Equilibrium

Equilibrium Basic Meaning

Balance between things

Types of Equilibrium

• Thermal Equilibrium — Heat flow from hot to cool, until temperature is equal

• Chemical Equilibrium — Balance between reactants and products in a chemical reaction

• Mechanical Equilibrium — An object which is in a state of no movement (no movement caused by itself)

• Physical Equilibrium — Balance between states of matter

Thermal Equilibrium

Heat flow from hot to cool, until temperature is equal

Mechanical Equilibrium

An object which is in a state of no movement (no movement caused by itself)

Physical Equilibrium

Balance between states of matter

Two Reaction Types

• Reversible — products can reform reactants

• Irreversible — Product’s cannot reform reactants

Reversible

• A reaction where the products can reform into the reactants

  • Double sided

Irreversible

• A reaction where the Product’s cannot reform reactants

  • Single sided

Types of Systems

• Open — Particles can move & Gain energy with/from surroundings

• Closed — Particles confined to container, no outside movement of particles, but energy transfer still possible

Open System

• Particles can move & Gain energy with/from surroundings

Closed System

• The Particles are confined to the container, cannot leave container

• Energy transfer still possible

• Closed System allows for reaching of Equilibrium

  • Prevents leaving of matter, keeps set concentration

Chemical Equilibrium

• Balance between reactants and products in a chemical reaction

2 Requirements:

• Rates of forward & reverse reactions are equal

• Concentrations of reactants & products remain constant

• It is Dynamic (Always occuring)

The Law of Mass Action

• once at equilibrium, the rate of the reaction in either direction is dependent only on the “active masses” of the reactants and products. 

• Rate (Forward) = Rate (Reverse)

• Leads to Equilibrium Constant

Equilibrium Constant (K_C)

• Only compounds in the gaseous (g) and aqueous (aq) states are allowed.

• All Values must be equilibrium values.

• The value of K depends on the particular reaction AND on the temperature.

• Concentration or Partial pressures

If K_C>1

• more products than reactants at EQ.

• Formation of Products is favored at Equilibrium

If K_C<1

• more reactants than products at EQ.

• Formation of Products is not favored

Uses of Chemical Equilibrium Equation

Calculate the Value of K

• If you have the equilibrium concentration or pressure values for the reaction components, you can calculate the K value

Determine if the Reaction is at Equilibrium

• If the ratio of products to reactants equals the K_eq value for a given reaction, then you know the system is at equilibrium

Find Unknown Equilibrium Concentrations

• If some concentrations of the reaction components are known at equilibrium, the the K value can be used to calculate an unknown.

Reaction Quotient (Q)

• Ratio of Products to Reactants

  • NOT ALWAYS AT EQULIBRIUM

• Same equation

When Q<K

• Less Products than at equilibrium

• Will favor forward reaction

When Q_c = K_c

• Then System is at equilibrium

When Q_c > K_c

• System has more products than equilibrium

• Will favor reverse reaction

Rule of Multipole Equilibria

• “If a reaction can be expressed as the sum of two or more reactions, the K for the overall reaction is the product of the K values for the individual reactions”

• Simply: You can manipulate elementary steps to get overall reactions. What ever you manipulate must be done to the K_C values, as well. Then you multiple the K_c values for steps, to get K_c for final

What happens to K_c when equation is flipped?

• K_c becomes reciprocal (K_c^-1)

What happens to K_c when equation multiplied?

• K goes to power of coefficient (nK_c)

5% rule

• For RICE tables

  • used when solving for x becomes polynomial

• Can make assumption that X is so small it does not need to be calculated

  • Remove it when added to another value NOT ALONE

• After solving, check X is less than 5% of Initial value (All moles)

• If not valid, solve polynomial

• Always add both initial concentrations (if 2)

Le Chateleir’s Principle

• when a reversible reaction at equilibrium is ‘stressed’ by a change in reaction conditions, the reaction direction must adjust to relieve the stress and bring the system back to equilibrium.

  • Simply: When something done to one side, the reaction must rebalance itself to re-reach new equilibrium

• Depends on equation for Q

  • How will equation change, directly afterwards

3 methods of Stress:

• Changes in Concentrations

• Changes in Temp

• Changes in Volume

Le Chateleir’s: Adding substance

• Will go in opposite direction of substance

  • + reactant → increase product

  • + product → increase reactant

Le Chateleir’s: Removing substance

• Will go in direction of substance

  • - reactant → decrease product

  • - product → decrease reactant

Le Chateleir’s: Effect of Temperature

• Temp = Energy

• Energy treated like addition/subtraction of substance

• ONLY METHOD TOP CHANGE K_C VALUE OF REACTION

How to change K_c value of an expression

• CAN ONLY BE DONE BY CHANGE IN TEMPERATURE

Le Chateleir’s: Effects of Volume

• Only affects gasses

• Based on Number of Moles on each side of reaction

• Decrease in Volume → Reaction favors direction with fewer moles

• Increase in Volume → Reaction favors direction with more moles

• Equal moles → No change in equilibrium, stays same

Effect of Catalyst on system at Equilbrium

• Has no Effect

Solubility Product Constant(K_sp)

• Measures how much a solid dissolves into ions at equilibrium

• Same equation as K_c

• Only for insoluble salts

• When Writing chemical Equation, Precipitate always Reactant

• Lower K_sp means less soluble

  • Vice Versa

Ion Solubility Product Q_sp

• Equivalent of the reaction Quotient (Q_c)

If Q_sp < K_sp

• Unsaturated

• Will not form precipitate

If Q_sp=K_sp

• Saturated

• Right amount of Ions present

if Q_sp >K_sp

• Supersaturated

• Precipitate Forms

Common Ion Effect

• Increasing the concentration of one of the ions by adding a second solution puts stress on the equilibrium.

Selective Precipitation

• chemical technique that separates ions in a solution by adding a reagent that causes one or more of the ions to precipitate. The remaining ions stay in solution. 

• Use specific other chemicals to form a precipitate → can analyze precipitate to see unknown elements

Vapor Pressure at Equilibrium

• Only affected by temperature