Scientific Tutor

How do you form an equilibrium equation from a chemical equation?

Now let us see how we apply this equilibrium equation to a chemical equation. It is usually best to see examples first and explain after.

 

Group 1 Examples:  Set up the equation for the equilibrium constant.  VIDEO Setting up the Equation for the Equilibrium Constant Examples 1.

N2(g) + 3 H2(g) <—-> 2 NH3(g)	K =	[ NH3 ]2
		[ N2 ] [ H2 ]3
1	1	1
2 H2O(g) <—-> 2 H2(g) + O2(g)	K =	[ H2 ]2 [ O2 ]
		[ H2O ]2

Lets analyze these group 1 examples above. So just like we talked about before, the products of the chemical equation become the top (numerator) of the equilibrium equation. The reactants of the chemical equation become the bottom (denominator) of the equilibrium equation. However, something new appears. What are the exponents (powers) that we see in the equilibrium equation? Where do they come from? Look at the first chemical equation in the example. What is the coefficient of NH_3(g)? What is the exponent of NH₃ in the equilibrium equation? Notice they are the same. That is because the coefficient from the chemical equation becomes the exponent in the equilibrium equation. Note if the coefficient is 1 then no exponent is shown in the equation. That is because anything to the first power is just itself.

 

How do the states of matter affect the equilibrium equation?

Unfortunately, there is one more possible trick that teachers or a test can throw at you. The equilibrium equation not only depends on the chemical equation but also the states of matter of each molecule in the chemical equation. For all things having to do with equilibrium only gas and aqueous count. Solids and liquids do not count for anything having to do with equilibrium. This link explains why solids and liquids don’t count. How does it work in the equilibrium equation? It means those things that are solid or liquids are completely removed from the equation. Notice all examples I have used so far have only had gases in them. The examples below have solids or liquids in them that are eliminated from the equilibrium equation.

 

Group 2 Examples:  Set up the equation for the equilibrium constant.  VIDEO Setting up the Equation for the Equilibrium Constant Examples 2.

 

C_4(s) + 4 O_2(g) <—-> 4 CO_2(g)

K =	[ CO2 ]4
	[ O2 ]4

 

 

2 Fe³⁺_(aq) + 3 CO₃^2-_(aq) <—-> Fe₂(CO₃)_3(s)

K =	1
	[ Fe3+ ]2 [ CO32- ]3

 

 

2 H₂O_(l) <—-> 2 H_2(g) + O_2(g)

K =	[ H2 ]2 [ O2 ]
	1

 

 

VIDEO Setting up Equilibrium Equations Demonstrated Example 1: What does the equation for the equilibrium constant look like?

CH_4(g) + 2 O_2(g) <—> CO_2(g) + 2 H₂O_(g)

 

What are the products?

Answer: CO₂ and H₂O

 

What is the coefficient and therefore exponent of CO₂?

Answer: 1

 

What is the coefficient and therefore exponent of H₂O?

Answer: 2

 

What does the top (numerator) of the equation look like?

Answer: [ CO₂ ] [ H₂O ]²

 

What are the reactants?

Answer: CH₄ and O₂

 

What is the coefficient and therefore exponent of CH₄?

Answer: 1

 

What is the coefficient and therefore exponent of O₂?

Answer: 2

 

What does the bottom (denominator) of the equation look like?

Answer: [ CH₄ ] [ O₂ ]²

 

What does the entire equilibrium equation look like?

COMPLETE ANSWER:

K =	[ CO2 ] [ H2O]2
	[ CH4 ] [ O2 ]2

 

 

 

PRACTICE PROBLEMS: What does the equation for the equilibrium constant look like?

 

MgBr_2(aq) + 2 NaI_(aq) <——> MgI_2(aq) + 2 NaBr_(aq)

K =	[ MgI2 ] [ NaBr ]2
	[ MgBr2 ] [ NaI ]2

 

2 C₄H_10(l) + 13 O_2(g) <—-> 8 CO_2(g) + 10 H₂O_(g)

K =	[ CO2 ]8 [ H2O ]10
	[ O2 ]13

 

6 C_4(s) + 12 O_2(g) + 24 H_2(g) <—-> 4 C₆H₁₂O_6(s)

K =	1
	[ O2 ]12 [ H2 ]24

 

6 Ag⁺_(s) + Ca₃(PO₄)_2(s) <—-> 3 Ca²⁺_(aq) + 2 Ag₃PO_4(s)

K =	[ Ca ]3
	1