Chemical Equilibrium

1. Chemical Equilibrium Chapter 17

2. Chemical Equilibrium • Chemical Equilibrium is a state of dynamic balance where the rate of the forward reaction is equal to the rate of the reverse reaction.

3. Le Chatelier’s Principle • When a system in equilibrium is subjected to a stress, the system readjusts to relieve the effect of the stress. • Stresses include Pressure, Temperature, Concentration of one or more reactants. • Leaving Cert 2008 Q7, 2007Q10a, 2006 Q11b,2005 Q9 2004 Q9(C) 2003 Q11 2002 Q10(C)

4. Experiments to investigate Le Chatelier’s principle using 3 named experiments. Example 1 Cobalt (II) Chloride, conc HCL. CoCl42- + 6H2O Co(H2O)62+ + 4Cl- BlueRed Addition of HCl - Addition of Water - Addition of Heat Exothermic CoCl42- + 6H2O Co(H2O)62+ + 4Cl- Increase in temperature- goes from red to blue therefore the reaction is ? Red to Blue Goes Blue to Red Exothermic

5. Example 2 Sodium Dichromate Cr2O72– + H2O 2CrO42– + 2H+ OrangeYellow Addition of an alkali(NaOH)- Why? Addition of an acid (HCL)- Why? Orange changes to Yellow Yellow to Orange

6. Example 3

7. Industrial Applications of Le Chatellier Manufacture of Ammonia(NH3) by the Haber Process N2 + 3H2 --> 2NH3 According to Le Chatelier a high pressure and low temperature are the best conditions to give a maximum yield of ammonia. In practice the reaction is carried out at a pressure of 200 atmospheres and a temperature of 500* C with an Iron catalyst.

8. HABER PROCESS

9. HABER PROCESS N2(g) + 3H2(g) 2NH3(g) : DH = - 92 kJ mol-1 Conditions Pressure 20000 kPa (200 atmospheres) Temperature 380-450°C Catalyst iron Equilibriumtheory favours low temperature exothermic reaction - higher yield at lower temperature high pressure decrease in number of gaseous molecules Kinetictheory favours high temperature greater average energy + more frequent collisions high pressure more frequent collisions for gaseous molecules catalyst lower activation energy Compromiseconditions Which is better? A low yield in a shorter time or a high yield over a longer period. The conditions used are a compromise with the catalyst enabling the rate to be kept up, even at a lower temperature.

10. IMPORTANT USES OF AMMONIA AND ITS COMPOUNDS MAKING FERTILISERS80% of the ammonia produced goes to make fertilisers such as ammonium nitrate (NITRAM) and ammonium sulphate NH3 + HNO3 ——> NH4NO3 2NH3 + H2SO4 ——> (NH4)2SO4 MAKING NITRIC ACIDammonia can be oxidised to nitric acid nitric acid is used to manufacture... fertilisers (ammonium nitrate) explosives (TNT) polyamide polymers (NYLON)

11. Example 2 Manufacture of Sulfuric Acid by the Contact process. Exothermic 2SO2 + O2 2SO3 What Conditions give the best Yield? According to Le Chatelier a high pressure and low temperature are the best conditions to give a maximum yield of sulphur trioxide. In practice the reaction is carried out at a pressure of 1 atmospheres and a temperature of 45O* C with a Vanadium Pentoxide (V2O5)catalyst.

12. The Equilibrium constant(Kc) Provided the Temperature remains constant there is a relationship between the concentration of the reactants and products in a equilibrium mixture. This constant is based on the law of Chemical Equilibrium. If Therefore Only a change in temperature changes the value of Kc

13. Example Write the equilibrium constant expression (Kc) for the following equation. Answer: Calculations involving the law of Chemical Equilibrium (See BOOK pages 242-245) Two types (a) Calculate KC (2004 Q 9) (b) Given KC calculate the equilibriumconcentrations (Revised sample paper Feb 2002 Q.9) Leaving Cert 2005 Q9(C) 2004 Q9(C) 2003 Q11 2002 Q10(C)

14. Question 7 2010 paper Find KC

15. Given KC find the equilibrium concentrations. 2003 Q11(a)