chapter 19

• What is the equilibrium constant K𝒸 used for?

  It is used to tell us the position of equilibrium in terms of the equilibrium concentrations.

  K𝒸 > 1, position is towards products

  K𝒸 = 1, position is in middle

  K𝒸 < 1, position is towards reactants
• How do you write a K�� expression?

  
• What should you not include in a K𝒸 expression?

  Don't include solids or liquids in K𝒸 expressions, since their concentrations are essentially constant.

  K𝒸 only includes species that are (g) or (aq)
• What is a homogeneous equilibrium?

  Where all species are in the same state or phase.

  
• What is a heterogeneous equilibrium?

  Where the species are in different states or phases.

  
• How do you calculate the units of Kc?

  -Substitute units into the expression for K𝒸.

  - Cancel common units and show the final units on a single line.

  
• [worked example- calculating K𝒸 from equilibrium amounts]

  
• What is Kₚ?

  The equilibrium constant in terms of partial pressures.

  For gases, it is easier to measure pressure than concentration.
• What is a mole fraction?

  The mole fraction of a gas is the same as its proportion by volume to the total volume of gases in a gas mixture. The sum of the mole fractions in a gas mixture must equal one.
• How do you calculate the mole fraction of a gas?

  
• [worked example- calculating mole fractions]

  
• What is partial pressure?

  The contribution the gas makes towards the total pressureP.The sum of the partial pressures of each gas equals the total pressure.
• How do you calculate partial pressure?

  
• [worked example- calculating partial pressures]

  
• How do you write a K

  ₚexpression?Similarly to K𝒸, but with partial pressures replacing concentration terms.

  
• What units are used for K

  ₚ?Usually atm, kPa, or Pa. These are calculated in the same way as with K𝒸.

  
• [worked example- calculating K

  ₚfor a homogeneous equilibrium]

  
• [worked example- calculating K

  ₚfor a heterogeneous equilibrium]

  
• What are the three rules of le Chatelier's principle (in relation to concentration, pressure, and temperature)?

  1- If the concentration of a species is increased, the equilibrium position shifts in the direction that reduces the concentration

  2- If the pressure is increased, the equilibrium position shifts towards the side with fewer gaseous moles

  3- If the temperature is increased, the equilibrium position shifts in the endothermic direction
• What is the effect of temperature on an equilibrium constant K?

  K will change according to the shift in equilibrium as dictated by le Chatelier's principle.(basically it changes exactly as you think it would, just apply le Chatelier's principle and roll with it)

  
• How do changes in concentration and pressure affect equilibrium constants?

  The value of an equilibrium constant K is unaffected by changes in concentration or pressure.
• Why does a change in pressure not affect K

  Increasing pressure causes an increase in concentration of ALL the reactants and products. Initially, Kₚ will either decrease or increase, depending on the consequential effect on the equation. However, eventually Kₚ will return to the original value as the equilibrium returns to normal.
• Why does a change in concentration not affect K𝒸?

  While K𝒸 will initially change according to the change in equilibrium position, eventually the equilibrium, and thus K𝒸, will return to the original value.
• How do catalysts affect equilibrium constants?

  They are unaffected, because:

  -Catalysts affect the rate of both the forward and reverse reactions by the same factor.

  Equilibrium is reached quicker but the equilibrium position is not changed.