AS/A-Level Chemistry - Intro to equilibrium constant/reaction quotient

 Remember equilibrium in A-Level Chemistry?

When a system is in dynamic equilibrium it has three main features: 

• The rate of the forward reaction equals the rate of the backward reaction. 
• At equilibrium there is no netchange in the concentrations. 
• Equilibrium can only be reached in a closed system.  

The Equilibrium Constant K_c 

Consider this reaction: 

N₂+ 3H₂ ⇌ 2NH₃ 

In this case K_c = [NH₃]² / [N₂] [H₂]³ 

• [N₂], [H₂]& [NH₃] are the concentration of reactants and products at equilibrium.  
• The powers [N₂], [H₂]& [NH₃] are raised to represents the number of moles in the balanced equation.   

K_c is a constant value assuming the temperature stays constant. Its value tells us the relative amounts of products and reactants at equilibrium: 

• A large K_c means the position of equilibrium is to the right. (favours forward reaction and products)
• A small K_c means the position of equilibrium is to the left. (favours backward reaction and reactants)

Therefore, for the hypothetical reaction: 

bB+cC⇌dD+eE

the equilibrium constant expression is written as 

K=[D]^d⋅[E]^e / [B]^b ⋅ [C]^c

Note: solids and pure liquids and solvents have an activity that has a fixed value of 1

Reaction quotient (Q)

Q, is the numerical value of the ratio of products to reactants at any point in the reaction. The reaction quotient is calculated the same way as is K, but is not necessarily equal to K. It is used to determine which way the reaction will proceed at any given point in time.

for bB+cC⇌dD+eE

Q=[D]^d⋅[E]^e / [B]^b ⋅ [C]^c

• If Q>K, then the reactions shifts to the left to reach equilibrium 
• If Q<K, then the reactions shifts to the right to reach equilibrium 
• If Q=K then the reaction is at equilibrium

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Drafted by Eunice (Chemistry)

References:

https://www.google.com/url?sa=i&url=https%3A%2F%2Fin.pinterest.com%2Fpin%2F837669599413238178%2F&psig=AOvVaw19MmQ9bOItBQTb9Z7a52Zu&ust=1625520364202000&source=images&cd=vfe&ved=0CAoQjRxqFwoTCKjv1rKtyvECFQAAAAAdAAAAABAq

https://www.google.com/url?sa=i&url=http%3A%2F%2Fpinkmonkey.com%2Fstudyguides%2Fsubjects%2Fchem%2Fchap11%2Fc1111101.asp&psig=AOvVaw1DLP0IfdPO_iji6BYMaqMP&ust=1625520632687000&source=images&cd=vfe&ved=0CAsQjhxqFwoTCIiR2bWuyvECFQAAAAAdAAAAABAW

https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Equilibria/Chemical_Equilibria/The_Equilibrium_Constant