AS Chemistry

1. AS Chemistry Enthalpy Changes

2. Learning Objectives • Candidates should be able to: • Explain that some chemical reactions are accompanied by energy changes, principally in the form of heat energy. • Construct (and interpret) a reaction pathway diagram, in terms of the enthalpy change of the reaction. • Calculate enthalpy changes from appropriate experimental results, including the use of the relationship: q=mcT. • Explain and use the terms enthalpy change of reaction and standard conditions, with particular reference to formation and combustion.

3. Starter activity Can you complete tasks 1 and 2 on your notes.

4. Exothermic

5. Endothermic

6. Simple calorimeter – flammable liquids

7. Bomb calorimeter

8. Simple calorimeter – chemicals in solution

10. AS Chemistry Hess’ Law and Enthalpy Cycles

11. Learning Objectives • Candidates should be able to: • apply Hess’ Law to construct simple energy cycles • carry out calculations involving such cycles

12. Starter activity Can you write out definitions for ΔHθc and ΔHθf ?

13. Standard enthalpy change of combustion, ΔHθc, 298 is the enthalpy change when 1 mole of a substance is burned completely in oxygen under standard conditions (100kPa and 298K), all reactants and products being in their standard states. Standard enthalpy change of formation, ΔHθf, 298 is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions (100kPa and 298K), all reactants and products being in their standard states.

14. Hess’ Law The enthalpy change of a reaction depends only on the initial and final state of the reaction and is independent of the route by which the reaction may occur.

15. ‘CRAP rule’ H = (Hc reactants) - (Hc products) H = (Hf products) - (Hf reactants)

19. AS Chemistry Bond Enthalpies

20. Learning Objectives Candidates should be able to apply Hess’ Law to construct simple energy cycles, and carry out calculations involving such cycles and relevant energy terms, with particular reference to average bond energies.

21. Starter activity Can you write equations for the ΔHθc and ΔHθfof glucose (C6H12O6)?

22. Bond breaking and bond making Chemical reactions involve bond breaking and bond making.

23. Bond energy The quantity of energy needed to break a particular bond in a molecule is called the bond dissociation enthalpy (Hdiss), or bond enthalpy for short. It refers to the enthalpy change when one mole of bonds of the same type are broken in gaseous molecules under standard conditions.

24. Bond energy H – H (g) H(g) + H(g)

25. Mean Bond Enthalpy The mean bond enthalpy is the amount of energy needed to break a covalent bond. They are average values taken from many different molecules

27. Bond breaking: Total endothermic value = (+347 x 1) + (+413 x 5) + (+358 x 1) + (+464 x 1) + (+498 x 3) = +4728 kJ Bond making: Total exothermic value = (-464 x 6) + (-805 x 4) = -6004 kJ Sum total of bond breaking and bond making: DHc = +4728 + - 6004 = -1276 kJ mol-1

28. AS Chemistry Kinetics

29. Learning Objectives • Candidates should be able to: • Explain and use the terms rate of reaction and activation energy. • Show understanding, including reference to the Boltzmann distribution, of what is meant by the term activation energy.

30. Starter activity Working in groups of 3, complete task 1.

31. Different rates of reaction Seconds Minutes Hours Days Weeks Months Years Decades Centuries Millennia dynamite exploding magnesium and acid cake baking fruit ripening plants growing rusting of iron erosion of rock crude oil forming

32. Rate of Reaction The rate of a reaction is found by measuring the amount in moles of a reactant which is used up, or the amount of product produced, in a given time. The units are often mol dm-3 s-1.

33. Changing the rate of a reaction There are five factors which can affect the rate of a reaction: • Surface Area • Concentration • Temperature • Use of a catalyst • Intensity of light

34. Collision Theory Reactions occur when the particles of reactants collide, provided they collide with a certain minimum amount of kinetic energy (and in the correct orientation).

35. Activation Energy - the minimum energy required for a reaction to occur.

36. A B Reactants Enthalpy Enthalpy Enthalpy Enthalpy Products Reactants Products Progress of Reaction Progress of Reaction Progress of Reaction Progress of Reaction C D Products Reactants Products Reactants

37. Maxwell-Boltzmann Distribution

39. AS Chemistry Effect of Temperature

40. Learning Objectives • Candidates should be able to: • Explain qualitatively, in terms of both of the Boltzmann distribution and of collision frequency, the effect of temperature change on the rate of reaction.

41. Starter activity Can you complete task 1?

42. Activation Energy

43. Increasing temperature

44. Increasing temperature

45. Temperature and Activation Energy

46. Temperature and Activation Energy

47. AS Chemistry Catalysis

48. Learning Objectives • Candidates should be able to: • explain that, in the presence of a catalyst, a reaction has a different mechanism, i.e. one of lower activation energy, and interpret this catalytic effect in terms of the Boltzmann distribution. • describe enzymes as biological catalysts (proteins) which may have specific activity.

49. Starter activity Answer past paper question.

50. Activation Energy