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Chemical Equilibrium: Reversible Reactions and Equilibrium Constants

Learn about reversible reactions and dynamic equilibrium in chemical equilibrium. Understand how equilibrium constants (Kc) determine the position of equilibrium and the relative concentrations of reactants and products. Practice calculations involving Kc.

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Chemical Equilibrium: Reversible Reactions and Equilibrium Constants

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  1. Ch 28 - Chemical equilibrium

  2. 2Mg + O2 2MgO Magnesium + oxygen Magnesium oxide • This reaction usually goes to completion, and can only go one direction.

  3. The Haber process • N2 + 3H2 2NH3 • This reaction is reversible – it goes both directions! The reactants react to form the product…… and at the same time the product is decomposing to form the reactants.

  4. Dynamic equilibrium Dynamic – reaction is still occurring Equilibrium –forward reaction and the backward reaction are going at the same speed. The symbol is used to mean dynamic equilibrium. In a reaction at equilibrium the concentrations of each reactant and product are constant (overall they are not changing!) The man stays in the same place!

  5. Equilibrium constant Kc The equilibrium constant tells us the position of the equilibrium of a reaction … It tells us whether the reaction mixture is made up of mostly reactants or mostly products!

  6. A large equilibrium constant Kc tells us that reaction lies on the right hand side – at equilibrium there is more products than reactants. 2SO2 + O2 2SO3 Say at equilibrium this reaction has an equilibrium constant of 45. This tells us that there is a higher concentration of products compared to reactants at equilibrium!

  7. A small equilibrium constant Kc tells us that reaction lies on the left hand side – at equilibrium there is more reactants than products N2O4 2NO2 Say at equilibrium this reaction has an equilibrium constant of 0.05. This tells us that there is a higher concentration of reactants compared to products at equilibrium!

  8. The formula for Kc In the reaction aA + bB cC + dD Kc = [C]c [D]d [A]a [B]b

  9. Check your learning… • What does reversible mean in terms of a chemical reaction? • Describe what is happening in a reaction when it reaches chemical equilibrium • Why is chemical equilibrium described as dynamic? • Tell what a large value of Kc means • Tell what a small value of Kc means

  10. Review of yesterday’s lesson.. • What is a reversible reaction? • What does equilibrium mean in terms of a chemical reaction? • Why are reactions at equilibrium said to be dynamic? • What is Kc, and what does it tell you about a reaction?

  11. Calculations involving Kc – EXAMPLE 1 • In a reaction between sulfur dioxide and oxygen • 2SO2 + 02 2SO3 It is found that at equilibrium the concentrations of SO2,02 and SO3 were 0.07mol-1L, 0.035mol-1L and 0.03mol-1L. Calculate the value of the equilibrium constant Kc

  12. Calculations involving Kc – EXAMPLE 2 • Mix together 180g of ethanoic acid and 138g of ethanol. At equilibrium there is 46g of ethanol in the reaction. Calculate Kc for the reaction C2H5OH CH3COOC2H5 + CH3COOH + H20

  13. Kc = [CH3COOC2H5 ][H20] [CH3COOH ][C2H5OH] Kc = [2][2] [1][1] Kc = 4

  14. Review of equilibrium.. • What is a reversible reaction? • What does equilibrium mean in terms of a chemical reaction? • Why are reactions at equilibrium said to be dynamic? • What is Kc, and what does it tell you about a reaction?

  15. 2007 Q10a • (i) Kc = [NH3]2 [N2][H2]3

  16. Kc = [NH3]2 [N2][H2]3 Kc = [2]2 [2][6]3 Kc = 1 108 Kc = 0.009

  17. 2004 Q9e)Calculations involving Kc – • In an experiment 6 moles of nitrogen and 18 moles of hydrogen were mixed and allowed to come to equilibrium in a 5L sealed vessel at a certain temperature. At equilibrium 6 moles of ammonia were present. Find the value for Kc N2 + 3H2 2NH3

  18. Kc = [NH3]2 [N2][H2]3 Kc = [1.2]2 [0.6][1.8]3 Kc = 0.4 L2/mol2

  19. Exam questions on this • 2010 Q7c • 2012 11 B – Not last part • 2008 Q7b

  20. Calculations involving Kc – 2010 Q7c • In an experiment 208.5g of Phosphorous chloride were placed in a 100L flask. The reaction was allowed to reach equilibrium. The mass of Chlorine gas present was 53.25g. Calculate the equilibrium constant for the reaction at 500K. PCl5 PCl3 +Cl2

  21. Kc = [PCl3] [Cl2] [PCl5] Kc = [0.0075] [0.0075] [0.0025] Kc =0.025 mol/L

  22. 2008 Q7) If 11 moles of hydrogen gas and 11 moles of iodine gas mixed at 764K. At equilibrium 17 moles of HI are present. H2 + I2 2HI

  23. I) Write the expression for Kc • Kc = [HI]2 • [H2][I2]

  24. H2 + I2 2HI

  25. Kc = [HI]2 [H2][I2] Kc = [17]2 [2.5][2.5] Kc = 46.25

  26. Finding equilibrium concentrations when Kc is given 2005 Q9c) The value of Kc is 4.0 at 373K. What mass of ethyl ethanoate would be present in the equilibrium mixture if 15g of ethanoic acid and 11.5g of ethanol were mixed and equilribrium established at this temperature?

  27. Kc = [CH3COOC2H5 ][H20] [CH3COOH ][C2H5OH] 4 = [CH3COOC2H5 ][H20] [CH3COOH ][C2H5OH]

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