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

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

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

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

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

Reaction Mechanisms: series of steps by which particles rearrange themselves to form products. Reaction Mechanisms: series of steps by which particles rearrange themselves to form products. Most products don’t form from a single collision Most products don’t form from a single collision Rate Determining Step: the slowest step. Rate Determining Step: the slowest step. Usually means that step has the highest activation energy Usually means that step has the highest activation energy Honors Extra practice hysical_Chemistry/Kinetics/Ra te_Laws/Reaction_Mechanis ms/Rate-Determining_Step

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

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

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

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. 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 Ex: steel wool vs. steel bar, block of wood vs. sawdust

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

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

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

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

Sample Regents Questions

Given the balanced equation representing a reaction: 2HCl (aq) + Na 2 S 2 O 3(aq) --> S (s) + H 2 SO 3(aq) + 2NaCl (aq) Decreasing the concentration of Na 2 S 2 O 3(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

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

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.

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

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)

Given the balanced equation representing a reaction: Zn (s) + 2HCl (aq) ==>H 2(g) + ZnCl 2(aq) Which set of reaction conditions produces H 2(g) at the fastest rate? (1) 1.0-g lump of Zn (s) in 50. mL of 0.5 M HCl at 20. o C (2) 1.0-g lump of Zn (s) in 50. mL of 0.5 M HCl at 30. o C (3) 1.0 g powdered Zn (s) in 50. mL of 1.0 M HCl at 20. o C (4) 1.0 g powdered Zn (s) in 50. mL of 1.0 M HCl at 30. o C

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)

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”

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

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?

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 LrRM&feature=player_embedded

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

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?

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

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

Which statement is true about energy in this reaction? (1) (1) The reaction is exothermic because it releases heat. (2) (2) The reaction is exothermic because it absorbs heat. (3) (3) The reaction is endothermic because it releases heat. (4) (4) The reaction is endothermic because it absorbs heat.

Which statement best describes how a catalyst increases the rate of a reaction? (1) (1) The catalyst provides an alternate reaction pathway with a higher activation energy. (2) (2) The catalyst provides an alternate reaction pathway with a lower activation energy. (3) (3) The catalyst provides the same reaction pathway with a higher activation energy. (4) (4) The catalyst provides the same reaction pathway with a lower activation energy.

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.

In a chemical reaction, the difference between the potential energy of the products and the potential energy of the reactants is defined as the (1) (1) activation energy (2) (2) ionization energy (3) (3) heat of reaction (4) (4) heat of vaporization

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

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.

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

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

Using Table I Table shows several types of reactions: Table shows several types of reactions: Top 6 reactions are combustions (- ∆H ) Top 6 reactions are combustions (- ∆H ) Middle reactions are synthesis 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

Using Table I Ex:2CO(g) + O 2 (g) → 2 CO KJ Exothermic reaction 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

Given the reaction: 2 H 2 (g) + O 2 (g) --> 2 H 2 O(l) kJ What is the approximate ΔH for the formation of 1 mole of H 2 O(l)? (1) (1) kJ (2) (2) kJ (3) (3) kJ (4) (4) kJ

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? Which lettered interval on the 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.

According to Table I, which salt releases energy as it dissolves? (1) (1) KNO 3 (2) (2) LiBr (3) (3) NH 4 NO 3 (4) (4) NaCl

Given the reaction: CH 4 (g) + 2 O 2 (g) --> 2 H 2 O(l) + CO 2 (g) What is the overall result when CH 4 (g) burns according to this reaction? (1) (1) Energy is absorbed and ΔH is negative. (2) (2) Energy is released and ΔH is negative. (3) (3) Energy is absorbed and ΔH is positive. (4) (4) Energy is released and ΔH is positive.

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