![• However, NOBr2 is an intermediate
• Unstable, unknown concentrations
• Rate law in respect to an intermediate is not desirable
• Represent [intermediate] in initial [reactants] ! With some
assumptions](https://image.slidesharecdn.com/unit5kineticspartiiupload-250130062710-795f5716/85/Unit-5-Kinetics-Part-II-III-upload-Collision-Model-Energy-Profile-Mechanisms-Rate-Law-Catalysis-39-320.jpg)
Unit 5 Kinetics Part II & III upload - Collision Model, Energy Profile, Mechanisms & Rate Law, Catalysis
- 1. Unit 5 Kinetics Part II & III Collision Model, Energy Profile, Mechanisms & Rate Law, Catalysis
- 4. Kinetic Molecular Theory • Particles are moving randomly • Temperature is related to the average kinetic energy of the particles • And thus temperature is related to the speed of the particles
- 5. The Collision Model • Molecules must collide to react. • Models molecules as projectiles moving in random directions with a fixed average speed that is determined by temperature. • As these projectiles collide, they bounce off one another, preserving their kinetic energy and momentum. • Sometimes, there can be Reaction Reaction = collisions of two particles with • enough energy to cause a reaction (activation energy) • correct orientation to form products
- 7. Effective collision – orientation • not every collision will result in a chemical reaction • ineffective collision occurs when • there isn't enough force, • molecules are moving too slowly, and/or the • molecules aren't aligned right
- 8. Collision energy => speed =>Temp • The faster the particles are moving, the higher chance they collide, the higher the rate of reaction
- 9. Activation energy • According to the collision model, the energy for the molecules to react comes from the kinetic energies of the colliding molecules • The kinetic energy of the molecules can be used to stretch, bend, and ultimately break bonds, leading to chemical reactions • If the kinetic energy is too low, the molecules simply bounce off – although they may be in the correct orientation • i.e. there is minimum of energy that particular molecule has to have in order for reaction to happen – Activation energy
- 10. Higher proportion of molecules have enough activation energy to react Reaction rate ⬆️ Activation Energy
- 14. Rate constant • The rate constant depends on • the magnitude of Activation Energy (Ea); generally, the lower the value of Ea is, the larger the rate constant and the faster the reaction. • the temperature – in general, higher temperature, higher rate of reaction
- 15. The Arrhenius Equation • k = rate constant • A = the frequency of correctly oriented collisions between the reacting species • Ea = activation energy (in terms of energy per mole) • R = universal gas constant • T = temperature (in Kelvin)
- 17. Factors affecting rate of reactions • Concentration • Increasing the concentration of reactants generally increases the rate of a reaction. • Higher chance of collisions between reactant molecules • Temperatures • ⬆️temp KE, particles KE > Activation energy, effective ⬆️ ⬆️ ⬆️ collision • Nature of the reactions • Some reactions just happen faster than other reactions by nature • Some reactants are more reactive • e. g. Burning is faster than rusting
- 18. • State of reactants • Gas > Liquid > Solid • Surface areas of reactants • ⬆️surface area effective collision ⬆️ • Presence of catalysts • A catalyst can be defined as a substance that increases the rate of the reaction without actually participating in the reaction. • increases the speed of reaction in both forward and reverse reaction by providing an alternate pathway which has lower activation energy.
- 20. • The reaction mechanism is the series of elementary steps by which a chemical reaction occurs. • The sum of the elementary steps must give the overall balanced equation for the reaction • The mechanism must agree with the experimentally determined rate law
- 21. Elementary reactions • An elementary reaction is a chemical reaction that occurs in a single step and involves only a single molecule or a group of atoms • the most basic type of chemical reaction • Can be first order or second order • involve the breaking of some bonds and the formation of other bonds
- 23. Elementary reaction’s rate law
- 24. • But if the reaction is an elementary reaction, you can use the molecularity to predict the order of reaction
- 26. Exercise • Assume the following reaction is an elementary reaction with a single step: • What would be the predicted Rate Law? • What is the overall order of reaction?
- 27. • Predicted Rate Law is • However, experimental data shows the reaction have the following rate law: • The reaction does NOT have a single step elementary mechanism.
- 30. Non elementary reaction example
- 34. Rate-Determining Step • In a multi-step reaction, the slowest step is the rate- determining step. It therefore determines the rate of the reaction. • The experimental rate law must agree with the rate- determining step
- 36. Rate Law of reaction with a Slow initial step
- 39. • However, NOBr2 is an intermediate • Unstable, unknown concentrations • Rate law in respect to an intermediate is not desirable • Represent [intermediate] in initial [reactants] ! With some assumptions
- 40. • NOBr2 either goes forward to becomes NOBr or goes backward to NO + Br2 • Because Step 2 is slow, we assume most falls back as reverse step 1 • Now you have Step 1 and Reverse-Step 1 happening at the same time Equilibrium ➡️
- 41. • Assuming it is exactly an equilibrium, • forward reaction rate = reverse reaction rate
- 42. Exercise • Which step in the reaction mechanism is the rate- determining (slowest) step?
- 43. Exercise
- 44. Catalyst • Catalyst: A substance that speeds up a reaction without being consumed • Increase the number of effective collisions – e.g. correct orientations • Provide a reaction path with lower Activation Energy • Enzyme: A large molecule (usually a protein) that catalyzes biological reactions. • Homogeneous catalyst: Present in the same phase as the reacting molecules. • Heterogeneous catalyst: Present in a different phase than the reacting molecules.
- 45. Catalysts lower the EA • Catalyst can covalently bonds with reactants in the first step to lower the activation energy
- 47. • Because activation is lowered, more particles have enough energy to react • Acid-base catalysis: Sometimes acid (H+) and base (OH-) can catalyse a reaction this way
- 48. Heterogeneous catalyst • exists in a phase different from the phase of the reactant molecules • usually as a solid in contact with either gaseous reactants or reactants in a liquid solution • Usually composed of Metal or Metal oxides • Initial step: adsorption of reactants