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Chemical Kinetics Unit 11 – Chapter 17
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Chemical Kinetics Studies reaction rate
How quickly or how slowly chemical reaction occurs Also, examines reaction mechanism How reaction occurs
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Reaction Rate The change in concentration of a reactant or a product per unit time Units: mol/Ls Concentration changes as reactants are consumed and products are formed
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Collision Theory Explains why reactions occur and how reaction rates can be changed Reacting substances (atoms, ions, formula units, molecules): Must collide in order to react Must collide with the proper orientation Must collide with sufficient energy to form the activated complex Activated complex: temporary, unstable arrangement of atoms that may form products, or may break apart, returning to reactants
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Collision Theory Activation energy: minimum amount of energy required to transform reacting particles into the activated complex (AKA transition state) and lead to reaction Symbolized as Ea Sources of activation energy: flame, spark, high temperature, radiation
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Ineffective vs. Effective Collisions
Particles may collide but not lead to reaction Because of incorrect orientation Because of insufficient energy
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Ineffective vs. Effective Collisions
Only when particles collide in proper orientation and with sufficient energy does reaction proceed +
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reactants → products + energy
Exothermic Processes or – ∆E Activated Complex reactants → products + energy
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Exothermic Processes Energy is ____________
The sign of ∆H is ____________ Products have ________ energy than the reactants The activation energy of the forward reaction is __________ than the activation energy of the reverse reaction released negative less lower
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Endothermic Processes
or + ∆E Activated Complex reactants + energy → products
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Endothermic Processes
Energy is ____________ The sign of ∆H is ____________ Products have ________ energy than the reactants The activation energy of the forward reaction is __________ than the activation energy of the reverse reaction absorbed positive more higher
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Chemical equilibrium Established when the forward reaction proceeds at the same rate as the reverse reaction Reactants are still transforming into products and products are still reverting to reactants But, the overall concentration of reactants and products is unchanged
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Spontaneity of Reaction
2nd Law of Thermodynamics: spontaneous processes always proceed in such a way that the entropy of the universe increases Related to Enthalpy (H): heat content of a system at constant pressure Entropy (S): degree of randomness or disorder or particles in a system Gibbs’ free energy (G): energy available to do useful work
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Spontaneity of Reaction
Type of reaction/process ∆Gsystem ∆Suniverse Spontaneous negative positive Nonspontaneous
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Spontaneity of Reaction
–∆G: reaction is spontaneous +∆G: reaction is nonspontaneous ∆G tells nothing about the reaction rate Only indicates whether a reaction will proceed
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Factors Affecting Reaction Rate
The Nature of Reactants Some substances react more readily than others, leading to faster reaction Recall periodic trend of reactivity for metals and nonmetals Which reaction would be faster? Ca + 2H2O → Ca(OH)2 + H2 2Na + 2H2O → 2NaOH + H2
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Factors Affecting Reaction Rate
Concentration Increasing the concentration of one or more reactants increases reaction rate Explained by collision theory Since more particles are present, there is a greater chance of the particles colliding
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Factors Affecting Reaction Rate
Concentration Burning Metals paper burns faster in pure oxygen than in air
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Factors Affecting Reaction Rate
Surface Area Increasing the surface area (decreasing particle size) increases reaction rate Explained by collision theory Greater surface area makes more of the substance available for collision with the other reactant steel wool rusts faster than a nail
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Surface Area
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Factors Affecting Reaction Rate
Temperature Increased temperature increases reaction rate Explained by collision theory Increased temperature Increased KE More frequent collisions Particles move faster More energetic collisions Increased likelihood of collisions occurring in proper orientation
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Factors Affecting Reaction Rate
Temperature Substances reacts faster in hot water than in cold water Food cooks faster at higher temperatures
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Rates of Reaction Collision Theory
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Factors Affecting Reaction Rate
Use of Catalysts Catalyst: substance that increases the reaction rate without being consumed in the reaction Reaction pathway for the catalyzed reaction has a lower activation energy (Ea) than the uncatalyzed reaction More particles now have enough energy to react Lower activation energy results in faster reaction
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Exothermic Reaction with Catalyst
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Endothermic Reaction with Catalyst
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Factors Affecting Reaction Rate
Use of Catalysts Catalysts can hold reactants together and help break bonds
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Catalysts Homogeneous catalyst: exists in the same phase as the reacting particles Heterogeneous catalyst: exists in a different phase than the reacting particles
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Catalytic Converters CO + Unburned Hydrocarbons + O2 CO2 + H2O
2NO + 2NO2 2N2 + 3O2
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Biological catalysts Enzymes
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Enzyme Catalysis Enzymes: large molecules – usually proteins – that catalyze biological reactions Chemical reactions in biological systems often occur too slowly to be effective Adjusting other factors, such as concentration or temperature, can destroy living cells
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Factors Affecting Reaction Rate
Use of Catalysts The substrate fits into the active site of the enzyme, much as a key fits into a lock The enzyme provides a platform on which the reaction can take place
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Factors Affecting Reaction Rate
Use of Catalysts Inhibitors: catalysts that slow the rate of reaction Food preservatives function as inhibitors, slowing the rate of oxidation
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Applications to Everyday Life
Cooking and freezing Food preservation Reducing air pollution Chemical production Body metabolism Termites and wood decay Medicine
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