2.9 Chemical equilibria

1. a. demonstrate an understanding that chemical equilibria are dynamic b. deduce the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium, eg extraction of methane from methane hydrate c. interpret the results of simple experiments to demonstrate the effect of a change of temperature, pressure and concentration on a system at equilibrium, eg i. iodine(I) chloride reacting with chlorine to form iodine(III) chloride, or ii. N2O4 ⇌2NO2. 2.9 Chemical equilibria Crowe2009 • Connector - Explain the terms: • Reversible reaction • Dynamic equilibrium

2. Dynamic equilibrium B A A reversible reaction is where products can, under appropriate conditions, turn back into reactants. • There will be a range of conditions over which both the forward and backward reaction will take place and this can lead to a state of balance with both reactants and products present in unchanging amounts. • This is called a dynamic equilibrium. A B these decompose these combine

3. Dynamic equilibrium Equilibrium – becauseof the unchanging amounts Dynamic – because reaction is still occurring It is rather like the situation where a man is walking the wrong way along a moving pavement or escalator. Neither have stopped but the man could remain in the same place for ever! The symbol is used to mean dynamic equilibrium. The man stays in the same place!

4. These reactions are reversible, but under the conditions normally used, they become one-way reactions. The products aren't left in contact with each other, so the reverse reaction can't happen.

5. Demo – chromate(VI)/dichromate(VII) reversible reaction 2CrO42- (aq) + 2H+ (aq) Cr2O72- (aq) + H2O (l) yelloworange Conc. soln. of Potassium dichromate 2M HCl 2M NaOH 250ml conical flask

6. Reversible reactions happening in a closed system A closed system is one in which no substances are either added to the system or lost from it. Energy can, however, be transferred in or out at will. Initially the iron and steam will react: Once iron(III) oxide and hydrogen have begun to form, they too will react: Since nothing can escape, eventually the rate of the first reaction will be the same as that of the second and a DYNAMIC EQUILIBRIUM is set up.

7. Dynamic Equilibria There are two reactions here, the forward reaction (left to right), and the reverse reaction (right to left). What are the reactants of the forward reaction? What are the products of the forward reaction? What are the reactants of the reverse reaction? What are the products of the reverse reaction?

8. Dynamic Equilibria There are two reactions here, the forward reaction (left to right), and the reverse reaction (right to left). At equilibrium, the rate of each reaction will be the same. What effect will this have on the amounts of A, B, C and D? Remember both reactions are still happening, but because they are doing so at the same rate the amounts of reactants and products remain constant. (It’s a bit like going up an escalator the wrong way, and remaining in the same position.)

9. At the beginning of the reaction, the concentrations of A and B were at their maximum. That means that the rate of the reaction was at its fastest. As A and B react, their concentrations fall. That means that they are less likely to collide and react, and so the rate of the forward reaction falls as time goes on.

10. In the beginning, there isn't any C and D, so there can't be any reaction between them. As time goes on, though, their concentrations in the mixture increase and they are more likely to collide and react. With time, the rate of the reaction between C and D increases:                                        

11. Eventually, the rates of the two reactions will become equal. A and B will be converting into C and D at exactly the same rate as C and D convert back into A and B again. At this point there won't be any further change in the amounts of A, B, C and D in the mixture. As fast as something is being removed, it is being replaced again by the reverse reaction. We have reached a position of dynamic equilibrium.

12. A summary A dynamic equilibrium occurs when you have a reversible reaction in a closed system. Nothing can be added to the system or taken away from it apart from energy. At equilibrium, the quantities of everything present in the mixture remain constant, although the reactions are still continuing. This is because the rates of the forward and the back reactions are equal. If you change the conditions in a way which changes the relative rates of the forward and back reactions you will change the position of equilibrium - in other words, change the proportions of the various substances present in the equilibrium mixture.

13. Le Chatelier's Principle If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. Suppose you have an equilibrium established between four substances A, B, C and D. What would happen if you changed the conditions by increasing the concentration of A?

14. What would happen if you changed the conditions by increasing the concentration of A? What would happen if you changed the conditions by decreasing the concentration of A? If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

15. What would happen if you changed the conditions by decreasing the concentration of A?

16. Pressure This applies to gas reactions. Here the rule depends upon the number of gas molecules on each side of the equation Get more gas molecules in backward direction 2NO2(g) N2O4 (g) Get less gas molecules in forward direction The higher the pressure the more the reaction moves in the direction with less gas molecules. • Increasing the pressure will give more N2O4 • Decreasing pressure gives more NO2at equilibrium..

17. Get more gas molecules in backward direction Look at the reaction of nitrogen and hydrogen to form ammonia. 3H2(g) + N2 (g)  2NH3 (g) Get less gas molecules in forward direction Which direction produces less gas molecules. Which direction do reactions move when compressed? Will high pressure give more or less NH3 in the equilbrium mixture? forward The side that has less gas molecules • Increasing the pressure will give more NH3 • Decreasing the pressure give less NH3 at equilibrium.. more

18. The effect of changing pressure in a gaseous equilibrium What would happen if you changed the conditions by increasing the pressure? If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

19. What would happen if you changed the conditions by decreasing the pressure? What happens if there are the same number of molecules on both sides of the equilibrium reaction?

20. Temperature & equilibrium Delta H, Enthalpy change 2NO2 N2O4 ΔH = -ve ΔH = -ve Gets hot going forward (exothermic) ΔH = +ve Gets cold going backward (endothermic) The rule is – whatever you do to the equilibrium, the system will change to oppose it Heating will give more NO2 in the equilibrium mixture Cooling would give more N2O4 in the equilibrium mixture..

21. The reaction of nitrogen and hydrogen to form ammonia (NH3) is exothermic. How will temperature affect the composition of the equilibrium mixture? Gets cold going backward (endothermic) 3H2 + N2 2NH3 ΔH = -ve Gets hot going forward (exothermic) Which direction is endothermic? Which direction does equilibrium move when heated? Will heating give more or less NH3 in the equilbrium mixture? backward backward less

22. The effect of temperature on the position of an equilibrium What type of reaction occurs in the forward direction? What would happen if you changed the conditions by increasing the temperature?

23. What would happen if you changed the conditions by increasing the temperature? Decreasing the temperature has the opposite effect. If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

24. Summary Increasing the temperature of a system in dynamic equilibrium favours the endothermic reaction. The system counteracts the change you have made by absorbing the extra heat. Decreasing the temperature of a system in dynamic equilibrium favours the exothermic reaction. The system counteracts the change you have made by producing more heat.

25. Effect of a catalyst on the position of equilibrium Adding a catalyst makes absolutely no difference to the position of equilibrium, and Le Chatelier's Principle doesn't apply to them. This is because a catalyst speeds up the forward and back reaction to the same extent. Because adding a catalyst doesn't affect the relative rates of the two reactions, it can't affect the position of equilibrium.

26. The Haber Process 3H2(g) + N2 (g)  2NH3 (g) H= -92kJ/mol • Is the forward reaction exothermic or endothermic? • Will heating the mixture give an equilibrium mixture with more or less ammonia? • Are there more gas molecules of reactant or product? • Will raising the pressure give an equilibrium mixture with more or less ammonia? exothermic less reactant more

27. The Haber Compromise 3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol • The aim of the chemical industry is not to make chemicals. It is to make money! • If we use low temperatures it takes ages to reach equilibrium. It’s better to get a 40% yield in 2 minutes than an 80% yield in 2 hours! • If we use very high pressures the cost of the equipment used increases drastically and there are also safety issues. Better 90% conversion at 200atm than 95% conversion at 600 atm. • Unchanged reactants can always be recycled.

28. The Haber Process

29. The Haber process Homework – produce a detailed summary of the Haber process, clearly explaining the reasons for the conditions used.

30. METHANE HYDRATE ICEA Possible Mechanism for Ice Age and Global Warming Cycles* Recent discoveries about the existence of a vast band of Methane Hydrate Ice along the world's continental Slopes, at approx. 500 meters depth, have revolutionized the theories of the Ice Age and Global Warming Cycles. The accumulation of Methane Ice leads to Ice Ages and the rapid melting and effervescence of this ice and gas leads to and equally rapid Global Warming. * http://www.utopiasprings.com/methane.htm

31. Stored methane in equilibrium methane hydrate (s) methane (g) + water (l) ∆H +ve Why is it bad news if Earth’s temperature rises? Position of equilibrium will move to right to oppose the change, methane gas is released, methane gas is a greenhouse gas, and would contribute to global warming What would happen if the pressure was increased? Methane is a gas and so position of equilibrium will move to left to oppose the change.

32. Iodine(I) chloride & iodine (III) chloride ICI (l) + Cl2(g) ICl 3(s) ∆H = +ve Why is it bad news if the Earth’s temperature rises? Position of equilibrium will move to right to oppose the change, releasing more methane, and methane is a greenhouse gas. What would happen if the pressure was increased? Methane is a gas and so position of equilibrium will move to left to oppose the change.

33. The contact process • What would be the effect of: • Increasing the temperature? • Increasing the pressure?

34. The contact process

35. http://www.chemguide.co.uk/physical/equilibmenu.html#top