49. Reversible Reactions & Dynamic Equilibrium
1. Reversible Reactions
Definition: A reaction where the products can react together to reform the original reactants. It is indicated by a double arrow (⇌).
Forward Reaction: Reactants → Products.
Backward Reaction: Products → Reactants.
Example: Ammonium chloride ⇌ ammonia + hydrogen chloride.
2. Dynamic Equilibrium
Reaching Equilibrium: * At the start, the forward reaction is fast, and the backward reaction is zero.
As products are made, the forward reaction slows down, and the backward reaction speeds up.
Eventually, the rates of the forward and backward reactions become exactly the same.
Key Features:
Both reactions are still happening, but they cancel each other out.
The concentrations of reactants and products remain constant (but not necessarily equal to each other).
It can only be reached in a closed system, meaning a sealed environment where no reactants or products can escape.
3. Position of Equilibrium
The "position" describes the relative amounts of reactants and products at equilibrium.
Equilibrium lies to the right: There are more products than reactants.
Equilibrium lies to the left: There are more reactants than products.
Changing the Position: The position can shift depending on conditions like temperature, pressure, or concentration.
4. Energy Changes
In a reversible reaction, if the forward reaction is endothermic, the backward reaction must be exothermic (and vice versa). The amount of energy transferred is the same in both directions.
Example (Hydrated Copper Sulfate):
Forward (Endothermic): Blue hydrated copper sulfate + heat → White anhydrous copper sulfate + water.
Backward (Exothermic): White anhydrous copper sulfate + water → Blue hydrated copper sulfate + heat.
5. Summary Checklist
Reversible reactions use the ⇌ symbol.
Equilibrium means the rates of the forward and backward reactions are equal.
Equilibrium requires a closed system.
Energy changes are opposite in the forward and backward directions.