Kinetics Flashcards
Collision theory
Particles must collide in the correct orientation
Particles must have sufficient minimum energy when colliding
Rate of reaction
Dependent on frequency of effective collisions
Factors affecting rate of reaction
- Temperature
- Concentration
- Surface area
- Catalyst
Concentration of aq or g reactants affecting rate
higher conc, more collisions per second per volume, higher probability of effective collisions/ higher frequency of effective collisions, rate of reaction increase
Surface Area of solid reactant affecting rate
smaller particle size, higher SA of contact between particles, more collisions, higher frequency of effective collisions, rate of reaction increase
Boltzmann distribution
- Temperature
2. Catalyst
Temperature affecting rate
temp decrease, ave KE of reacting particles decrease,fraction of particles with KE> or equal to Ea decrease, less collisions, lower frequency of effective collisions, rate of reaction decreases
Catalyst
speeds up rate of reaction
provides an alternative path with lower Ea for reaction to occur
remains chemically unchanged after the end of a reaction
exerts no effect on enthalpy change of reaction
Catalyst affecting rate
provides alternative pathway with lower Ea, fraction of molecules with KE> or equal to Ea increase, more collisions, frequency of effective collisions increase, rate of reaction increase
Energy profile diagram
- shows stability of reactants/products
- number of steps
- Magnitude of Ea in reaction
Steps in reaction
- Single
2. Two
Single step
Reaction takes place in a single step
there will only be a single transition state (hump)
Two steps
Reaction takes place in 2 steps
there will be two transition states (2 humps)
a reaction intermediate is formed and is short lived as it is consumed in the subsequent step. Stable enough to be isolated for a short period of time
Activation energy
Minimum amount of energy required to initiate a chemical reaction. A reaction which occurs in a different pathway will have different activation energy
Transition state
Most unstable state hence appears at the maximum energy. at this stage it is equally likely to form reactants and products.
Types of catalyst
- Heterogeneous
- Homogeneous
- Auto
- Biological
Heterogeneous
Catalyst and reactants are in a different phases
usually involving solid catalyst and gaseous reactants. Increases rate of reaction by weakening existing bonds in all reactant particles by bringing them closer hence providing alternative pathway of lower Ea
usually one step reaction
Examples: Fe or Fe2O3 (Haber process)
Pt, Pd or Rh (catalytic converters) remove harmful gases and converts them to unharmful gases through redox reactions
Mode of action in heterogenous catalyst
- Adsorption
- Reaction
- Desorption
Adsorption
reactant particles adsorbed onto surface of catalyst (stick to) weak interactions formed between reactant and catalyst
weakening existing bonds in all reactant particles by bringing them closer hence providing alternative pathway of lower Ea
Reaction
occurs at faster rate on surface of catalyst as reactants brought closer to one another and existing interactions in reactant molecules weakened lowering Ea