Reactivity 2.3. SL How far_ The extent of chemical change (1).pptx

IB chemistry

1. Lecture notes SL/HL By Ms. Anoosha Qaisar

2. R2.3—How far? The extent of chemical change Guiding Question : How can the extent of a reversible reaction be influenced?

3. Reversible Reactions and the Haber Process Reversible reactions can proceed in both forward and reverse directions. The extent of reaction (how far the reaction goes in each direction) is important in industrial processes. Haber Process Reaction: N 2 (g) + 3H 2 (g) ↔ 2NH 3 (g) + heat Industrial conditions: High temperature (400–450°C), high pressure (150–200 atm), and an iron catalyst optimize ammonia production.

4. Ammonia applications: Mainly used for fertilizers, which are crucial for agriculture. Also used in explosives, pharmaceuticals, and refrigeration. Industrial relevance: The continuous removal of ammonia shifts equilibrium, maximizing yield (based on Le Chatelier’s principle). This careful control of conditions is vital for efficiency.

5. dynamic equilibrium Content Statement A state of dynamic equilibrium is reached in a closed system when the rates of forward and backward reactions are equal Learning outcome Describe the characteristics of a physical and chemical system at equilibrium.

6. SYSTEM AT EQUILIBRIUM Reversible Reaction A reversible reaction is a chemical reaction in which the products can react to reform the reactants. System at equilibrium: The system is at equilibrium when the rate of the forward reaction equals the rate of the reverse reaction, and the concentration of produ cts and reactants remains unchanged. Arrows going both directions ( ⮀ ) indicates equilibrium in a chemical equation. 2HgO (s) ⮀ 2Hg (l) + O 2(g)

7. CHARACTERISTICS OF SYSTEM AT EQUILIBRIUM Characteristics of a physical system at equilibrium A physical system is one in which a chemical reaction is not occurring. Equilibrium will only occur in a “closed” system. A typical example is the evaporation-condensation cycle of a volatile liquid in a closed container. - When the rate of evaporation equals the rate of condensation, the system is at equilibrium

8. A state of dynamic equilibrium is reached in a closed system when the rates of forward and backward reactions are equal.

9. Examples of physical system at equilibrium

10. A chemical system is one in which a chemical reaction is occurring. For the system to reach equilibrium, the forward reaction must equal that of the reverse reaction. EQUILIBRIUM IN CHEMICAL SYSTEMS

11. 1. Equilibrium is dynamic – the reaction has not stopped but both reactions or processes are still occurring on the microscopic level even though it looks like the reaction or process has stopped. 2. Equilibrium is achieved in a closed system so that no matter can escape to the surroundings. 3. The concentrations of reactants and products remain constant at equilibrium – they are being produced or consumed at an equal rate. CHARACTERISTICS OF THE EQUILIBRIUM STATE

12. 4 . There is no macroscopic change in properties – meaning the observable properties do not change at equilibrium because they depend on the concentration of the mixture components. 5. Equilibrium can be reached from either direction – meaning that at the same conditions, equilibrium will be reached whether the reaction started with all reactants, all products or a mixture of both. 6. It is important to note that even though the concentrations of reactants and products are constant at equilibrium, this does not imply they are equal.

13. EQUILIBRIUM POSITION The equilibrium position is the proportion of the reactants and products in the equilibrium mixture. A reaction with an equilibrium mixture with mostly products is said to “lie to the right”. A reaction with an equilibrium mixture with mostly reactants is said to “lie to the left”. We can quantitatively compare equilibrium mixtures with different conditions by using the equilibrium constant K c . There can be numerous equilibrium positions, but only one equilibrium constant, K c , at a specified temperature.

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17. The equilibrium constant (Kc) is a ratio that provides information about the relative concentrations of reactants and products in a reaction at equilibrium. It quantifies the extent to which a reaction proceeds before reaching equilibrium. The value of Kc is determined experimentally, and it varies with temperature. For a general reaction: aA + bB ⇌ cC + dD the equilibrium constant expression is: equilibrium constant (Kc)

18. Square brackets indicate the concentration of each substance in mol/dm³. Numerator : Products. Denominator : Reactants. Each concentration is raised to the power of its coefficient from the balanced equation. For the Haber process:

19. PRODUCT FAVORED EQUILIBRIUM Large values for K signify the reaction is “product favored” When equilibrium is achieved, most of the reactants have been converted to products. K > 1

20. REACTANT FAVORED EQUILIBRIUM Small values for K signify the reaction is “reactant favored” When equilibrium is achieved, very little reactant has been converted to product. K < 1

21. The equilibrium law describes how the equilibrium constant, K, can be determined from the stoichiometry of a reaction.

22. In Aqueous solutions, pure substances are omitted in the K c This is not true if all species are of the same state:

23. The magnitude of K c gives information on the extent of reaction The reaction quotient, Q, enables us to predict the direction of reaction Reaction Quotient Q – concentrations of the reactants and products at one moment in time when the reaction is not at equilibrium, substituted into the equilibrium constant expression K c = 49.5

24. Relationships between K c for different equations of a reaction

25. The equilibrium constant K c has a fixed value at the same temperature. You can derive K c from different sets of data for the same reaction at constant temperature. All K c values should be equal no matter how you arrived at the value since only temperature affects K c .

26. CONCLUSIONS ABOUT EQUILIBRIUM EXPRESSIONS If an equation is written in reverse, the K expression is the reciprocal of the original K. 2NO 2 (g) ⮀ 2NO(g) + O 2 (g ) 2NO(g) + O 2 (g) ⮀ 2NO 2 (g)

27. The equilibrium constant and reaction stoichiometry When the balanced equation for a reaction is multiplied by a factor n , the equilibrium expression for the new reaction is the original expression, raised to the nth power. 2NO 2 (g) ⮀ 2NO(g) + O 2 (g ) NO 2 (g) ⮀ NO(g) + ½O 2 (g )

28. Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium.

29. R2.3.4—Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium.

30. Le-chatelier’s Principle When a system at equilibrium is placed under stress, the system will undergo a change in such a way as to relieve that stress. Translated: The system undergoes a temporary shift to restore equilibrium. 1. When you add a substance or heat, the system shifts to the opposite side. 2. When you take out a substance or heat, the system shifts to the side of the takeout. When you increase pressure, the system shifts to the side with the least number of gaseous molecules.

31. Le Chatelier’s Principle ' When a system at equilibrium is subjected to a change, the system will respond to minimise the effect of the change .' changes to the concentrations of reactants or products changes to the pressure of a system involving gaseous reactants and products changes to the temperature the addition of a catalyst. Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium. Apply Le Châtelier’s principle to predict and explain responses to changes of systems at equilibrium.

32. Effect on Equilibrium Position : Shift left : Increases reactant concentration. Shift right : Increases product concentration. Examples of System Adjustments Concentration : Removing a product shifts equilibrium right (toward more products). Adding a reactant shifts equilibrium right (toward more products). Pressure (for gases): Increase in pressure : Shifts toward the side with fewer gas molecules. Decrease in pressure : Shifts toward the side with more gas molecules. Temperature : Endothermic reaction : Increased temperature shifts equilibrium right . Exothermic reaction : Increased temperature shifts equilibrium left .

33. Changes in concentration N 2 (g) + 3H 2 (g) ⇌ 2NH 3 (g)

34. Changes in pressure The pressure of a system at equilibrium can be changed by either: adding or removing a gaseous reactant or product, or changing the volume of the reaction vessel. If an equilibrium mixture of gases is subjected to an increase in pressure, the equilibrium position will shift to the side with the lowest number of gaseous molecules.

35. Changes in temperature if the temperature is increased , the equilibrium position will shift in the direction that will cause a lowering of the temperature – the endothermic direction. if the temperature is decreased, the equilibrium position will shift in the exothermic direction, generating heat and increasing the temperature.

36. Adding a catalyst A catalyst increases both forward and backward reactions equally by lowering the activation energy

37. Le Chatelier Example #1 A closed container of ice and water is at equilibrium. Then, the temperature is raised. Ice + Energy ⮀ Water The system temporarily shifts to the _______ to restore equilibrium. right

38. Le Chatelier Example #2 A closed container of N 2 O 4 and NO 2 is at equilibrium. NO 2 is added to the container. N 2 O 4 (g) + Energy ⮀ 2 NO 2 (g) The system temporarily shifts to the _______ to restore equilibrium. left

39. INDUSTRIAL APPLICATIONS Remember the only thing that can change the value of K c is a change in temperature. In reactions involving the manufacture of a chemical, the goal is to obtain the highest yield possible. We use our knowledge of Le Chatelier’s principle and kinetics to maximize the yield.

40. Concentration : the reactants nitrogen and hydrogen are supplied in the molar ratio 1 : 3 in accordance with their stoichiometry in the equation. The product ammonia is removed as it forms, thus helping to pull the equilibrium to the right and increasing the yield. Pressure : as the forward reaction involves a decrease in the number of gas molecules, it will be favoured by a high pressure. The usual pressure used in the Haber process is about 2 × 10 7 Pa. OPTIMUM CONDITIONS FOR HABER PROCESS

41. OPTIMUM CONDITIONS FOR HABER PROCESS Temperature: as the forward reaction is exothermic, it will be favoured by a lower temperature. However, too low a temperature would cause the reaction to be uneconomically slow, and so a moderate temperature of about 450 °C is used.

42. Catalyst: a catalyst will speed up the rate of production and so help to compensate for the moderate temperature used. A catalyst of finely divided iron is used, with small amounts of aluminium and magnesium oxides added to improve its activity. More recently, ruthenium has become the catalyst of choice, and this has helped reduce the energy requirement. OPTIMUM CONDITIONS FOR HABER PROCESS http://www.spaceflight.esa.int/impress/text/education/Images/Catalysis/Image_004.png

43. Contact Process S (s) + O 2 (g) ⇄ SO 2 (g) 2 SO2 (g) + O2 (g) ⇄ 2 SO3 (g) ∆ H = –196 kJ mol SO 3 (g) + H 2 O (l) ⇄ H 2 SO 4 (l)