1. Reversible Reactions and Equilibrium
Section 19.2
Reversible Reactions and Equilibrium

2. Reversible Reactions
Has both a forward and a reverse reaction (at the same time)
Reactants → Products
Reactants ← Products
Indicated by a double arrow
Ex: 2 SO2 + O2 ↔ 2SO3
There is no net change: the reaction reaches chemical equilibrium
Always the same rate of change backwards and forwards
Equilibrium position is based on concentration of reactants and products
Indicates whether reactants or products are favored at equilibrium
Does not mean 50:50 reactants and products

3. Reversible Reactions
Even though the rates for the forwards and reverse reactions are equal, the concentrations of components on each side may not be equal.
An Equilibrium position may be shown:
A B or A B
1% % % %
It depends on which side is favorable

4. Equilibrium Constants (Keq)
Keq is a ratio of product concentration to reactants concentration at equilibrium. Each constant is raised to the power of its coefficient in the equation.
aA + bB cC + dD
𝐾𝑒𝑞= 𝐶 𝑐 𝑥 𝐷 𝑑 𝐴 𝑎 𝑥 𝐵 𝑏
*If Keq > 0, products are favored
*If Keq < 0, reactants are favored

5. Ex 1: Dinitrogen tetraoxide and nitrogen dioxide exist in equilibrium
Ex 1: Dinitrogen tetraoxide and nitrogen dioxide exist in equilibrium. N2O NO2 A liter of the gas mixture at 10oC and equilibrium contains mol N2O4 and mol NO2. What is the equilibrium constant?

6. Ex 2: 1 mol of hydrogen and 1 mol of Iodine are sealed in a lL flask and allowed to react at 450oC. At equilibrium, 1.56 mol HI is present with some reacant gases. Calculate Keq for the reaction H2 + I HI

7. Le Chatelier’s Principle
Henri Le Chatelier, a french chemist, studied how stresses ( or changes) could disrupt the delicate balance of equilibrium.
He found that when systems have been disrupted, they make adjustments to restore equilibrium.
When equilibrium is restored, everything is different (ie: concentration of products and reactants are different)

8. Disruptions/ Stresses of Equilibrium
Concentration
Adding or removing reactants or products disrupts equilibrium.
Ex: H2CO CO2 + H2O
21% %
If you add CO2, rxn will shift to the left and more H2CO3 is produced.
If you remove some product, rxn will shift towards the products.
This causes a void on the product side, so the reactants work to make up the product.

9. Disruptions/ Stresses of Equilibrium
Temperature
Increase in temperature causes equilibrium to shift in the direction that absorbs heat.
When heat is Reactant:
↑ heat = ↑ in products
↓ heat = ↑ in reactants
When heat is a Product:
↑ heat= ↑ in reactants
↓ heat = ↑ in products

10. Disruptions/ Stresses in Equilibrium
Pressure
A change in the pressure will change the number of moles of reactants or products.
↑ Pressure= formation of more product
A smaller space = more collisions
↓ Pressure = formation of more reactants