R2.3 How far?

• Physical equilibrium is the process of physical change in which the forward reaction and reverse reaction have the same rate
• Static equilibrium is when reaction has reached equilibrium at rest and completion of reaction (ex. acid + base reaction)
• Dynamic equilibrium is when both the forward and reverse reaction are still occurring but now at constant, identical rates
• All dynamic equilibriums need to occur in closed systems
• At physical equilibrium, macroscopic properties (like color) remain unchanged and constant
• Reactant and product concentrations remain constant during forward and reverse reaction upon reaching equilibrium. Note this DOES NOT mean that reactant and product concentrations are identical to each other they are respectively constant.
• Concentration of product gathered triggers the reverse reaction, keeping the system at equilibrium as products are being used as they are made and vise versa for the reactants
• Kc is the equilibrium constant- representing the extent of a reaction, meaning the composition of reactants and products in the reaction mixture at equilibrium. Valid at the same state, at the same temperature
• In the equation:

  aA+bB --> cC+dD

  Kc = ([C]^c * [D]^d)/([A]^a * [B]^b)
• If Kc=1 then there is an equal amount of products and reactants present at equilibrium
• As Kc becomes bigger, the equilibrium "lies to the right" meaning it is product-favored

  
  

  Kc>1 = higher concentration of products

  Kc>>1 = almost exclusively products and reactions goes to completion
• As Kc becomes smaller, the equilibrium "lies to the left" meaning it is reactant-favored

  Kc<1 = higher concentration of reactants

  Kc<<1 = almost exclusively reactants and forward reaction barely proceeds
• What is Le Châtelier's Principle?

  A system at equilibrium reacts to a change in ways it minimizes its effect
• If reactant is added or product is removed, the reaction shifts towards the right to reach equilibrium as more products are formed with greater concentrations of reactants creating more products
• If products is added or reactants are removed, the the reaction shifts towards the left to reach equilibrium as more reactants are formed with greater concentrations of products decomposing to create more reactants
• Only in gases does pressure change affect reaching equilibrium. When pressure increases, the reaction shifts towards the side with less gas moles to cancel out the pressure's effect as the reaction moves towards the side of the reaction with the least amount of increase in space occupancy and collisions with an increase in pressure. If pressure decreases, the opposite happens
• Temperature is the only change that can affect Kc value and as heat is added, it moves towards the endothermic side of the reaction so as to give more heat to the system so that more heat can be released and the reactions can balance out. If heat is removed, the reaction will proceed in the exothermic direction
• Temperature affects Kc because it changes the heat of the reaction and scales it up or down and the position at which the reaction is at the most stable temperature (Kc) has shifted.
• Pressure and concentration do not affect equilibrium as it simply triggers the reaction to react to the change in a way that cancels its affect and allows it to reach its prior equilibrium again
• Catalyst have no effect on position of equilibrium but increase rate of reaction
• The reaction quotient (Q) is found using the same formula as Kc yet the concentration values are not those found at equilibrium, but at any point of the reaction.
• Comparing Q and Kc helps us find the direction of equilibrium. When Q > K, the equilibrium will be to the left and favoring reactants (as then, denominator is higher); when Q < K, the equilibrium will be to the right and favoring products (as then, numerator is higher); and when Q=Kc, the reaction is at equilibrium
• The first way to find equilibrium concentrations is with all initial concentrations and one equilibrium concentration. It's difference from the initial concentration is carried over to the other concentrations (note the coefficient- if the change is 0.30 for a coefficient of 2, the change will be 0.15 for a coefficient of 1 and 0.60 for a coefficient of 3). The difference is subtracted from the reactants, and added to the products.

  Can be represented in an ICE chart (initial, change, equilibrium) KEY: RED = GIVEN

  
• If initial concentration of a molecule is not given, it is assumed to be 0 mol dm-3
• Another way to find the equilibrium concentrations is from all the initial concentrations and the value of Kc. 'X' can be used to represent change and the values can then be plugged into Kc's equation and solved algebraically.

  REMINDERS:

  1. When Kc is very small, initial concentration of reactants = equilibrium concentration of reactants

  
• REMINDER: Consider reaction vessel in noting down concentration values in calculating the composition of the equilibrium mixture:

  ex. 2 mol + 4 mol --> 8 mol + 6 mol in 2 dm3 vessel => 1 mol + 2 mol --> 4 mol + 6 mol in 1 dm3 vessel

  Essential they need to be divided by the amount needed for the vessel to = 1 dm3
• When Gibbs energy is at its minimum and entropy is at its maximum is when equilibrium is reached
• As spontaneity increases, equilibrium is product favored and lies to the right
• As spontaneity decreases, equilibrium is reactant favored and lies to the left
• ΔG=-RTln(Kc) where R is the gas constant (8.31 J K-1 mol -1) and T is the temperature in Kelvin
• Kc=e ^ -(ΔG/RT) where R is the gas constant (8.31 J K-1 mol-1), T is temperature in Kelvin, and ΔG must be expressed in Joules
• MEMORIZE this as gibbs energy graphs!

  
• Arrhenius' factor tells us temperature and the use of a catalyst affect the rate constant K
• A generalization is that a 10 degrees Celsius increase in a reaction causes its rate to double
• Logarithmic scales are used to group together very distant values into one scale
• Arrhenius' Equation helps mathematically evaluate the relation between temperature and rate of reaction as temperature does not change concentration but only the value of Kc
• When activation energy is higher (endo, usually), temperature increases have a greater effect on the reaction than when activation energy is lower (exo, usually)
• lnK=(-Ea/RT)+lnA is the logarithmic expression of Arrhenius' equation
• ln (k1/k2)=(Ea/RT)(1/T2-1/T1) --> Most applicable Arrhenius' equation
• Arrhenius' equation is often used to relate to a geometric requirement for the reaction