1. Chemical Kinetics
Deals with rates and mechanisms of chemical reactions.

2. Collision Theory:
in order to react, particles must effectively collide with each other.
During effective collisions, electrons shift between atoms
old bonds are broken and new ones form.

3. What makes a collision effective?
Particles must have:
the right speed or kinetic energy
the correct orientation to each other.
Correct Orientation:

4. Activation Energy: minimum amount of energy needed for a reaction to occur

5. Reaction Rate: speed a reaction occurs.
Rate depends on frequency and effectiveness of collisions between reactant particles.
Measured by how fast reactants are used up or product produced.

6. Rate Determining Step: the slowest step.
Reaction Mechanisms: series of steps by which particles rearrange themselves to form products.
Most products don’t form from a single collision
Rate Determining Step: the slowest step.
Usually means that step has the highest activation energy

7. Factors Affecting Reaction Rate
Temperature:
Increasing temp. increases
reaction rate
Faster particles move = greater frequency of collisions
Greater chance collision will be effective
More particles will have enough energy to overcome activation energy

8. Factors Affecting Reaction Rate
Concentration:
Greater conc. of reactants = greater chance particles will collide effectively
More conc. = less space between particles
Ex: Zn in 1M HCl versus Zn in 6M HCl

9. Factors Affecting Reaction Rate
Pressure: (gases only)
Compressing a gas will increase concentration of reactant particles.
Increase in concentration allows for more collisions

10. Factors Affecting Reaction Rate
Surface Area: (of a solid reactant)
Greater the surface area, the more the reactant particles are exposed to each other for possible collisions.
Ex: steel wool vs. steel bar, block of wood vs. sawdust

11. Ways to increase surface area:
Grind into fine powder
Stir or agitate to disburse particles

12. Factors Affecting Reaction Rate
Nature of Reactants:
Simpler reactants will react faster than ones with many complex bonds.
Fastest to React:
(aq) ionic compounds
have simple ions that are disbursed in solution
Covalents are usually slower to react.
Gases usually react faster than
substance in the liquid or solid phases.

13. Factors Affecting Reaction Rate
Catalysts:
speed up the rate of a chemical reaction without being altered. (ex: enzymes)
Works by providing an alternate reaction pathway that require less energy.
Lowers the amount of activation energy needed to start reaction

14. Inhibitors: slow down chemical reactions
Can tie up reactants in side reactions
Can block active site in enzymatic catalysts
Can provide alternate reaction pathway with higher activation energy requirements.

15. Sample Regents Questions

16. Given the balanced equation representing a reaction:
2HCl(aq) + Na2S2O3(aq)--> S(s) + H2SO3(aq) + 2NaCl(aq)
Decreasing the concentration of Na2S2O3(aq) decreases the rate of reaction because the
(1) activation energy decreases (2) activation energy increases (3) frequency of effective collisions decreases (4) frequency of effective collisions increases

17. As the temperature of a chemical reaction in the gas phase is increased, the rate of the reaction increases because
(1) fewer particle collisions occur (2) more effective particle collisions occur (3) the required activation energy increases (4) the concentration of the reactants increases

18. Why can an increase in temperature lead to more effective collisions between reactant particles and an increase in the rate of a chemical reaction?
(1) The activation energy of the reaction increases. (2) The activation energy of the reaction decreases. (3) The number of molecules with sufficient energy to react increases. (4) The number of molecules with sufficient energy to react decreases.

19. At 20. °C, a 1. 2-gram sample of Mg ribbon reacts rapidly with 10
At 20.°C, a 1.2-gram sample of Mg ribbon reacts rapidly with 10.0 milliliters of 1.0 M HCl(aq). Which change in conditions would have caused the reaction to proceed more slowly?
(1) increasing the initial temperature to 25°C (2) decreasing the concentration of HCl to 0.1 M (3) using 1.2 g of powdered Mg (4) using 2.4 g of Mg ribbon

20. A 5. 0-gram sample of zinc and a 50
A 5.0-gram sample of zinc and a 50.-milliliter sample of hydrochloric acid are used in a chemical reaction. Which combination of these samples has the fastest reaction rate?
(1) a zinc strip and 1.0 M HCl(aq) (2) a zinc strip and 3.0 M HCl(aq) (3) zinc powder and 1.0 M HCl(aq) (4) zinc powder and 3.0 M HCl(aq)

21. Thermodynamics Study of the energy involved in chemical reactions.
Particles must have a certain energy to react
During collisions KE (of motion) is converted to PE (stored energy in chemical bonds)

22. PE Diagrams Show how PE changes during a chemical rxn. PE Reactants
PE Products
Activation Energy:
energy needed to start
energy required to form “activated complex”

23. Activated Complex: intermediate between reactant and product
Found at top of energy hill
Must be formed for rxn to occur
Unstable and short lived (very high in energy)
Contain bonds in process of being broken/formed

24. ∆H = PE products – PE reactants
Heat of Reaction (∆H)
Overall net change in energy (enthalpy) for reaction
Difference between PE of products and reactants
∆H = PE products – PE reactants
What is ∆H for this reaction?

25. Endothermic vs. Exothermic
Exothermic: (∆H negative)
Products have less energy than reactants
Energy given off
Endothermic: (∆H positive)
Products have more energy than reactants
Energy taken in
Usually requires more activation energy than exothermic reactions

26. Determine the following: PE reactants PE Products PE activated complex
Forward activation energy
Reverse activation energy
∆H for forward rxn
Exo or endo?

27. Affects of a Catalyst Catalysts lower:
activation energy for both the forward and
reverse reactions
Energy of the activated complex
How much energy was saved by using a catalyst?

28. Catalysts Don’t Change ∆H
They do NOT EFFECT:
PE of reactants or products.
∆H (overall heat lost or gained)

29. Which interval on this diagram represents the difference between the potential energy of the products and the potential energy of the reactants?   
(1) 1     (2) 2    (3) 3      (4) 4

30. Which statement is true about energy in this reaction?
The reaction is exothermic because it releases heat.     
The reaction is exothermic because it absorbs heat.
The reaction is endothermic because it releases heat.
The reaction is endothermic because it absorbs heat.

31. Which statement best describes how a catalyst increases the rate of a reaction?
The catalyst provides an alternate reaction pathway with a higher activation energy.
The catalyst provides an alternate reaction pathway with a lower activation energy.
The catalyst provides the same reaction pathway with a higher activation energy.
The catalyst provides the same reaction pathway with a lower activation energy.

32. Which statement correctly describes the energy changes that occur in the forward reaction?
(1) The activation energy is 10. kJ and the reaction is endothermic.
(2) The activation energy is 10. kJ and the reaction is exothermic.
(3) The activation energy is 50. kJ and the reaction is endothermic.
(4) The activation energy is 50. kJ and the reaction is exothermic.

33. In a chemical reaction, the difference between the potential energy of the products and the potential energy of the reactants is defined as the
activation energy
ionization energy
heat of reaction
heat of vaporization

34. Which information about a chemical reaction is provided by a potential energy diagram?
(1) the oxidation states of the reactants and products
(2) the average kinetic energy of the reactants and products
(3) the change in solubility of the reacting substances
(4) the energy released or absorbed during the reaction

35. Which statement correctly describes an endothermic chemical reaction?
(1) The products have higher potential energy than the reactants, and the ΔH is negative.
(2) The products have higher potential energy than the reactants, and the ΔH is positive.
(3) The products have lower potential energy than the reactants, and the ΔH is negative.
(4) The products have lower potential energy than the reactants, and the ΔH is positive.

36. S(s) + O2(g) → SO2(g) + energy
Given the reaction:
S(s) + O2(g) → SO2(g) + energy 
Which diagram best represents the potential energy
changes for this reaction?

37. Reference Table I Heats of Reaction Table (∆H ):
Shows various reactions and whether they are endothermic or exothermic.
- ∆H exo
+ ∆H endo
The energy involved is for the relative number of moles indicated in the balanced equation

38. Using Table I Table shows several types of reactions:
Top 6 reactions are combustions (- ∆H )
Middle reactions are synthesis
∆H values are showing ∆H of “formation” of the compound from its elements
Bottom reactions show dissolving of various ionic salts (some are exo, some endo)
Last reaction is neutralization

39. Using Table I Ex: 2CO(g) + O2(g) → 2 CO2 + 566 KJ Exothermic reaction
∆H = KJ
The reverse of this reaction would be endothermic ∆H = +566 KJ (←)
If just one mole of CO used, half the energy would be produced ∆H = - 283KJ

40. 2 H2(g) + O2(g) --> 2 H2O(l) + 571.6 kJ
Given the reaction:
2 H2(g) + O2(g) --> 2 H2O(l) kJ
What is the approximate ΔH for the
formation of 1 mole of H2O(l)?
kJ        
kJ  
kJ     kJ

41. Chemical cold packs are often used
to reduce swelling after an athletic
injury. The diagram represents the
potential energy changes when a
cold pack is activated.
Which lettered interval on the
diagram represents the potential
energy of the products? 
diagram represents the heat of
reaction?
Identify a reactant listed in
Reference Table I that could be
mixed with water for use in a
chemical cold pack.

42. According to Table I, which salt releases
energy as it dissolves? 
KNO3       
LiBr
NH4NO3 
NaCl

43. CH4(g) + 2 O2(g) --> 2 H2O(l) + CO2(g)
Given the reaction:
CH4(g) + 2 O2(g) --> 2 H2O(l) + CO2(g)
What is the overall result when CH4(g) burns
according to this reaction?
Energy is absorbed and ΔH is negative.      
Energy is released and ΔH is negative.
Energy is absorbed and ΔH is positive.  
Energy is released and ΔH is positive.

44. According to Table I, which potential energy diagram best represents the reaction that forms H2O(l) from its elements?
(1)
(2)
(3)
(4)