The Rates and Mechanisms of Chemical Reactions Kinetics The Rates and Mechanisms of Chemical Reactions SC4. Obtain, evaluate, and communicate information about how to refine the design of a chemical system by applying engineering principles to manipulate the factors that affect a chemical reaction. b. Construct an argument using collision theory and transition state theory to explain the role of activation energy in chemical reactions. c. Construct an explanation of the effects of a catalyst on chemical reactions and apply it to everyday examples

We Are Talking About Reaction Rates Speed of any event is measured by a change that occurs per unit of time. The speed of reaction (i.e. reaction rate) is measured as a change in concentration (Molarity; M) of a reactant or product over a certain timescale. Time is the independent variable (x-axis) and concentration is the dependent variable (y-axis) Reaction rate is expressed in M/s

The 3 Fundamental Questions of Chemical Reactions What happens? Answer given by balanced chemical equation and stoichiometry To what extent does it happen? Answer deals with chemical equilibrium which we will study in a later unit How fast and by what mechanism? Chemical kinetics

Why? Important Examples Chemical Kinetics is very important for biological (your life), environmental (our lives) and economic (industry) processes. Biological: Large proteins (aka Enzymes) increase the rates of numerous reactions essential to life. Environmental: The maintenance or depletion of the ozone layer depends on the relative rates of reactions that produce or destroy ozone. Economic: The synthesis of ammonia (NH3) from N2 and H2 depends on rates of reactions. Fertilizer industries use catalysts to speed up these rates for economic reasons.

Exothermic Reaction Coordinate Diagram Process where energy is released as it proceeds. Heat is given off to surroundings. Reactants  Products + Energy

Endothermic Reaction Coordinate Diagram Process where energy is absorbed as it proceeds. Heat is consumed and surroundings become cooler. Reactants + Energy  Products

Factors that affect KINETICS All based on COLLISION THEORY: Collision theory: For a reaction to occur, the atoms or molecules must collide with one another with enough energy (activation energy) and must collide in the right orientation. Enough particles have to collide with enough activation energy and in the right orientation, or they will not react. The particles will just bounce off each other.

Factors that affect KINETICS Concentration of reactants Temperature Presence of a catalyst Surface area Agitation Nature of reactants

Factor 1: Concentration Collision theory: For a reaction to occur, the atoms or molecules must collide with one another with enough energy (activation energy) and must collide in the right orientation. FACTORS: Concentration of reactants If you increase concentration (Molarity), the rate of reaction increases. Why? There are more molecules which increases the number of collisions altogether; however, there are better chances that molecules will collide in the right orientation.

Factor 2: Temperature FACTORS: Temperature Collision theory: For a reaction to occur, the atoms or molecules must collide with one another with enough energy (activation energy) and must collide in the right orientation. FACTORS: Temperature Temperature is an averaged kinetic energy of molecules so if you increase temperature, you increase kinetic energy. This means you increase the number of collisions Heat supplies the energy to allow the reaction to proceed (i.e. overcoming the activation energy barrier)

Factor 3: Presence of Catalyst Collision theory: For a reaction to occur, the atoms or molecules must collide with one another with enough energy (activation energy) and must collide in the right orientation. FACTORS: Presence of a catalyst Catalyst assist a reaction and increase the reaction rate without being consumed in the reaction. Adding a catalyst decreases the activation energy which means more molecules will have enough energy to react.

Factor 3: Presence of Catalyst

Factor 4: Surface Area FACTORS: Surface Area Collision theory: For a reaction to occur, the atoms or molecules must collide with one another with enough energy (activation energy) and must collide in the right orientation. FACTORS: Surface Area Increased surface areas of molecules/particles will increase the rate of reaction. This means to break into smaller particle sizes by grinding or crushing a mixture of reactants. More places to react give better chances for collisions in the right orientation.

Factor 5: Agitation FACTORS: Agitation Collision theory: For a reaction to occur, the atoms or molecules must collide with one another with enough energy (activation energy) and must collide in the right orientation. FACTORS: Agitation Stirring or shaking a reaction will increase the reaction rate. When agitating, you are introducing energy into the reaction and thus giving molecules/particles more energy to react (overcome activation energy barrier). Your mechanical energy is converted to kinetic energy.

Factor 6: Nature of Reactants Collision theory: For a reaction to occur, the atoms or molecules must collide with one another with enough energy (activation energy) and must collide in the right orientation. FACTORS: Nature of reactants Reactants whose bonds are weaker have a lower activation energy and thus a higher rate of reaction. All chemical reactions involve bond breaking and bond making. Bond breaking occurs on reactant side. Collisions between reactants that require less kinetic energy are needed to break weaker bonds (i.e. smaller activation energy)