OZ - kinetics *02 Flashcards
what is activation enthalpy?
the energy that pairs of molecules must possess to react when they collide
how must particles collide to react
collision theory
collide in the right direction (facing each other the right way)
collide with the minimum amount of energy needed (activation energy)
enthalpy profile diagram - endo
reactants below products
enthalpy profile diagram - exo
reactants above products
Boltzmann distribution graphs - what does it show?
shows how molecules in a gas dont all have the same amount of energy
Boltzmann distribution graphs - what does it look like?
curve starts at (0,0) cause no molecules have zero energy
a few molecules are moving very slowly
most molecules are moving at a moderate speed
some molecules have more energy than the activation enthalpy
—> these are the only ones that can react
increasing temp. effect on rate - Boltzmann Distribution graph
particles, on average, have more kinetic energy and will move faster
so a greater proportion of molecules will have the activation energy and be able to react
- changes the shape of the curve —> it pushes to the right
increasing temp. effect on rate - collision theory
particles on average have more kinetic energy and will move faster, so a greater proportion of molecules will have the activation energy and be able to react
so the frequency of collision that result in a reaction will increase
—> more successful collision per unit time (increases rate)
also because more kinetic energy there are more collisions that take place
(collide more are more likely to react when they collide)
increasing conc. effect on reaction rate
increasing conc. of reactants in solution = the particles are closer together on average
so they collide more often
more collision = more chances to react
increasing pressure effect on reaction rate
increase pressure of a gas = particles closer together on average
they collide more often = more chances to react
catalysts effect on reaction rate
catalysts lower the activation enthalpy by providing a different way for the bonds to be broken and remade
activation enthalpy lower = more particles have enough energy to react
how do you find the rate of reaction - experiments
monitor loss of reactant or formation of product
- vol. of gas produced
- loss of mass
- change in pH
- temperature change
- taking samples at regular intervals and analysing them by titration
how do you find the rate of reaction - graph
use data from the experiment to plot a graph
- time on x-axis, change on y-axis
to find the reaction at a particular time
- draw a tangent to the curve at that time: gradient of tangent = rate of reaction
- rate units = change / unit of time
rate of reaction - how does it change over a experiment?
rate always fastest at the beginning
as a reaction continues the conc. of reactants decreases
so there will be less frequent collisions between reactant particles and the rate will decrease
role of catalysts
to increase rate of reaction by providing an alternative reaction pathway with a lower activation enthaply
catalyst chemically unchanged at the end of the reaction
catalysts - two types
homogenous
heterogenous
homogeneous catalyst - what?
a catalyst that is in the same state as the reactants
homogeneous catalyst - how does it work?
forms intermediates
- speeds up rate by forming one or more intermediate compounds with the reactants.
- –> the products are then formed from the intermediate compounds - activation energy needed to form intermediates and then products LOWER than activation energy needed to make products directly from reactants
- the catalyst is reformed at the end and is chemically unchanged
homogeneous catalyst - enthalpy profile
profile would have two humps
- lower than the uncatalysed reaction hump!
intermediates are formed at the trough between the humps
homogeneous catalyst - Boltzmann Distribution graph
line for activation energy moves back
- so there are molecules that have enough energy to react in catalysed reaction but not enough to react when uncatalysed
the curve stays the same
homogeneous catalyst - example
chlorine radical
breakdown of ozone
