Year 12 Chemistry

1. Year 12 Chemistry Term 2 - 2012

2. Reversible Reactions • Some chemical reactions are reversible. • Water can become liquid, solid or gas depending on the circumstances. • We saw in the previous unit that the reaction to form an ester could either move forward to produce the ester and water or in the right conditions move backward to produce the alcohol and the carboxylic acid. • This reaction is known as a reversible reaction.

3. Reversible Reactions • Without a something push it to completion this reaction will reach a point where it contains both products and reactants. • When the reaction reaches this point both reactions are occurring at equal rates. This means there is no overall change in the amount of each item present. This is called dynamic equilibrium. • We write this reaction as: Reactant ↔ Product

4. Equilibrium • Equilibrium is the term given to an object in a state of balance. • Static equilibrium is like a tug of war where both sides are equal strength. The rope does not move because the force on both sides is equal • Dynamic equilibrium is like a sports game, there is always a certain number of players on the field but the players themselves can change.

5. Reversible Reactions • Chapter 1 Page 203

6. Reversible Reactions • For reversible reactions we have: • Forward reactions - the reaction going as written from left to right. • Reverse or backward reactions - the reaction going opposite to the way it is written from right to left. • What is the product of the reverse reaction for the following reversible reaction? Pb(NO3)2(s)↔ Pb2+ + 2NO3-(aq)

7. Reversible Reactions • Reversible Reaction - A reaction which can go forward or backward depending on the circumstances. • Dynamic Equilibrium - The state where the concentration of products and reactants in a reaction remains stable, the forward and reverse reactions are occurring at the same rate. • Forward Reaction - A reversible reaction which is occurring from left to right. • Backward / Reverse Reaction - A reversible reaction which is occurring from right to left.

8. Types of Reversible Reactions • All precipitation reactions are reversible • Potassium chromate ↔ Potassium dichromate • Esters • Gases

9. How Does Equilibrium Occur? Ag+(aq) + Fe2+(aq) ↔ Fe3+ + Ag(s) • When the reaction above begins we have large concentrations of Ag+ and Fe2+. • Reactants decrease as products increase • As products increase they begin to react • Eventually reach state of equilibrium where both reactions are occurring at the same rate.

10. Br2 (g) + 2NO(g) ↔ 2NOBr (g)

11. Catalysts • How do catalysts affect equilibrium? • Draw graph:

12. Exercises

13. Exercises

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16. Exercises

17. Carbon Dioxide Equilibrium Read page 207- 208. Write down equation of dissolution of CO2 in H2O. What is the relationship between solubility and pressure for gases in liquids. Does the reaction lie to the right or left?

18. Solubility and Pressure for Gases in Liquids • As the pressure increases ↑ solubility increases ↑ • As pressure decreases ↓ solubility decreases ↓ • As solubility increases ↑ temperature decreases ↓ • As solubility decreases ↓ temperature increases ↑ • This is the opposite of solids and liquids in which temperature normally increases ↑ as solubility increases ↑.

19. Equilibrium Position • The extent to which the reaction has gone in the forward or reverse direction. • Lies to the left Reverse reaction is favoured Most of reactant is still resent Small amount of product • Lies to the right Forward reaction is favoured Most of the reactants has converted to product

20. Le Chatelier’s Principle If a system in equilibrium is disturbed, the system adjusts itself so as to minimize the disturbance

21. Le Chatelier’s Principle Disturbance to a system may include: • Concentration • Pressure • Temperature • Note a system at equilibrium is not disturbed by adding more solid to it. This is because the concentration of ions is not changed when a solid is added. • Concentration not amount effects equilibrium

22. Carbon dioxide and carbonic acid Read page 210 – 211. Predict the direction of the equilibrium if: • The plunger was pulled creating more volume • The temperature of the system decreased • Adding sodium hydroxide to the mix.

23. Forcing Reactions to Completion Water liquid and gas equilibrium:

24. Forcing Reactions to Completion H2O(l)  H2O(g) Water in a terrarium reaches equilibrium because it is a closed system water evaporates while vapour condenses. Water in wet clothes attempts to establish equilibrium however as liquid evaporates, it is carried by wind and diffuses into the atmosphere and so dryness will eventually occur.

25. Forcing Reactions to Completion When synthesizing chemicals, chemists may wish to push a reversible reaction to completion to obtain the maximum amount of a product. A common way to force reactions to completion is to remove a product as it is produced. Addition of an excess of cheap or common reactant is another way.

26. Exercises page 212

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36. Condition For Equilibrium • There is a quantitative relationship between reactants and products at equilibrium. • Using • When the reaction is at equilibrium at a constant temperature the expression [I3-] / [I2] [I-] has a constant value.

37. Condition for Equilibrium • See table 2.1 page 240.

38. Conditions for Equilibrium • Table shows a clear relationship between the reactants and products in an equilibrium reaction. • For any equilibrium reaction there is a function of the concentration of the species which has a constant value at equilibrium • This constant is given the symbol K and known as the equilibrium constant.

39. Equilibrium Constant • In an equilibrium reaction where aA + bB ↔ cC + dD • When the reaction is at equilibrium the expression: Has a constant value, regardless of the starting concentrations of the substances involved. This is called the equilibrium constant K

40. Equilibrium Constant Vs Reaction Quotient = K The above equation at constant temperature is known as the equilibrium expression Q= Alternatively the above equation at constant temperature is known as the reaction quotient

41. Equilibrium Constant Vs Reaction Quotient • The equilibrium constant [K] is the relationship between the products and reactants at equilibrium. • The reaction quotient [Q] is the relationship between the products and reactants at any given point during the reaction. • When Q = K the reaction is at equilibrium. • If Q ≠ K the reaction is not at equilibrium.

42. Equilibrium Constant Vs Reaction Quotient • It is possible to use Q to determine which direction a reaction if occurring. • If Q < K the reaction goes from left to right until Q = K • If Q > K the reaction goes from right to left until Q = K • If Q = K the reaction is at equilibrium.

43. Reaction Quotient

44. Rules for Writing Equilibrium Expressions • Both Q and K are always written with the products in the numerator and the reactants in the denominator. K or Q = K or Q =

45. Rules for Writing Equilibrium Expressions • Coefficients of products or reactants are written as powers for that product or reactant. eg:

46. Units for Equilibrium Constants • Reaction quotients have units. • mol/L • To calculate the value of the unit: • Cancel units above and below the division symbol. If the remainder is below the division symbol the units will be in mol/L-1 etc. If the remainder is above the division symbol units will be in mol/L1 etc.

47. Exercises • Page 244 • Questions 1 - 4

48. Temperature and Equilibrium Constant • The equilibrium constant K only remains stable if temperature is stable. If temperature changes then the value of the equilibrium constant will change. • If reaction is exothermic and temperature increases: Reaction will move from right to left K value will decrease. • If reaction is endothermic and temperature increases: Reaction will move from left to right K value will increase.

49. Temperature and Equilibrium Constant • For an exothermic reaction: K decreases as temperature increases. • For an endothermic reaction: K increases at temperature increases.

50. Exercises • Questions 16 and 17 • Page 252.