ][O 2 ] ²

This equation equals three different equilibrium constants:

Kc = which is the general equilibrium constant, used with molarity as the measurement of concentration

Ksp = which is the constant for a solubility product, used with molarity as the measurement of concentration

Kp = which the constant when using pressure instead of concentration, when all reactants and products are gases.

From here, you can do any equilibrium calculation and find any equilibrium constant. In order to solve for the __**equilibrium constant**__, "k", two pieces of information must be made available.


 * The balanced equation for the reaction, with the phyiscal states (solid, liquid, gas, or aqueous) clearly labeled. This is because solids and pure liquids are exempt from equilibrium constant equations.
 * The concentrations of each of the species in the equation, or enough information to find them. These values will be plugged into the equilibrium constant expression.

Once you have these two things, you can get started: If the given equilibrium calculation is in this form,

aA + bB ↔ cC + dD

whereas the lowercase letters stand for the balanced equation's mole ratios, then the equilibrium constant equation of the forward reaction will be in this form:

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

assuming none of the species' physical states are not solids or pure liquids. The reverse reaction is therefore in this form:

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

For Activity 1, we will use the equation below.

SiH4(g) + 2 O2(g) ↔ SiO2(g) + 2 H2O(g)

**Activity 1**

1. Set up the equilibrium constant equation.

Kc = [SiO2] x [H2O]² / [SiH4] x [O2]²

2. Plug in the given values. The given values are:

[SiH4]=0.45M; [O2]=0.25M; [SiO2]=0.15M; and [H2O]=0.10M

Kc = (0.15) x (0.10)² / (0.45) x (0.25)²

Kc = 0.053

This is the equilibrium constant of the forward reaction. Now you can do it yourself! =)


 * 4. What is the equilibrium constant for the reverse reaction?**

In order to solve for __**equilibrium concentrations**__, the exact opposite needs to be done. The following information is needed:


 * The equilibrium constant "k"
 * The balanced equation for the reaction, with the phyiscal states (solid, liquid, gas, or aqueous) clearly labeled.

Once you have both these things, you can get started:

To find the equilibrium concentrations in a reaction where a precipitate forms, we use the Solubility Product Constant "Ksp".

Using the equation

aA(s) ↔ cC + dD

the solubility product constant will be in this form

Ksp = [C]^c x [D]^d

Then, based on the mole ratios of the balanced equation, we can plug the values in of the concentrations in the form of "x". For example, if the products of the equation have a one-to-one ratio, then the solubility product constant will be

Ksp = (x) x (x)

Therefore, Ksp = x

If a two-to-one ratio:

Ksp = (x) x (2x)²

Therefore, Ksp = 4x³

For Activity 2, we will use the equation below:

CaCl2(s) ↔ Ca+2(aq) + 2 Cl-(aq)

**Activity 2**

1. Set up the solubility product constant equation

Ksp = [Ca+2] x [Cl-]²

2. Plug in the given Ksp value. Ksp = 6.0 x 10^-37

6.0 x 10^-37 = [Ca+2] x [Cl-]²

3. Identify the mole ratios of the products and plug in the concentration values in the form of "x".

6.0 x 10^-37 = (x) x (2x)² 6.0 x 10^-37 = 4x³


 * 4. Solve for "x", and you have the concentration of Ca+2. Doubling this result will give you the concentration of Cl-. Now you can find equilibrium concentrations!** =)

An ICE Table is an organizational tool used in the stoichiometry of equilibrium problems. ICE stands for initial, change, equilibrium.
 * ICE Tables**

When solving an ICE problem the reactants/products go on the top. For this reason, the ICE Table is sometimes referred to as the RICE table where the R stands for Reaction.

The equation must be balanced before one can use the ICE table and ICE Problems require values to be in the form of molar concentration. If your values are in moles you can divide them by 1.00 Liter to convert the moles to molars.

Take, for example, the following problem:   At 35 degrees Celsius, 2.0 mol of pure NOCl(g) is introduced into a 2.0 L flask. The NOCl(g) partially decomposes according to the following equilibrium equation: NOCl(g) → NO(g) + Cl2(g)

At equilibrium, the concentration of NO(g) is 0.032 mol/L. Use an ICE table to determine equilibrium concentrations of NOCL(g) and Cl2(g) at this temperature. <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">

To find the changes in concentration given the concentration at Equilibrium: 1. ****<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Figure out the balanced chemical equation for the chemical equation and set up an ICE Table using your given information. ** <span style="font-size: 12pt; color: black; line-height: 115%; font-family: 'Times New Roman','serif';"> <span style="font-size: 12pt; color: black; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">2NOCl(g) → 2NO(g) + Cl2
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; msofareastfontfamily: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; msolist: Ignore;">

<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> Change = Equilibrium - initial ** <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"><span style="color: #ff0000; font-family: 'Times New Roman', Times, serif;"> Change in NO=.032-0.0=.032 3. ****<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Using stoichiometry calculate the change in concentration for the other reactants/products in the reaction. ** **<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Change= (Change found in Step#2) * mole ratio ** <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> Change in NOCL = .032 * 1/1 =.032M Change in Cl2 = .032 * 1/2= .016M
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">2 NOCl || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">2 NO || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">Cl2 ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">Initial || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">2.0 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">0.0 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">0.0 M ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">Change || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Equilibrium <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">.032 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; msofareastfontfamily: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; msolist: Ignore;">2. ****<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Calculate the change in concentration among the reactants with a given equilibrium concentration. **
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; msofareastfontfamily: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; msolist: Ignore;">

<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> Now that we have calculated the values for the change in concentration, we might as well finish out the table by finding the remaining equilibrium concentrations.
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">2 NOCl || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">2 NO || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">Cl2 ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">Initial || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">2.0 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">0.0 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">0.0M ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">Change || .032 M || .032 M || .016 M ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Equilibrium <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">.032 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;"> ||

Note that the change in concentration of the products/reactants is determined as positive or negative depending on what side of the equation the reactions starts on.

Ex: If the reaction starts on the reactants side, then the change in the concentration of the reactants will be negative because reactants will be lost over the course of the reaction. Conversely, the product side will be positive because you’ll be gaining product as your losing reactants. This is because the reactants are going towards the creation of the product.

Equilibrium= initial + change ** <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> Equilibrium of NOCl= 2.0 + (- .032) =1.968 M Equilibrium of Cl2 = 0.0 + .016= .016 M
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; msofareastfontfamily: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; msolist: Ignore;">4. ****<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Calculate the equilibrium concentrations. **
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">

Now consider the following problem:

<span style="font-size: 120%; font-family: 'Times New Roman', Times, serif;">In a 1.00 L vessel, 0.200 mol of nitrogen gas and 0.200 mol of oxygen gas were reacted at 773 K to produce nitrogen monoxide gas. The percent reaction was found to be 12.5%. <span style="font-size: 120%; font-family: 'Times New Roman', Times, serif;">Calculate all three of the equilibrium concentration s. <span style="font-size: 120%; font-family: 'Times New Roman', Times, serif;"> <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> <span style="font-size: 120%; font-family: 'Times New Roman', Times, serif;">N2(g)+ O2(g) <span style="font-size: 12pt; color: black; line-height: 115%; font-family: 'Times New Roman','serif'; mso-ansi-language: ES;">→ 2NO(g) <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> Percent reaction is the ratio of the amount of reactant that reacts to the initial amount of reactant in the system. This value is given as a percent.
 * 1.Write the equation and set up the ICE table**
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">N2 || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">O2 || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2NO ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Initial || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2.00 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2.00M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">0.00M ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Change || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Equilibrium || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> ||

% reaction= amount of substance reacted / initial amount of substance 2. ****<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Using the % reaction calculate the change in concentration for the other products/ reactants. ** Change= initial * % reaction ** <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> Change in N2= 2.00 * (12.5/100) = .025 M
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; msofareastfontfamily: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; msolist: Ignore;">
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">

Change= (Change found in Step#2) * mole ratio ** <span style="display: block; font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; text-align: center;"> <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Change in O2= .025 * (1/1) = .025 M Change in NO= .025 * (2/1) = .050 M
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; msofareastfontfamily: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; msolist: Ignore;">3. ****<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Find the change in concentration for the other products/reactants. **
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">

<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> Equilibrium= initial + change ** <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> Equilibrium of N2 = 2.00 + (-.025) = 1.75 M Equilibrium of O2 = 2.00 + (-.025) = 1.75 M Equilibrium of 2NO = 0.00 + .050 = .050 M <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> And that's all there is to ICE tables! If you have any other questions, the video below should be helpful. media type="custom" key="3982361"
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">N2 || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">O2 || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2NO ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Initial || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2.00 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2.00 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">0.00 M ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Change || <span style="font-size: 12pt; color: #ff0000; line-height: 115%; font-family: 'Times New Roman','serif';">.025 M || <span style="font-size: 12pt; color: #ff0000; line-height: 115%; font-family: 'Times New Roman','serif';">.025 M || <span style="font-size: 12pt; color: #ff0000; line-height: 115%; font-family: 'Times New Roman', Times, serif;">.050 M ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Equilibrium || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif'; msofareastfontfamily: 'Times New Roman'; mso-fareast-font-family: 'Times New Roman'; msolist: Ignore;">4. ****<span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Find the equilibrium concentrations. **
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';"> || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">N2 || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">O2 || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2NO ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Initial || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2.00 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">2.00 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">0.00 M ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Change || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">.025 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">.025 M || <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">.050 M ||
 * <span style="font-size: 12pt; line-height: 115%; font-family: 'Times New Roman','serif';">Equilibrium || <span style="font-size: 12pt; color: #ff0000; line-height: 115%; font-family: 'Times New Roman', Times, serif;">1<span style="font-size: 12pt; color: #ff0000; line-height: 115%; font-family: 'Times New Roman', Times, serif;">.75 M || <span style="font-size: 12pt; color: #ff0000; line-height: 115%; font-family: 'Times New Roman','serif';">1.75 M || <span style="font-size: 12pt; color: #ff0000; line-height: 115%; font-family: 'Times New Roman', Times, serif;">.050 M ||