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Chemical Equilibrium. Chapter 18. Chemical Equilibrium. Section 18-1 Pp. 589 - 591. Equilibrium is…. Equilibrium is not static Opposing processes occur at the same time and at the same rate Rate of forward reaction = Rate of reverse reaction For example 1: water at 0.0°C

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chemical equilibrium1
Chemical Equilibrium
  • Section 18-1
  • Pp. 589 - 591
equilibrium is
Equilibrium is…
  • Equilibrium is not static
  • Opposing processes occur at the same time and at the same rate
  • Rate of forward reaction = Rate of reverse reaction
  • For example 1: water at 0.0°C
      • At 0.0, some liquid H2O is freezing and some ice (solid) H2O is melting. Their rates are equal, so equilibrium
  • For example 2: adding sugar molecules into water
      • At equilibrium, the rate of sugar molecules in solution crystallizing equals the rate of sugar crystals dissolving
reversible reactions
Reversible Reactions
  • Every reaction can proceed:
      • Reactants  Products OR
      • Reactants  Products
  • Def. – chemrxn in which products can react to re-form reactants
  • 2HCl  H2 (g)+ Cl2 (g)
      • Here most HCl is decomposing into hydrogen gas & chlorine gas
      • However, some H2 and Cl2 are synthesizing into HCl
      • At equilibrium, these rates are equal
conclusion
Conclusion
  • A reversible chemical reaction is in chemical equilibrium when rate of forward reaction = rate of reverse reaction
  • The concentrations of the reactants and products are static
  • [Reactants] DOES NOT = [Products] !!!
  • Usually see double arrows to indicate reversibility:
equilibrium lies to the right
Equilibrium lies to the right?
  • Some reactions “favor” the formation of products
      • At equilibrium: higher concentrations of Products than of Reactants
      • [Products] > [Reactants]
      • Sometimes the forward arrow will be longer than the reverse arrow, to indicate “product favored”
  • Some reactions “favor” the formation of reactants
      • At equilibrium: [Products] < [Reactants]
      • Sometimes the reverse arrow will be longer than the forward arrow, to indicate “reactant favored”
equilibrium constant
Equilibrium Constant
  • Given the general reaction:

nA + mBxC + yD

  • Initially, there is only A & B but no C or D, so the forward reaction rate is at its maximum.
  • Over time, C + D accumulate, so forward rate slows & reverse rxn rate increases
  • Eventually the two rates become equal to each other
    • Equilibrium !!!
equilibrium constant1
Equilibrium Constant
  • After equilibrium is reached, the individual concentrations of A, B, C, and D undergo no further change if conditions remain the same.
    • A ratio of their concentrations should also remain constant.
  • The equilibrium constant is designated by the letter K.
slide10

The constant K is independent of the initial concentrations.

  • Kis dependent on the temperature of the system.

The Equilibrium Constant

  • The numerical value of K for a particular equilibrium system is obtained experimentally.
slide11

If the value of K is small, the reactants are favored.

  • A large value of K indicates that the products are favored.
  • Only the concentrations of substances that can actually change are included in K.
    • Pure solids and liquids are omitted because their concentrations cannot change.
shifting equilibrium
Shifting Equilibrium
  • Section 18.2
  • Pp. 598 - 601
le chatelier s principle
Le Chatelier’s Principle
  • A system is happily at equilibrium, then a change in concentration, pressure, or temperature occurs
  • What will happen to the equilibrium?
  • Def. – if a system in equilibrium is stressed, then the equilibrium is shifted in a way to relieve that stress
  • 3 main Stressors: pressure, concentration, and temperature
pressure change
Pressure Change
  • N2(g) + 3H2(g)  2 NH3(g)
      • What does the (g) stand for?
      • How many moles of gas on the left? On the right?
  • So if there was a pressure INCREASE:
      • Causes an increase in gas concentration
      • Which side will experience the biggest change in gas pressure (concentration)?
      • How would the system relieve the stress?
      • Shifts toward side with fewer moles!!
      • Pressure increase = shift to side with fewer moles
pressure change page 2
Pressure Change (Page 2)
  • What is pressure was DECREASED?
      • Shifted to side with more moles of gas
  • INVERSE relationship between pressure change and side with number of moles
      • INCREASE pressure = shift to side with FEWER moles
      • DECREASE pressure = shift to side with MORE moles
pressure change k eq
Pressure change & Keq
  • Even though changes in pressure may shift the equilibrium position, they do not affect the value of the equilibrium constant.
  • Increasing pressure by adding a gas that is not a reactant or a product cannot affect the equilibrium position of the reaction system.
concentration change
Concentration Change
  • If one side INCREASES concentration, then the system will shift the reaction to the opposite side
  • If one side DECREASES concentration, then the system will shift the reaction to its side
    • If we increase the H2 concentration, what will happen?
      • Will N2 increase or decrease? What about NH3?
    • If we decrease the N2 concentration, what will happen?
      • Will H2 increase or decrease? What about NH3?
concentration k eq
Concentration & Keq
  • Changes in concentration have no effect on the value of the equilibrium constant.
      • Such changes have an equal effect on the numerator and the denominator of the chemical equilibrium expression.
temperature change
Temperature Change
  • Reactions are either exothermic or endothermic
  • If endothermic, then energy is a REACTANT
  • If exothermic, then energy is a PRODUCT
  • So if a reaction is ENDOTHERMIC, then increasing temperature shifts equilibrium to products
      • Increase TEMP  Shift to PRODUCTS
      • Decrease TEMP  Shift to REACTANTS
temperature change1
Temperature Change
  • If a reaction is exothermic, then heat is a PRODUCT
  • In an EXOTHERMIC rxn, when temp is INCREASED, then the rxn shifts to REACTANTS
  • In an EXOTHERMIC rxn, when temp is DECREASED, then the rxn shifts to PRODUCTS
  • The value of the equilibrium constant (Keq) for a given system is affected by the temperature.
haber process
Haber Process
  • Artificial production of ammonia

N2(g) + 3H2(g)  2 NH3(g) +92 kJ

  • In this reaction, we want to produce as maximum amount of NH3 (Ammonia)
  • There are how many moles of reactants? Products?
      • So how do we create the maximum product concentration by changing the pressure?
haber process page 2
Haber Process (Page 2)

N2(g) + 3H2(g)  2 NH3(g) +92 kJ

  • Temperature
      • Is the reaction endothermic or exothermic?
      • Is heat a reactant or product?
      • So what should be done to maximize products?
  • Concentration
      • How would me maximize products by altering concentrations?
last example
Last Example
  • Bicarbonate Buffer in mammalian blood

H2CO3  H+ + HCO3-

  • What happens when pH decreases? What happens to the [H+]? So what happens to the equilibrium?
      • What about the [H2CO3] & [HCO3-]
  • What happens when pH increases? What happens to the [H+]? So what happens to the equilibrium?
      • What about the [H2CO3] & [HCO3-]