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Equilibrium: Calculating Equilibrium Constant,K: (Knowing Equilibrium Info)
Procedures for Calculating any Equilibrium Constant Problem
Step 1 : Write down balanced chemical reaction
Step 2 : Determine and write down the Equilibrium Constant Expression (K=[products]E/[reactant]E) where E = Equilibrium conditions.
Step 3: Plug in and do math to solve for unknown. (3 Levels of Difficulty)
Level 1 : Simplest Difficulty
- Problems gives all equilibrium concentration and you are solving for K (would be nice to figure out units also). See example below.
- Problem gives all equilibrium concentrations except one and gives value of K (you need to determine units of K) and solving for last equilibrium concentration., ([Example of Work for Simple Equilibrium Problems]. Remember, you need to manipulate the equation, in this case, equilibrium constant expression into a form with your unknown on one side of equal sign and all other variable on other side of equal sign.
Level 2 : ICE Box without "x" (have one piece of information at equilibrium)
Level 3 : ICE Box with "x" (will not do in 1st year, but will in AP Chem)
- Sometimes they give all equilibrium concentrations and look for Equilibrium constant (K).
- Sometime they give K and you are looking for one of the species' equilibrium concentration (they give all other species' equilibrium concentrations)
- Sometimes they give concentration of one reactant and/or product at equilibrium and all other species' initial concentrations and you are calculating K.
- If this occurs, all you Wizards of Stoichiometry need to set-up ICE Box table.
- i. Change mole and container of initial conditions into concentration (Molarity, M).
- ii. Convert one piece of equilibrium to mole or M reacted.
- iii. Take M reacted and use stoichiometry to determine amount of all other species reacted or producted in rxn
- iv. Use math on table to determine equilibrium concentration of all species.
Example 1
Calculate the Equilibrium constant for the following reversible reaction @25o:
2 NO(g) + 1 O2 (g) <==> 2 NO2 (g)
where the equilibrium concentration of NO,O2,and NO2 are 2.00M,4.00M,and 3.00M, respectively.
Answer:
Step 1: Balanced chemical reaction. See question
Step 2: Write down Equilibrium constant expression:
K= {NO2 (g)}2 / {O2 (g)}1{NO(g)}2
But write it like this :
Step 3: plug in known information into Equilibrium constant expression directly since you know all equilibrium concentration.
K = (3.00M)2 / {(2.00M)2)(4.00M)1}
K=0.5625 = 0.563 (for K, we don't usually have units)