EQUILIBRIA Cambridge AS level unit 7 Lesson 1

This is used for Cambridge AS level studies

1. UNIT-7 EQUILIBRIA

2. Chemical Equilibria Reversible reaction Some reactions go to completion where the reactants are used up to form the products and the reaction stops when all of the reactants are used up In  reversible reactions  the products can react to reform the original reactants To show a reversible reaction, two opposing half arrows are used: ⇌ Dynamic equilibrium In a  dynamic equilibrium  the reactants and products are  dynamic  (they are constantly moving) In a dynamic equilibrium the  rate  of the  forward  reaction is the same as the rate of the  backward  reaction in a  closed system , and the  concentrations  of the  reactants  and  products  is  constant

3. Le Chaterlier's Principle Position of the equilibrium The  position of the equilibrium  refers to the relative amounts of products and reactants in an equilibrium mixture. When the position of equilibrium shifts to the  left , it means the concentration of  reactants  increases When the position of equilibrium shifts to the  right , it means the concentration of  products  increases Le Chatelier’s principle Le Chatelier’s principle  says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

4. Effects of concentration

5. Effects of pressure Changes in pressure only affect reactions where the reactants or products are gases https://javalab.org/en/le_chateliers_principle_pressure_en /

6. Effects of temperature Effects of temperature table

7. Effects of catalysts A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the  forward  and  reverse  reaction  equally ) Catalysts only cause a reaction to reach its equilibrium  faster Catalysts therefore have  no effect  on the  position of the equilibrium  once this is reached

8. Equilibrium Constant Equilibrium expression & constant The  equilibrium expression  is an expression that links the  equilibrium constant ,  K c , to the  concentrations of reactants and products  at equilibrium taking the  stoichiometry  of the equation into account

9. Partial pressure For reactions involving mixtures of gases, the equilibrium constant  K p  is used as it is easier to measure the  pressure  than the concentration for gases The  partial pressure  of a gas is the pressure that the gas would have if it was in the container all by itself The  total pressure  is the sum of the  partial pressure

10. Equilibrium expressions involving partial pressures Equilibrium expressions in terms of partial pressures are written similarly to those involving concentrations with a few differences:

11. Equilibrium Constant Calculations Calculations involving  K c In the equilibrium expression each figure within a square bracket represents the concentration in  mol dm -3 The  units  of  K c  therefore depend on the form of the equilibrium expression Some questions give the  number of moles  of each of the reactants and products at equilibrium together with the volume of the reaction mixture The concentrations of the reactants and products can then be calculated from the number of moles and total volume

12. Calculations involving  K p In the equilibrium expression the  p  represent the partial pressure of the reactants and products in  Pa The  units  of  K p   therefore depend on the form of the equilibrium expression K p   = 9.1 x 10 -6  Pa -1

13. Haber & Contact Processes Equilibrium reactions are involved in some stages of large-scale production of certain chemicals An understanding of equilibrium and Le Chatelier’s principle is therefore very important in the chemical industry Haber process The Haber process involves the synthesis of ammonia according to: N­ 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g)       ΔH r  = -92 kJ mol -1 Le Chatelier’s principle is used to get the best yield of ammonia Maximising the ammonia yield Pressure An increase in  pressure  will result in the equilibrium shifting in the direction of the fewest molecules of gas formed to reduce the pressure In this case, the equilibrium shifts towards the right so the yield of ammonia increases An increase in pressure will cause the particles to be closer together and therefore increasing the number of successful collisions leading to an increased reaction rate Very high pressures are expensive to produce therefore a  compromise pressure  of 200 atm is chosen

14. Temperature To get the maximum yield of ammonia the position of equilibrium should be shifted as far as possible to the right as possible Since the Haber process is an exothermic reaction, according to Le Chatelier’s principle the equilibrium will shift to the right if the temperature is lowered A decrease in  temperature  will decrease the energy of the surroundings so the reaction will go in the direction in which energy is released to counteract this Since the reaction is exothermic, the equilibrium shifts to the right However,  at a low temperature the gases won’t have enough kinetic energy to collide and react and therefore equilibrium would not be reached therefore  compromise temperature  of 400-450  o C is used in the Haber process A heat exchanger  warms the incoming gas mixture to give molecules more  kinetic energy  such that the gas molecules  collide more frequently  increasing the likelihood of a reaction

15. Catalysts In the absence of a catalyst the reaction is so slow that hardly anything happens in a reasonable time! Adding an iron catalyst speeds up the rate of reaction Contact process The Contact process involves the synthesis of sulfuric acid according to: 2SO­ 2 (g) + O 2 (g) ⇌ 2SO 3 (g)    ΔH r  = -197 kJ mol -1 Le Chatelier’s principle is used to get the best yield of sulfuric acid Maximising the sulfuric acid yield 1. Pressure An increase in  pressure  will result in the equilibrium shifting in the direction of the fewest molecules of gas formed to reduce the pressure In this case, the equilibrium shifts towards the right so the yield of sulfuric acid increases In practice, the reaction is carried out at only 1 atm This is because  Kp   for this reaction is already very high meaning that the position of the equilibrium is already far over to the right Higher pressures than 1 atm will be unnecessary and expensive

16. 2. Temperature The same principle applies to increasing the temperature in the Contact process as in the Haber process A  compromise temperature  of 450  o C is used 3. Removing sulfuric acid SO 3  is removed by absorbing it in 98% sulfuric acid The SO 3  reacts with the solution and more H 2 SO 4  is formed 4. Catalysts The Contact process uses vanadium(V) oxide as a catalyst to increase the rate of reaction

17. Acids & Bases

18.   Brønsted –Lowry Theory The Brønsted -Lowry Theory  defines acids and bases in terms of proton transfer between chemical compounds A  Brønsted -Lowry acid  is a species that  gives away  a  proton  (H + ) A  Brønsted -Lowry base  is a species that  accepts  a proton (H + ) using its  lone pair of electrons

19. Acid & Base Dissociation

20. https://acswebcontent.acs.org/ChemistryInContextSuite/applets/ph-scale/latest/ph-scale_en.html

21. 10 pH Scale

22. Strength of Acids & Bases

23. Neutralisation Reactions

24. pH Titration Curves What are pH titration curves? Titration is a technique used in neutralisation reactions between acids and alkalis to determine the concentration of the unknown solution It involves adding  a titrant  of known concentration from a burette into a conical flask containing the  analyte  of unknown concentration An indicator is added which will change colour at the  endpoint  of the titration The endpoint is the point at which equal number of moles of  titrant  and  analyte  react with each other The equivalence point is halfway the  vertical region  of the curve Equivalence point → moles of alkali = moles of acid This is also known as the  equivalence point  and this is the point at which  neutralisation   takes place http://www.chem.uiuc.edu/webFunChem/titrations/IntroTitrate.htm

25. Strong acid + strong alkali pH titration curve Initially there are only H +  ions present in solution from the dissociation of the strong acid (HCl) (initial pH about 1-2) As the  volume of strong alkali (NaOH) added increases , the pH of the HCl solution slightly increases too as more and more H +  ions react with the OH -  from the NaOH to form water The change in pH is not that much until the volume added gets close to the equivalence point The pH surges upwards very steeply The  equivalence point  is the point at which all H +  ions have been neutralised (therefore pH is 7 at equivalence point) Adding more NaOH will increase the pH as now there is an  excess in OH -  ions  (final pH about 13-14)

26. The pH titration curve for HCl added to a NaOH has the  same shape The initial pH and final pH are the other way around The equivalence point is still 7

27. Strong acid + weak alkali pH titration curve Initially, there are  only H +  ions present  in solution from the dissociation of the strong acid (HCl) (initial pH about 1-2) As the volume of weak alkali (NH 3 ) added increases, the  pH of the analyte solution slightly increases too  as more and more H +  ions react with the NH 3 The change in pH is not that much until the volume added gets close to the equivalence point The  equivalence point is the point at which all H +  ions have been neutralised by the NH 3  however the equivalence point is not neutral, but the solution is still acidic (pH about 5.5) This is because all H +  have reacted with NH 3  to form NH 4 +  which is a relatively strong acid, causing the solution to be acidic As more of the NH 3  is added, the pH increases to above 7 but below that of a strong alkali as NH 3  is a weak alkali

28. The pH titration curve for strong acid added to a weak alkali has the same shape The initial and final pH are the other way around The  equivalence point is still about 5.5

29. Weak acid + strong alkali pH titration curve Initially there are  only H +  ions present in solution  from the dissociation of the weak acid (CH 3 COOH, ethanoic acid) (initial pH about 2-3) As the volume of strong alkali (NaOH) added increases, the  pH of the ethanoic acid solution slightly increases too  as more and more H +  ions react with the OH -  from the NaOH to form water The change in pH is not that much until the volume added gets close to the equivalence point The pH surges upwards very steeply The  equivalence point is the point at which all H +  ions have been neutralised by the OH -  ions  however the equivalence points is not neutral, but the solution is slightly basic (pH about 9) This is because all H +  in CH 3 COOH have reacted with OH -  however, CH 3 COO -  is a relatively strong base, causing the solution to be basic As more of the NaOH is added, the pH increases to about 13-14

30. The pH titration curve for weak acid added to a strong alkali has the same shape The initial and final pH are the other way around The  equivalence point is still about 9

31. Weak acid + weak alkali pH titration curve Initially there are only H +  ions present in solution from the dissociation of the weak acid (CH 3 COOH, ethanoic acid) (initial pH about 2-3) In these pH titration curves, there is no vertical region There is a  ‘point of inflexion’  at the equivalence point The curve does not provide much other information

32. Indicators used in Titration Indicators  are substances that change colour when they are added to acidic or alkaline solutions When choosing the appropriate indicator, the pH of the equivalence point is very important The two most common indicators that are used in titrations are  methyl orange  and  phenolphthalein Both indicators change colour over a specific pH range

33. Choosing indicators for titrations Strong acid and strong alkali The colour change for  both indicators  takes place at a pH range that falls within the vertical region of the curve Therefore, either indicator can be used

34. Strong acid and weak alkali Only  methyl orange  will change colour at a pH close to the equivalence point and within the vertical region of the curve

35. Weak acid and strong alkali Now, only  phenolphthalein  will change colour at a pH close to the equivalence point and within the vertical region of the curve The pH range at which methyl orange changes colour falls below the curve

36. Weak acid and weak alkali Neither indicator is useful, and a different method should be considered