1. How Fast Does A Reaction Occur?
KINETICS
How Fast Does A Reaction Occur?

2. Energy Diagrams
Reactants always start a reaction so they are on the left side of the diagram.
Products are on the right.
Reactants
Products

3. Exothermic Reactions
The exothermic reaction gives off heat because the products are at a lower energy level than the reactants.

4. Graph of an Exothermic Reaction

5. Graph of an Exothermic Reaction
In an exothermic graph, the reactants have greater energy than the products.
The change in energy (ΔH) is a negative value.
Products
Reactants

6. Exothermic Reactions
When climbing a ladder, you are more stable near the bottom than at the top.
In an exothermic reaction, products are more stable because they are lower in energy.

7. H2(g) + Cl2(g) → 2HCl(g) + heat
Exothermic Reactions
For an exothermic reaction, heat is a product and is found on the right side of the arrow in a chemical reaction.
H2(g) + Cl2(g) → 2HCl(g) + heat

8. Endothermic Reactions
The endothermic reaction absorbs heat because the products are at a higher energy level than the reactants.

9. Graph of an Endothermic Reaction

10. Graph of an Endothermic Reaction
In an endothermic graph, the products have greater energy than the reactants.
The change in energy is a positive value.
Products
Reactants

11. Endothermic Reactions
In an endothermic reaction, reactants are more stable because they have lower energy.

12. Endothermic Reactions
For an endothermic reaction, heat is a reactant and is found on the left side of the arrow in a chemical reaction.
heat + C (s) + H2O (g) → CO (g) + H2 (g)

13. Exothermic And Endothermic Reactions
Scientists have observed that the energy released in the formation of a compound from its elements is always identical to the energy required to decompose that compound into its elements.

14. Energy Diagrams Activation energy activated complex
Activation energy is the minimum amount of energy that reacting particles must have to form the activated complex.

15. Energy Diagrams activated complex
The activated complex is a short-lived, unstable arrangement of atoms that may break apart and re-form the reactants or may form products.

16. Energy Diagrams Activation energy
To calculate the activation energy, take the top of the peak and subtract the reactants.

17. Energy Diagrams Enthalpy/heat of reaction (ΔH)
The enthalpy or heat of reaction (ΔH) is the amount of heat released or absorbed in the reaction.

18. Energy Diagrams Enthalpy/heat of reaction (ΔH)
To determine ΔH, take the energy of the products and subtract the energy of the reactants.

19. Problem
1) The heat content of the reactants of the forward reaction is ______ kilojoules.
(80)

20. Problem
2) The heat content of the products of the forward reaction is ______ kilojoules.
(160)

21. Problem
3) The heat content of the activated complex of the forward reaction is about _____ kilojoules.
(240)

22. Problem
4) The activation energy of the forward reaction is _____ kilojoules.
(240 – 80 = 160)

23. Problem
5) The heat of reaction (ΔH) of the forward reaction is _____ kilojoules.
(160 – 80 = 80)

24. Problem 6) The forward reaction is (endothermic or exothermic).
(endothermic - products are higher in energy; ΔH is positive)

25. Problem
7) The heat content of the reactants of the forward reaction is ______ kJ.
(40)

26. Problem
8) The heat content of the products of the forward reaction is about ______ kilojoules.
(20)

27. Problem
9) The heat content of the activated complex of the forward reaction is about _____ kilojoules.
(100)

28. Problem
10) The activation energy of the forward reaction is about _____ kilojoules.
(100 – 40 = 60)

29. Problem
11) The enthalpy change (ΔH) of the forward reaction is ____ kilojoules.
(20 – 40 = – 20)

30. Problem 12) The forward reaction is (endothermic or exothermic).
(exothermic - products are lower in energy; ΔH is negative)

31. Problem
13) The heat content of the reactants of the reverse reaction is ______ kilojoules.
(20)

32. Problem
14) The heat content of the products of the reverse reaction is ______ kilojoules.
(40)

33. Problem
15) The enthalpy change (ΔH) of the reverse reaction is ____ kilojoules.
(40 – 20 = + 20)

34. Forward or Reverse Favored?
Remember that enthalpy is the heat of the reaction (products minus reactants).
The enthalpy factor favors the forward reaction (the products) if ΔH is negative because the products are lower in energy and are thus more stable.
The reverse reaction is favored if ΔH is positive.

35. Forward or Reverse Favored?
Entropy is the degree of randomness or disorder.
The entropy factor favors the side of the reaction more gas molecules because gases have more entropy than liquids and solids.

36. Forward or Reverse Favored?
The forward reaction (product side) is favored if there are more gas molecules on the right side of the arrow.
The reverse reaction (reactant side) is favored if there are more gas molecules on the left side of the arrow.

37. Problem 2H2O (l) → 2H2 (g) + O2 (g) ΔH = +572 kJ
16) Does the enthalpy factor favor the forward or the reverse reaction? Why?
(ΔH is positive. Reactants are lower in energy and thus more stable. Reactants are favored, so enthalpy favors the reverse reaction.)

38. Problem 2H2O (l) → 2H2 (g) + O2 (g) ΔH = +572 kJ
17) Does the entropy factor favor the forward or the reverse reaction? Why?
(There are 0 gas moles on the left. There are 3 gas moles on the right. Entropy favors the side with more gas moles. Therefore, the product side is favored, i.e. the forward reaction.)

39. CO (g) + NO2 (g) → CO2 (g) + NO (g)
Problem
CO (g) + NO2 (g) → CO2 (g) + NO (g)
ΔH = kJ
18) Does the enthalpy factor favor the forward or the reverse reaction? Why?
(The forward reaction is favored because ΔH is negative. Products are lower in energy and thus more stable. )

40. CO (g) + NO2 (g) → CO2 (g) + NO (g) ΔH = - 226 kJ
Problem
CO (g) + NO2 (g) → CO2 (g) + NO (g) ΔH = kJ
19) Does the entropy factor favor the forward or the reverse reaction? Why?
(There are 2 gas moles on the left. There are 2 gas moles on the right. Therefore, neither side is favored.)

41. H2(g) + Cl2(g) ↔ 2HCl(g) + heat
Problem
H2(g) + Cl2(g) ↔ 2HCl(g) + heat
20) Does the enthalpy factor favor the forward or the reverse reaction? Why?
(Heat is a product which means the reaction is exothermic The forward reaction is favored because ΔH is negative. Products are lower in energy and thus more stable. )

42. H2(g) + Cl2(g) ↔ 2HCl(g) + heat
Problem
H2(g) + Cl2(g) ↔ 2HCl(g) + heat
21) Does the entropy factor favor the forward or the reverse reaction? Why?
(There are 2 gas moles on the left. There are 2 gas moles on the right. Therefore, neither side is favored.)

43. Energy Diagrams
The activation energy can be lowered by adding a catalyst.
effect of the catalyst

44. Energy Diagrams
The catalyst lowers the activation energy by providing an alternate pathway for the reaction to occur.
Enzymes are biological catalysts.

45. Expressing Reaction Rates
As you know, some chemical reactions are fast and others are slow; however, fast and slow are inexact, relative terms.
Chemists often need to be more specific.

46. Expressing Reaction Rates
We generally define the average rate of an action or process to be the change in a given quantity during a specific period of time.
The rate of a reaction can be described by the increase in the concentration of the products with time or the decrease in the concentration of the reactants with time.

47. Expressing Reaction Rates
Reaction rates cannot be calculated from balanced equations as stoichiometric amounts can.
Reaction rates are determined experimentally by measuring the concentrations of reactants and/or products in an actual chemical reaction.

48. Collision Theory
According to the collision theory, atoms, ions, and molecules must collide with each other in order to react.

49. Collision Theory
The following three statements summarize the collision theory.
1. Particles must collide in order to react.

50. Collision Theory
2. The particles must collide with the correct orientation.

51. Collision Theory
3. The particles must collide with enough energy to form an unstable activated complex, also called a transition state, which is an intermediate particle made up of the joined reactants.

52. Collision Theory
activated complex

53. Collision Theory
The minimum amount of energy that colliding particles must have in order to form an activated complex is called the activation energy of the reaction.
Particles that collide with less energy than the activation energy cannot form an activated complex.

54. Collision Theory
In an exothermic reaction, molecules collide with enough energy to overcome the activation energy barrier, form an activated complex, then release energy and form products at a lower energy level.

55. Energy
Reactants
Products
Reaction coordinate

56. Activated Complex or Transition State
Energy
Reactants
Products
Reaction coordinate

57. Activation Energy
Energy
Reactants
Products
Reaction coordinate

58. Reactants
Energy
Overall energy change
Products
Reaction coordinate

59. Collision Theory
In the reverse endothermic reaction, the reactant molecules lying at a low energy level must absorb energy to overcome the activation energy barrier and form high-energy products.

60. Factors Affecting Reaction Rates
The reaction rate for almost any chemical reaction can be modified by varying the conditions of the reaction.

61. Factors Affecting Reaction Rates
The reactive nature of the reactants affects the rate of a chemical reaction. Some reactions are naturally slow because the bonds involved are strong and unreactive or the electrons are tightly held.

62. Factors Affecting Reaction Rates
cont., The greater the number of bonds being broken and made (like double or triple bonds, the slower the reaction. 62

63. Factors Affecting Reaction Rates
Another important factor that affects the rate of a chemical reaction is the concentration of the reactants. Reactions speed up when the concentrations of reacting particles are increased. Increasing the number of reactants increases probability of collisions.

64. Factors Affecting Reaction Rates
continued. The rate of gaseous reactions can be increased by pumping more gas into the reaction container.

65. Factors Affecting Reaction Rates
Surface area of the reactants affects the rate of a chemical reaction. Increasing the surface area of reactants provides more opportunity for collisions with other reactants, thereby increasing the reaction rate.

66. Factors Affecting Reaction Rates
Temperature affects the rate of a chemical reaction. Generally, increasing the temperature at which a reaction occurs increases the reaction rate. Raising the temperature raises both the collision frequency and the collision energy.

67. Factors Affecting Reaction Rates
Adding a catalyst affects the rate of a chemical reaction. A catalyst is a substance that increases the rate of a chemical reaction without itself being consumed in the reaction. In fact, catalysts are not included in the chemical equation.

68. Factors Affecting Reaction Rates
A catalyst lowers the activation energy of a reaction by providing an alternate pathway for the reaction to occur (the blue line).

69. Factors Affecting Reaction Rates
Compressing gases affects the rate of a chemical reaction. When two gases react, compressing the gases (increasing the pressure) generally increases the rate of the reaction.