23
AG Definitions Rate of reactions
AG
Definitions
Rate of reactions
AG
Rates of Reactions
• The rate of reaction is • the change in concentration• per unit time of any one reactant or
product.
Exam Q (Hons)2003 Q72004 Q82007/Q92011 /Q5
AG
Factors affecting rate• nature of reactants
• particle size
• concentration
• temperature
• catalysts
AG
Equations to know
• Write each equation then check • Decomposition of hydrogen peroxide using
manganese dioxide as catalystMnO2
• 2H2O2 2H2O + O2
• Sodium thiosulfate and hydrochloric acid• Na2S2O7 + 2HCl S + 2NaCl + SO2 +H2O
AG
Rate Graphs• Concentration v. ( 1 /Time )• or• Temp v. ( 1 /Time )
• ( 1 /Time )used as Rate and Time inversely related
• (shorter time means faster rate)
• be careful with units of 1/time
AG
Catalyst
• is a substance that alters the rate of reaction
• but is not consumed in the reaction.
Exam Q (Hons)2003 Q7‘07/Q9
AG
Homogeneous catalysis
• occurs when the reactants and the catalyst are in the same phase.
• example =?liquids
• KI catalyses 2H2O2 2H2O + O2 (iodine snake)
• And any enzyme
AG
Heterogeneous catalysis
• occurs when the reactants and the catalyst are in different phases.(NB must be phases not states)
• example = ?• Al2O3 (solid)catalyses • ethanol (gas) ethene
Exam Q (Hons)
‘07/Q4
Methanol methanal using platinum
AG
Autocatalysis
• occurs when one of the products of the reaction catalyses the reaction.
• Example = ?• Mn2+ ions in KMnO4 titrations
(purple changes to colourless more quickly as titration proceeds)
AG
Mechanism of Catalysis• Intermediate Formation theory
• Surface Adsorption theory
• Know details of each and evidence of intermediate formation theory
AG
Enzymes
• Are biological catalysts made of protein • Examples of homogeneous catalysis• Need to know 2 examples
– Amylase catalyses conversion of starch to maltose– Catalase catalyses conversion of hydrogen peroxide to
hydrogen and water
AG
Learning Check• Do I know Definition for
1. Rate of reaction2. Catalyst 3. Homogeneous catalysis 4. Heterogeneous catalysis 5. Auto catalysis 6. Two mechanisms of catalysis
Press enter to continue
AG
Catalytic converter• Catalysts = ?• Pt + Pd + Rh on honeycomb surface
(ceramic) • Gases in
CO NO NO2
hydrocarbons • Gases out
CO2 and N2 and H20
AG
Learning Check• Do I know
1. 3 metals in Catalytic converter 2. 4 wastes in exhaust fumes 3. Problem of each4. What each is converted to 5. What poisons catalytic converter 6. Type of catalysis occuring in catalytic converter
Press enter to continue
AG
Collision Theory
• for a reaction to occur the reacting particles must collide with each other
• a collision only results in a product being formed if a certain minimum energy is exceeded (called activation energy)
AG
Effective Collision
• Is one in which a reaction occurs
• The activation energy has been reached or exceeded.
Exam Q (Hons)2009 Q9
AG
Activation Energy
• is the minimum energy which colliding particles must have for a reaction to occur (minimum energy required for effective collisions between particles)
Exam Q (Hons)2006/Q72009/Q9
AG
Activation Energy 2• Catalysts lower the activation energy of a
reaction
Without catalyst
with catalyst
Compare E act
AG
Energy Profile Diagram
• Sketch an energy profile diagram for an endothermic reaction.
• Press enter when ready and• It should look like this
AG
EndothermicEnergy Profile Diagram
0 2 4 6 8 10 120
1
2
3
4
5
6
7
Energy
Reactants
Products
Energy InActivation
Energy
Note – axes should be labelled Time (x) and energy (y)Curve should be smooth !
AG
Energy Profile Diagram
• Sketch an energy profile diagram for an exothermic reaction.
• Press enter when ready and• It should look like this
AG
ExothermicEnergy Profile Diagram
0 2 4 6 8 10 120
1
2
3
4
5
6
7
Energy
Reactants
Products
Energy Out
Activation Energy
Note – axes should be labelled Time (x) and energy (y)Curve should be smooth !
AG
Learning Check• Do I know Definition for
1. Effective collision2. Activation energy
Can I draw energy profile diagram for 1. Exothermic reaction2. Endothermic reaction3. Either of above with catalyst
The End
