7.3 Chemical Kinetics Objectives 3 and 4 Chemistry

Essential Questions What is chemical kinetics? What is a reaction rate? What is collision theory and how does it relate to rate of reactions? What is activation energy and how does it relate to rate of reaction? How do you draw and interpret energy diagrams? What affects reaction rates and why?

Chemical Kinetics Do all reactions occur at the same speed? No different reactions have different rates Chemical kinetics focuses on the speed or rate of reactions A reaction rate is measure of how quickly reactants are converted into products.

Reaction Rate (don’t need to write) 18.1 Rates of chemical reactions are often measured as a change in the number of moles during an interval of time. As time passes, the amount of reactant (red squares) decreases and the amount of product (blue spheres) increases. Rates of chemical reactions are often measured as a change in the number of moles during an interval of time. Interpreting Diagrams Assuming equal time intervals between the boxes, how can you tell that the rate of conversion of reactant to product is not constant throughout this reaction?

Collisions Atoms/molecules must collide in order to react Do all collisions result in a reaction? No because only some collision successfully lead to formation of products

Collision Theory Collision theory states that atoms, ions, and molecules can react to form products when they collide with both enough kinetic energy and correct orientation If both conditions are not met there is no reaction Reaction rate and the number of collisions are proportional

Effective Collision (cont)

Activation Energy Activation energy (Ea) is the minimum amount of energy required to react Ea is the kinetic energy required for the collision to result in a reaction Reaction rate is inversely proportional to Ea

Activated Complex/Transition State Brief moment where particles are in a transition state They are not reactants or products, but one big complex

Exothermic Energy Diagram Energy of reactants greater than energy of products ΔH = Hproducts – Hreactants & Ea = activated - reactants c is the enthalpy change (∆H) and would be -100 KJ a is the Ea and would be 50 KJ

Endothermic Energy Diagram Energy of products greater than energy of reactants c is the enthalpy change (∆H) and would be 200 KJ a is the Ea and would be 300 KJ

Energy Diagram Practice Label reactants, products, ∆H, Ea What is the value for Ea and ∆H? Is the reaction exo or endo? Ea = 10 KJ; ∆H = -25 KJ Exo R Ea ∆H P

Rate of Reactions What affects the rate of a reaction? Particle size/surface area Concentration Catalyst Temperature

Particle Size For solids and liquids the more surface area (smaller particle size) the faster the reaction rate More surface area allows for more collisions to occur which is why the rate increases

Concentration The greater the concentration the greater the reaction rate Why? There is more that can react and more collisions

Catalyst Catalyst are agents that increase the rate of reaction Catalyst are not reactants because they are still left at the end of a reaction

Catalyst (cont) Catalyst lowers Ea required to react by providing a different mechanism or path for a reaction to occur that requires less energy

Catalyst (cont) Why does lowering the Ea cause a rxn’s rate to increase? The lower the Ea the less energy required to react which makes it easier for it react and thus speeds up the reaction

Temperature Reaction rate increases and decreases with temperature If you increase temperature will the reaction go faster or slower? Why? Faster because the molecules move faster and collide more often leading to more reactions and have more energy when they collide.

Essential Questions What is chemical kinetics? What is a reaction rate? What is collision theory and how does it relate to rate of reactions? What is activation energy and how does it relate to rate of reaction? How do you draw and interpret energy diagrams? What affects reaction rates and why?

7.3 Tracked Assignment 7.3 Worksheet