Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16General Chemistry Topic 6:Kinetics & Reaction RatesObjectives: By the end of this topic, you should:1. Be familiar with the concepts of kinetic and potential energy.2. Understand the concept of activation energy, and the energy needed to complete a chemical reaction.3. Understand the terms exothermic and endothermic, and be able to correlate them with heat flow between a system and its surroundings. 4. Be able to write the reaction rate expression for a simple reaction. 5. Understand the effects of temperature, concentrations and catalysts on reaction rates. Suggested reading: DTC, Chapter 7, pp. 234-242 (226-234)Homework problems: 7.21 (7.21), 7.22 (7.22), 7.49 (7.47), 7.50 (7.48), 7.55 (7.59), 7.56 (7.60), 7.59 (7.55), 7.64 (7.62)Kinetics and reaction ratesFor a chemical Rx to take place, molecules of the reactants must contact each other (collide) with enough energy to react and form new bonds.If molecules collide with enough energy to break existing bonds and form new bonds, the chemical Rx will occur.Activation Energy – energy required to break existing bonds and/or form new ones (allow the Rx to take place)If collision energy is less than activation energy, molecules bounce apart without reacting.Kinetics is the study of the rate (speed) at which chemical Rx occur.All molecules, ions, and atoms are constantly in motion (except at absolute zero). They move around and collide with each other and with other particles.As an example, consider the reaction of carbon (C) and oxygen (O2) to form carbon dioxide (CO2). C + O2 CO2What is involved in this reaction?Activation energy and chemical RxEnergy (both potential and kinetic) is involved in chemical RxThe carbon atoms and oxygen molecules must contact each other (collide) with enough energy for a reaction to occurThe electron configurations of the C and O atoms must be altered. - in the reactants, O atoms are covalently bonded together with a double bond (2 shared pairs of e-) - in the product, carbon dioxide, there are double bonds linking the 2 O atoms to CC O O+OCO+Activation energy and chemical RxC atoms and O2 molecules have a certain kinetic energy (related totheir speed) and potential energy (related to their chemical form). When they collide, kinetic energy is transformed to potential energyrelated to arrangements of electrons about C and O nuclei. - an unstable arrangement (unstable intermediate) is formed. Unstable intermediate may revert to the original configuration - C atom and O2 molecule – no CO2 is formed - energy level less than activation energyUnstable intermediate may transform into a stable (new) configuration - CO2 is formed - energy level greater than activation energyCollision Energylow highFraction of collisionsEactActivation energy and chemical RxMost collisions havethis much energyIn order for a reaction to occur, the combined energies of the reactants must reach a minimum value - the activation energy, which is specific to each reaction.Only a small fraction of all collisions produce enough energy to be successful.Only collisions with this much (or more) energyproduce the productRate of Rx  “green area” under curveC atoms and O2 molecules have a certain kinetic energy (related totheir motion) and potential energy (related to their chemical form). REVIEW: Activation energy and chemical RxC O O+Unstable intermediate may transform into a stable (NEW) configuration - CO2 is formed - energy level greater than activation energyOCOC + O2 CO2When they collide, kinetic energy is transformed to potential energyrelated to arrangements of electrons about C and O nuclei. - an unstable arrangement (unstable intermediate) is formed. UnstableIntermediateUnstable intermediate may revert to the original configuration - C atom and O2 molecule – no CO2 is formed - energy level less than activation energyCollision Energylow highFraction of collisionsEactREVIEW: Activation energy and chemical RxIn order for a reaction to occur, the combined energies of the reactants must reach a minimum value - the activation energy, which is specific to each reaction.Activation energy is amount of energy required to form new productOnly a small fraction of all collisions produce enough energy to be successful.Any collisions with this much (or more) energyproduce the productRate of Rx  “green area” under curveThe reaction rate tells us how often the collisions between reactants is resulting in formation of the stable (new) product.A number of factors affect the reaction rate: The rate of the Rx is proportional to the concentrations of the reactants. reaction rate  [C] [O2]  means “proportional to” or reaction rate = k [C] [O2] k = rate constant of the RxReaction rates and rate constantsAnother example: 2 CO + O2 2 CO2rate = k [CO]2 [O2] – because 2 molecules of CO react with 1 O2Reaction rates increase as concentrations of reactants increase. - collisions aren’t more energetic - more collisions (and so more that have > activation energy)1Temperature affects Rx ratesTemperature – reaction rates increase with increasing temperature.Temperature is actually a measure of the average kinetic energy of atoms and molecules in a sample.Increasing the temperature increases their kinetic energies, so the average energy of collision is greaterThis means the fraction of collisions which exceed the activation energy is also greater. This means the reaction goes faster (more reactants become products)An increase of 10C generally doubles or triples most reaction rates.Collision Energylow highFraction of collisionsEactAny collisions with thismuch (or more) energyproduce the productIncreased temperature(increased collision energy)Increased temperature causes more energetic collisionsReal reason why warming things up speeds things upReaction rate is faster at higher temperature because morecollisions are energetic enough to allow Rx (> Eact).2Collision Energylow highFraction of collisionsEactAny collisions with this much (or more) energyproduce the productCatalysts increase Rx rates by lowering activation energyCatalysts lower activation energy, so more collisions have enoughenergy to react. More successful collisions means faster Rx.Catalysts participate in chemical Rx, but are not themselves alteredPt &