Title: Lesson 4 The Equilibrium Law

Learning Objectives:

• Know how to quantify Kc using equilibrium concentrations

• Know how to determine Kc when given initial concentrations using the ICE method

• Know how to determine equilibrium concentrations using Kc

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The equilibrium law

So far we know how to derive the equilibrium constant Kc and considered what the value tells us about the composition of an equilibrium mixture.

Now we will quantify these values...

We can use the equilibrium law to quantify equilibrium constant or the concentrations of reactants and products in an equilibrium.

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Quantifying concentrations of reactants or products from data on initial concentrations of other components

Step 1: Write the balanced equation.

Step 2: Under the equation, write the values of each component using three rows: initial, change, and equilibrium.

Step 3: Write the expression for Kc from the balanced equation. Substitute values for equilibrium concentration and calculate Kc.

Initial – Change – Equilibrium = ICE method

ASG has a dance, and lets 100 boy-girl couples into the gym. Throughout the evening some couples have fights and break apart, forming single boys and single girls. Of course some of these singles form new couples. At the end of the evening there are 12 single girls. Calculate the equilibrium numbers.

1 Couple 1 girl + 1 boy

Initial 100 0 0

Let’s start with a silly, non-chem example…

ASG has a dance, and lets 100 couples into the gym. Throughout the evening some couples have fights and break apart, forming single boys and single girls. Of course some of these singles form new couples. At the end of the evening there are 12 single girls. Calculate the equilibrium numbers.

1 Couple 1 girl + 1 boy

Initial 100 0 0

1212Equilibrium

ASG has a dance, and lets 100 couples into the gym. Throughout the evening some couples have fights and break apart, forming single boys and single girls. Of course some of these singles form new couples. At the end of the evening there are 12 single girls. Calculate the equilibrium numbers.

1 Couple 1 girl + 1 boy

Initial 100 0 0

Change -12 +12 +12

121288Equilibrium

ASG has a dance, and lets 100 couples into the gym. Throughout the evening some couples have fights and break apart, forming single boys and single girls. Of course some of these singles form new couples. At the end of the evening there are 12 single girls. Calculate the equilibrium numbers.

Equilibrium 88 12 12

1 Couple 1 girl + 1 boy

88

)12)(12(

)(

))((

couples

boysgirlsk = 1.64

Example:

4 moles of H2 gas and 6 moles of Cl2 gas are pumped into a 2 liter tank at 30C. At some time later, it is found that there are 2 moles of HCl gas in the tank. Calculate the Equilibrium Constant.

H2 + Cl2 2HCl

Example:

4 moles of H2 gas and 6 moles of Cl2 gas are pumped into a 2 liter tank at 30C. At some time later, it is found that there are 2 moles of HCl gas in the tank. Calculate the Equilibrium Constant.

Initial Concentration

H2 + Cl2 2HCl

liter

mol

2

4

liter

mol

2

60

Example:

4 moles of H2 gas and 6 moles of Cl2 gas are pumped into a 2 liter tank at 30C. At some time later, it is found that there are 2 moles of HCl gas in the tank. Calculate the Equilibrium Constant.

Initial Concentration

H2 + Cl2 2HCl

liter

mol

2

60[2]

Example:

4 moles of H2 gas and 6 moles of Cl2 gas are pumped into a 2 liter tank at 30C. At some time later, it is found that there are 2 moles of HCl gas in the tank. Calculate the Equilibrium Constant.

Initial Concentration

H2 + Cl2 2HCl

Change

Equilibrium Conc.

0[2] [3]

liter

mol

2

2

Example:

4 moles of H2 gas and 6 moles of Cl2 gas are pumped into a 2 liter tank at 30C. At some time later, it is found that there are 2 moles of HCl gas in the tank. Calculate the Equilibrium Constant.

Initial Concentration

H2 + Cl2 2HCl

Change

Equilibrium Conc.

0[2] [3]

[1]

+1- ½ - ½

[1.5] [2.5]

]][[][

22

2

ClHHCl

Kc ]5.2][5.1[

]0.1[ 2

= 0.267 0.3

The changes that occur must be in the same ratio as the co-efficients in the balanced equation

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Calculating equilibrium concentrations from the equilibrium constant

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More complex when you’re only given Kc and initial concentrations...

Continued on next slide...

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Calculating equilibrium concentrations when Kc is very small... In some reactions Kc is very small, less than 10-3. This represents a reaction in which the forward reaction has

hardly proceeded.

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Solutions

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