Presentation Transcript
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