Rates of Reaction :Rates of Reaction Chapter 26


Measuring Rates of Reaction :Measuring Rates of Reaction No too interested in accurate timing More interested in comparisons


Rates as rx proceeds? :Rates as rx proceeds? Many reactions tend to slow down Occurs as reactants get used up


Factors Affecting Rx Rates :Factors Affecting Rx Rates Collision Theory


Demolition Derby Illustration :Demolition Derby Illustration Rate of wipeouts depends on: How crowded the field is How fast the cars travel Size of the cars Strength of the cars Geometry of collisions (head-on or side-on etc)


Demolition Derby Illustration :Demolition Derby Illustration Application to reactions: How crowded concentration Speed temperature Size surface area Strength stability of reactants Geometry accounts for randomness


Explanations :Explanations Concentration – particles are closer and more likely to collide  faster rx Temperature – faster – more collision impact  faster rx Surface area – more chance of contact  faster rx Strength & Geometry – can’t do much to change this – depends on the substances


Activation Energy :Activation Energy Most rxs not spontaneous Usually require something to start Petrol – tiny spark Coal – roaring fire Provides an energy barrier for the rx Higher temp  more particles have energy to overcome the energy barrier


Distribution of particle energy :Distribution of particle energy Energy of particles (average speed) Number of particles (frequency) A B Normal distribution at 25oC


Distribution of particle energy :Distribution of particle energy Energy of particles (average speed) Number of particles (frequency) A B Normal distribution at 55oC


Activation Energy –Exothermic Rx :Activation Energy –Exothermic Rx Energy Reactants Products Before rx After rx Activation Energy H


Activation Energy –Endothermic Rx :Activation Energy –Endothermic Rx Energy Reactants Products Before rx After rx ActivationEnergy H


Activation complex :Activation complex Intermediate step – more energy Activation complex Eg. H2 + ½ O2  H2O First H2 and ½ O2 split into individual atoms


Reaction Mechanisms :Reaction Mechanisms Single step rx – only 1 activation complex Many rxs involve multiple steps Substances formed after each step – intermediates Each step also has an activation complex with its own activation energy


Multiple Step Rx :Multiple Step Rx Energy ActivationEnergy H Intermediates Activated complexes


Rate determining step :Rate determining step Multi-step rx – step with highest activation energy slowest Rate determining step


Catalysts :Catalysts Speeds up a rx by providing an alternate pathway with a lower activation energy Does not get used up in the rx – can be used again and again Small amount of catalyst can make a big difference Can speed up a rx by 108 to 1011 times Catalysts may be damaged physically


Hurdles analogy :Hurdles analogy Uncatalysed pathway – few particles have enough energy to go over Catalysed pathway – most particles have enough energy to go over


Catalysts :Catalysts Catalysts may be altered physicallyeg may melt, break into bits etc Catalysts may be poisoned – if a chemical attaches to it and prevents the catalysis


Catalysis and the Haber process :Catalysis and the Haber process ½ N2 + 1½ H2  NH3 Very slow reaction Catalyst of iron and a mixture of other substances works best Requires a large surface area Depend on adsorbtion – sticking to surface Catalyst can last for 8 years Can be poisoned by H2S


Enzymes :Enzymes p 439


Enzymes cf Catalysts :Enzymes cf Catalysts Enzymes - proteins “Biological catalysts? Specificity Mild conditions Denaturing Image from www-news.uchicago.edu/.../06/061011.ied.shtml Human insulin degrading enzyme


How enzymes work :How enzymes work Substrate Active site Shape Specific Images from: www.chem4kids.com/files/bio_enzymes.html


Catalysts in Everyday Life :Catalysts in Everyday Life Examples For polymerisation For hydrogenation For producing ammonia For reducing air pollution from cars


Catalytic Converter :Catalytic Converter


Catalytic Converter :Catalytic Converter


The end :The end