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Answers to the Kc questions.

Answer 1. (a)

C2F4 = 0.865 mol   and  HCl = 1.73 mol

(b) Kc = [C2F4] [HCl]2 / [CHClF2 ] 2

(c) Kc = (0.865/23.2) x (1.73/23.2)2 / (0.27/23.2)2  =  1.53 mol dm -3 

(d) Yield would increase
     Equilibrium opposes temperature increase
     Moves in the endothermic direction

(e) Causes ozone depletion/decomposition/damage

Answer 2.

Answer 3. (a)

EQM amount A = 0.25 – 0.015 = 0.235 mol
EQM amount B = 0.25 – (2 x 0.015) = 0.22 mol

(b) 0.13 dm 6 mol -2

(c) [H2O] / conc of water is (effectively) constant
     (because it it so much larger than the other concentrations)

(d) Initial amount ClCH2CHO = 4.71 / 78.5 = 0.06 mol

     EQM amount ClCH2CHO = (0.06 – x) mol
    EQM amount ClCH2CH(OH)2 = x mol

     37(0.06 – x) = x
     2.22 = 38x
     x = 0.058421

     [ClCH2CH(OH)2 ] = 0.058421 / 0.05 = 1.17 moldm -3

Answer 4.

Answer 5. C

Answer 7.
B. 1 and 2 only.

Answer 6.

(a) 1–2 atm pressure, vanadium pentoxide catalyst
(b) Lower temperatures give a better yield but are slower 
     (or converse)
     450°C is a compromise between yield and rate

 

Answer 8.

Answer 10.

Answer 9.

Explanation for operational differences.
Temperature
• Low temperature for maximum yield: (∆H –ve \ exothermic)
• High temperature to increase rate
Pressure
• High pressure for maximum yield (fewer (gaseous) moles/molecules
  of products)
• High pressure expensive to generate
OR high pressure is a safety hazard

(b) (i) Equilibrium (position) shifts to the left (as T is decreased)
         AND (forward) reaction is endothermic

(ii) Student 2 is correct AND
same number of gas particles/ gas(eous) molecules/moles
of gas on each side (of equation)

The equilibrium constant (Kc) is a fundamental concept in chemistry that provides valuable information about the composition of a chemical system at equilibrium. Here’s a simple outline of how Kc is useful:

1. Defining the Equilibrium Constant:

  • The equilibrium constant (Kc) is a ratio of the concentrations of the products to the reactants, each raised to the power of their stoichiometric coefficients, at equilibrium.
  • For a general reaction: aA  +  bB  ⇌  cC  +  dD, the equilibrium constant expression is:
    Kc = [C] c [D] d / [A] a [B] b

2. Predicting the Direction of the Reaction:

  • If Q < Kc: The reaction will proceed in the forward direction (toward products) to reach equilibrium.
  • If Q > Kc : The reaction will proceed in the reverse direction (toward reactants) to reach equilibrium.
  • If Q = Kc : The reaction is already at equilibrium.

3. Calculating Equilibrium Concentrations:

  • Kc allows chemists to calculate the concentrations of reactants and products at equilibrium.
    Given the initial concentrations and the value of Kc, one can set up an ICE (Initial, Change, Equilibrium) table to solve for unknown concentrations.

4. Understanding Reaction Extent:

  • The magnitude of Kc indicates the extent to which a reaction proceeds.
    • Large Kc (≫ 1): Indicates that the reaction heavily favors the formation of products.
    • Small Kc (≪ 1): Indicates that the reaction favours the reactants, with little product formed at equilibrium.

5. Effect of Temperature:

  • The value of Kc is temperature-dependent. By knowing Kc at different temperatures, one can understand how the equilibrium position shifts with temperature changes.
    • For exothermic reactions: Increasing temperature decreases Kc.
    • For endothermic reactions: Increasing temperature increases Kc.

6. Le Chatelier’s Principle:

  • Kc helps in applying Le Chatelier’s Principle to predict how a system at equilibrium responds to changes in concentration, pressure, or temperature.
    This aids in optimising conditions for desired outcomes in industrial and laboratory settings.

7. Gibbs Free Energy and Kc:

  • There is a relationship between the Gibbs free energy change
    (ΔG) and Kc: ΔG∘ = − RT ln⁡Kc  where is the gas constant and is the temperature in Kelvin.
    • If ΔG∘ < 0 : Kc > 1 and the reaction is product-favoured.
    • If ΔG∘ > 0 :  Kc < 1 and the reaction is reactant-favoured.

Summary:

The equilibrium constant Kc is a crucial tool in chemistry for predicting the direction of a reaction, calculating equilibrium concentrations, understanding the extent of reactions, analysing the effect of temperature, applying Le Chatelier’s Principle, and relating to Gibbs free energy. Its value provides deep insights into the behaviour of chemical systems at equilibrium, guiding both theoretical understanding and practical applications.

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