Kinetics & Equilibrium Ted.ed

Review  A chemical equation describes a chemical change (reaction). 2NO(g) + O 2 (g)  2NO 2 (g)BEFOREReaction!Reaction!After

Reaction Rates HHHHow fast does a reaction go to completion? –R–R–R–Reactions occur at different rates EX: Burning of hydrogen  fast EX: Rusting of iron  slow EX: Convert coal to diamond  very slow RRRReaction rate – the “speed” of a reaction CCCCollision theory: a reaction can occur when molecules collide if: –T–T–T–They have enough kinetic energy –T–T–T–They are colliding frequently –T–T–T–They have the right orientation

Collision Theory  Activation Energy – the minimum amount of energy required to start a reaction. –The energy of a molecular collision must be at least as much as the activation energy. –Anything less, the molecules will simply bounce apart without a reaction

Potential Energy Diagram  All reactions have an energy profile: Potential Energy Diagram Reaction Coordinate  Energy  Activation Energy 1.Reactants absorb energy in order to collide and react 2.The activated complex releases energy to become products 3.If the products have less energy than the reactants, then  Energy was released  Reaction is Exothermic reactants Activated complex products

Potential Energy Diagram Reactions that gain more energy than released are called endothermic Reactions that gain more energy than released are called endothermic The energy of the products is higher than the reactants The energy of the products is higher than the reactants If energy is absorbed, then the reaction is endothermic If energy is absorbed, then the reaction is endothermic Reaction Coordinate  Energy  Reactants Products +H+H Activation Energy

Factors that Affect Rates of Reaction 1.Temperature – average kinetic energy –Reactions run faster at high temperatures.  Increasing temperature means faster molecules  More collisions per second  Collisions with greater energy  Activation energy can be reached

Factors that Affect Rates of Reaction 2.Concentration –Increasing solute concentration increases the rate of reaction –Higher concentration means more molecules colliding  Ex: Zn(s) + 2HCl(aq) ZnCl 2 (aq) + H 2 (g)  Ex: Zn(s) + 2HCl(aq) → ZnCl 2 (aq) + H 2 (g)  6.00 M HCl solution will “eat” Zinc much faster than 1.00 M HCl solution –Explains why we usually mix reactants with one reactant in great excess, it keeps reaction rate high for entire reaction.

Concentration Mg + H 2 SO 4  MgSO 4 + H 2 Set-Up 1 Add 1 mole of Mg to 1 mole of H 2 SO 4. Result Mg and H 2 SO 4 are both completely used up, but the reaction gets very slow close to the end. Set-Up 2 Add 1 mole of Mg to 5 moles of H 2 SO 4. Result 4 moles of H 2 SO 4 remain, but the reaction stays fast all the way through.

Concentration  Which bumper car ride will result in more collisions per minute for the red car?

Factors that Affect Rates of Reaction 3.Particle Size –The smaller the reactant particles, the faster the reaction. –Smaller particles have greater surface area  Grinding and/or dissolving solids will increase the surface area

Particle Size  Aluminum powder bears a warning that it is an explosion hazard. Aluminum foil bears no such warning. Why is Al powder so dangerous when Al foil is not? –Powdered Al has a much greater surface area exposed to oxygen. –The oxidation of aluminum is highly exothermic. –With powdered aluminum, a highly exothermic reaction can occur at a much greater rate.

Particle Size These two blocks have the same total volume, but the one on the right has been broken into 1000 equal pieces. This increases its surface area by 60 times. 1 meter 10 cm

Particle Size Available to react Not available to react

Factors that Affect Rates of Reaction 4.C atalyst aaaa substance that speeds up a reaction by lowering the activation energy of a reaction –N–N–N–Not consumed in the reaction. –W–W–W–Written above the reaction arrow. EEEExample: Platinum metal is often used as a catalyst to convert methanol vapors into formaldehyde. CH 4 OCH 2 O + H 2

Catalysts  Enzymes are proteins that act as biological catalysts. –speed up many biochemical reactions that would take far too long to happen naturally. –Enzymes function best in specific temperature ranges.  Human enzymes work best around 37ºC.  Too high a temperature will 'denature' an enzyme and destroy its functionality

Catalysts and Activation Energy Reaction Coordinate  Potential Energy  Uncatalyzed Catalyzed

Factors that Affect Rates of Reaction 5.Chemical Nature of Reactants  breaking bonds absorb energy and slows reactions  Reactants with many bonds react slower Combustion of sucrose  C 12 H 22 O O 2 → 12 CO H 2 O  Many bonds to break  Reacts slowly  2Na + Cl 2 → 2NaCl  Few bonds to break  Reacts quickly