Equilibrium constant expressions are the ratios of products to reactants in a chemical reaction. These are determined by how the chemicals involved in the reaction interact with each other.
The equilibrium constant expression for a given reaction is written as follows: Kc= [products]/[reactants], where Kc is the equilibrium constant, [products] is the number of products present at equilibrium, and [reactants] is the number of reactants present at equilibrium.
There are three main types of equilbrium constants: concentration, thermodynamic, and kinetic. Concentration constants relate to how much of a substance there is at equilibrium; thermodynamic constants relate to internal forces within a reaction; and kinetic constants relate to how quickly a reaction occurs.
This article will focus on identifying the proper form of the concentration-based equilibrium-constant expression for an equation.
Identify the reacting species
Before you can find the equilibrium constant expression, you need to identify the reacting species. For this reaction, the reacting species are nitrogen gas (N2) and oxygen gas (O2).
The way you determine which species reacts is by assigning which molecule is in excess. In this case, oxygen is in excess, so it reacts with nitrogen to form nitride oxide.
Sometimes it can be tricky to determine which molecules are reacting species. If you are unsure, look up information on how to identify them!
Equilibrium-constant expressions are not only used for gases, but also liquids. You will need to adjust the expression slightly for liquids, however. More details on that below!
The general form of the equilibrium-constant expression for a pure reactant or product is Ka(X=Y) where A=X, B=Y, and K a = [(A)(B)]/[(A)+(B)] = [([A].[B])/(A+B)].
This equation states that at equilibrium concentration of A equals concentration of B.
Example: Finding the Equilibrium Constant Expression for a Pure Reactant or Product.
Assume that Nitrogen Gas (N2) is introduced into an atmosphere containing Oxygen Gas (O2). What is the equilibrium constant expression?
Solution: Since N2 enters the atmosphere already containing O2, it must be in excess. Therefore, we can write an expression using N2 as the [([A].[B])/(A+B)]. This means that at equilibrium there is equal amounts of N2 and O 2 .
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Identify the product species
After identifying the reactants, you must next determine the product species. The product species is what is produced when all of the reactants are combined.
In this equation, the product species is nitrogen (N) and oxygen (O) gas. When the nitric oxide (NO) and oxygen (O2) molecules combine, they form nitrogen (N) and oxygen (O) gas.
The number of molecules of each gas produced is determined by the relative size of each gas’s concentration in relation to the other gases’ concentrations. This is why it is important to identify all of the species in the equation!
Proper identification requires determining how many molecules of each substance are produced per second. This information determines how many molecules are present in a given volume after a certain amount of time.
Assign all coefficients to one side of the equation
Once you have the equation, you must identify the proper form of the equilibrium-constant expression for the equation.
First, you must assign all of the coefficients to one side of the equation. This is done by multiplying each side of the equation by the same coefficient.
For example, if there is a constant k on only one side of the equation, then k must be assigned to only one side of the equation when you multiply it by itself.
Second, you must divide both sides of the equation by 2no2, where n is a constant. This will create an expression with only one non-zero denominator. You can then replace this denominator with a 1 to make it disappear.
The last step is to add and subtract 1 from both sides of the equal sign.
Eliminate all variables by plugging in values
Once you have determined the correct form of the equilibrium-constant expression, you can now eliminate all of the variables in the reaction. To do this, you must plug in values for the concentrations of all of the reactants and products.
For example, if the concentration of nitrogen (N2) is 1 M, then you would need to plug in 1 M for N2 in the equilibrium-constant expression. You would then need to solve for the remaining variable (oxygen).
By doing this, you have eliminated all variables and are left with only one chemical equation: N2(g) + O2(g) ⇌ 2NO(g).
Write the equilibrium-constant expression
When writing the equilibrium-constant expression for the reaction of nitrogen (N2) and oxygen (o2) to form nitric oxide (NO), make sure to identify the correct radical for each atom.
The nitrogen atom radical is N+, and the oxygen atom radical is O-. These must be included in the expression, as they dictate how many molecules of each gas are needed to create a neutral solution of nitric oxide.
The ratio of these atoms in the reaction determines how many molecules of nitric oxide are formed. The N2 and O2 must fully react to form NO, so they cannot be either left over or insufficient for forming NO.
The equation is also written as N2(g)+O2(g)⇌2NO(g), which shows that there must be two molecules of nitric oxide produced per molecule of nitrogen and oxygen used in the reaction.
Make sure that all sides of the equation are equal by using algebraic manipulation
Once you have the equation set up, you will need to make sure that both sides of the equation are equal. To do this, you will need to manipulate the equation using algebra.
For example, if you had the following equation: 2N2(g)+3O2(g)⇌6NO(g)
And wanted to make sure that both sides of the equation were equal, you would first divide both sides by 2N2(g) and then subtract 3O2(g) from both sides. You would then take the O2(g) out of the second side and put it over on the other side. Then, you would take 6NO(g) and put it on the other side.
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