Equilibrium

Reversible Reactions Many reactions are reversible. Many reactions are reversible. Once product is formed, it can go back and reform reactants. Once product is formed, it can go back and reform reactants. (2:45 min)

Equilibrium A state of balance exists between the forward and reverse reactions. A state of balance exists between the forward and reverse reactions. Rate of forward rxn = Rate of reverse rxn

Equilibrium Concentration of reactants and products become constant at equilibrium. Concentration of reactants and products become constant at equilibrium. This does not mean the amount or quantities are equal to each other. This does not mean the amount or quantities are equal to each other. Concentrations constant, not equal Concentrations constant, not equal Con Con, Requal!!! Con Con, Requal!!!

Equilbrium Must have a closed system. Must have a closed system. This is so no reactants or products (especially gases) can escape. This is so no reactants or products (especially gases) can escape. If they did you could not reach equilibrium If they did you could not reach equilibrium The reaction will instead “go to completion” in one direction The reaction will instead “go to completion” in one direction (1:25 min)

Equilibrium Dynamic Process: the reactions (forward/reverse) have not stopped. Dynamic Process: the reactions (forward/reverse) have not stopped. You can no longer see changes in materials because the forward/reverse rxns happening at same rate. You can no longer see changes in materials because the forward/reverse rxns happening at same rate. (1:47 min) good (1:47 min) good

Parking Lot Equilibrium Interactive: Parking Lot Equilibrium Interactive: hill.com/sites/ /student_view0/cha pter14/animations_center.html# hill.com/sites/ /student_view0/cha pter14/animations_center.html# hill.com/sites/ /student_view0/cha pter14/animations_center.html# hill.com/sites/ /student_view0/cha pter14/animations_center.html#

Types of Equilibrium Phase Equilibrium: Gas/liquid: Rate of vaporization = rate of condensation Ex: H 2 O (l) ↔ H 2 O (g) Results in constant amount of vapor at a specific temperature we measure as “equilibrium vapor pressure”

Phase Equilibrium: Solid/liquid: Rate of melting (fusion) = rate of solidification Ex: H 2 O (s) ↔ H 2 O (l)

Types of Equilibrium Solution Equilibrium: Rate of dissolving = rate of crystallization Ex: NaCl (s) ↔ NaCl (aq) Ex: NaCl (s) ↔ NaCl (aq) Occurs in saturated solutions Occurs in saturated solutions Solution holds maximum solute it can at that temp. Solution holds maximum solute it can at that temp. Even if you add more solute the conc. will not change Even if you add more solute the conc. will not change

Types of Equilibrium Chemical Equilibrium: Happens during chemical reactions Forward/reverse reactions occur at same rate. Forward/reverse reactions occur at same rate. Concentrations become constant Concentrations become constant No longer see observable changes No longer see observable changes Ex: N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g)

Going to Completion Reactions will not reach equilbrium if: You remove product preventing reverse reaction If system left open and gas leaves If an insoluble product forms (ppt) If water is formed

Helpful Videos Equilibrium Overview (Good Review) (18min) Equilibrium Overview (Good Review) (18min)

Practice Questions

Which balanced equation represents a phase equilibrium? (1) H 2 (g) + I 2 (g) ↔ 2HI(g) (2) 2NO 2 (g) ↔ N 2 O 4 (g) (3) Cl 2 (g) ↔ Cl 2 (l) (4) 3O 2 (g) ↔ 2O 3 (g)

Which statement about a system at equilibrium is true? (1) The forward reaction rate is less than the reverse reaction rate. (2) The forward reaction rate is greater than the reverse reaction rate. (3) The forward reaction rate is equal to the reverse reaction rate. (4) The forward reaction rate stops and the reverse reaction rate continues.

A chemical reaction is at equilibrium. Compared to the rate of the forward reaction, the rate of the reverse reaction is (1) faster and more reactant is produced (2) faster and more product is produced (3) the same and the reaction has stopped (4) the same and the reaction continues in both directions

Which quantities must be equal for a chemical reaction at equilibrium? (1) the activation energies of the forward and reverse reactions (2) the rates of the forward and reverse reactions (3) the concentrations of the reactants and products (4) the potential energies of the reactants and products

Given the equation representing a phase change at equilibrium: H 2 O(s) ↔ H 2 O(l) Which statement describes this equilibrium? (1) The H 2 O (s) melts faster than the H 2 O(l) freezes. (2) The H 2 O (l) freezes faster than the H 2 O (s) melts. (3) The mass of H 2 O(s) must equal the mass of H 2 O(l). (4) The mass of H 2 O (l) and the mass of H 2 O (s) each remain constant.