2. Chemical Equilibrium Chapter 17

3. 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.

4. 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)

5. Experiments to investigate Le Chatelier’s principle using 3 named experiments. Example 1 Cobalt (II) Chloride, conc HCL. CoCl 4 2- + 6H 2 O Co(H 2 O) 6 2+ + 4Cl - Blue Red Addition of HCl - Addition of Water - Addition of Heat Exothermic CoCl 4 2- + 6H 2 O Co(H 2 O) 6 2+ + 4Cl - Increase in temperature- goes from red to blue therefore the reaction is ? Red to Blue Goes Blue to Red Exothermic

6. Example 2 Sodium Dichromate Cr 2 O 7 2– + H 2 O 2CrO 4 2– + 2H + Orange Yellow Addition of an alkali(NaOH)- Why? Addition of an acid (HCL)- Why? Orange changes to Yellow Yellow to Orange

7. Example 3

8. Industrial Applications of Le Chatellier Manufacture of Ammonia(NH 3 ) by the Haber Process N 2 + 3H 2 --> 2NH 3 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.

9. HABER PROCESS

10. N 2 (g) + 3H 2 (g) 2NH 3 (g) :  = - 92 kJ mol -1 ConditionsPressure20000 kPa (200 atmospheres) Temperature380-450°C Catalystiron Equilibrium theory favours low temperature exothermic reaction - higher yield at lower temperature high pressure decrease in number of gaseous molecules Kinetic theory favours high temperature greater average energy + more frequent collisions high pressure more frequent collisions for gaseous molecules catalyst lower activation energy Compromise conditions 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. HABER PROCESS

11. 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 NH 3 + HNO 3 ——> NH 4 NO 3 2NH 3 + H 2 SO 4 ——> (NH 4 ) 2 SO 4 MAKING NITRIC ACIDammonia can be oxidised to nitric acid nitric acid is used to manufacture... fertilisers (ammonium nitrate) explosives (TNT) polyamide polymers (NYLON)

12. Example 2 Manufacture of Sulfuric Acid by the Contact process. Exothermic 2SO 2 + O 2 2SO 3 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 (V 2 O 5 )catalyst.

13. 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

14. 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 K C (2004 Q 9) (b) Given K C calculate the equilibriumconcentrations (Revised sample paper Feb 2002 Q.9) Leaving Cert 2005 Q9(C) 2004 Q9(C) 2003 Q11 2002 Q10(C)

15. Question 7 2010 paper Find K C

16. Given K C find the equilibrium concentrations. 2003 Q11(a)