kinetics Flashcards
what is collision theory?
the reactant species must collide with sufficient energy for a reaction to occur (a successful collision)
what is the activation energy?
the minimum amount of energy which particles need to collide and start a reaction
what is the transition state in a reaction?
the intermediate- where the bonds are half made and half broken
what is a ‘species’ in a chemical reaction?
a useful term which which covers any sort of particle- molecule, ion or free radical
what does the maxwell-boltzmann curve show?
the spread of energies that molecules of a gas or liquid have at a particular temperature
what is the x and y axis on a maxwell boltzmann curve?
y= number of molecules with a particular energy
x= energy
what are the two energies marked on a maxwell-boltzmann curve?
most probable energy and mean energy (not at the peak of the curve)
why should the energy distrubution curve go through the origin?
there are no molecules with no energy
why do a few particles have low energies?
collisions cause some particles to slow down
what does the area under the curve represent?
total number of particles
a few particles have energy greater than the Ea, but the curve never meets the x axis, why?
there is no maximum energy for molecules
how does the distribution curve change as temperature increases?
the distribution shifts towards having more molecules with higher energies (shifts right) so more particles have Ea
both Emp and mean energy shift to right also, but number of molecules with those energies decreases
at higher temps molecules have wider ranges of energies
total area under curve remains constant as number of particles is constant.
what effect dies removing half the molecules have on the distribution curve?
decrease number of particles with Ea
area under curve halves
Emp and Ea stay same
what effect does decreasing the volume of the container have on the distribution curve?
no effect because their are still the same number of molecules
and same number of molecules with enough energy to react
what effect does adding a catalyst have on the distribution curve?
curve stays same
Ea is lowered so more particles have Ea
Emp stays same
what effect does adding an inert gas have on the distribution curve?
no effect because there are still the same number of molecules and same number of molecules with enough energy to react
what are the 5 factors affecting rate of reaction?
-temp
-conc
-pressure
-catalyst
-surface area
how does increasing temp increase rate of reaction?
particles have more kinetic energy so increases frequency of collisions. proportion of collisions with enough energy to meet the Ea also increases
how does increasing concentration increase rate? (same for increasing pressure)
there are more reacting particles per unit of volume so frequency of successful collisions is greater
how does a catalyst increase rate of reaction?
catalysts are not used up in a reaction. They provide an alternative route for the reaction which has a lower Ea so more particles have the Ea
how does increasing surface area by powdering the reactants increase rate?
a reaction only occurs if the particles in the gas or liquid collide with particles in the solid. Increasing SA increases chances of collisions as more particles exposed.
what can catalysts react to form?
intermediates
what are the two types of catalysts?
heterogenous and homogenous
explain the difference between a heterogenous and homogenous catalyst?
heterogenous- when the catalyst is in a different phase (state) from the reactants
homogenous- when the catalyst is in the same phase (state) as the reactants.
give an example of a heterogenous catalyst and a homogenous one?
hetero- haber process uses a solid iron catalyst but reactants are gases
homo- Cl radical in the atmospheric destruction of ozone is in a gas state and so are reactants
what is the equation and catalyst in the haber process?
N2 +3H2 (reversible arrows) 2NH3
Fe catalyst
what do catalytic converters in cars do and what are they shaped with?
remove pollutants from exhaust fumes. Honeycomb structure with thin layer of Pd, Pt and Rh metals
what are the 4 steps of adsorption and desorption in heterogenous catalysis?
- reactant gases adsorb (not absorb) to the solid catalyst
- the internal covalent bonds weaken and break
- new covalent bonds in the products form
- the products desorb from the solid catalyst and leave it unchanged.
where are zeolite catalysts used and what is their structure?
in the petrochemical industry
open pore structure that ions and molecules can fit into. They are then confined within this space which changes their structure and reactivity.
what do enzymes do?
speed up metabolic reactions by acting as a biological catalyst
