Reaction rates and chemical equilibrium Flashcards
in order for a reaction to occur between particles
- particles must collide and must be in a correct orientation
- collisions must be sufficiently energetic for the activation energy barrier to be overcome and for bonds to be broken
what is ea
the minimum energy required for a reaction to occur
if particles have E
no reaction occurs
if particles have E>EA
reaction should occur
boltzmann distribution curve
- distribution curve at 0,0
- very few particles have high energy
- no maximum energy so line does not meet x axis
- only particles with E>EA can react on collision, shown by the shaded area on the right of the Ea
rate of reaction
the change in concentration of reactants or products with time
if concentration of reactant increases
- rate increases
- more particles in a given volume
- collision frequency increases
as pressure increases
particles are closer together - greater number in a given volume and collide more frequently
surface area
- reducing particle size for the same mass increases the surface area of a solid
- more particles exposed on the surface, greater frequency of collisions between the solid particles and mobile reactants, increasing rate
at higher temperatures
- paarticles have more energy so greater number of particles have E>EA
- greater proportion of collisions lead to a successful reaction
- particles move faster and collide more frequently
catalyst
- speeds up rate but remains chemically unchanged
- provides an alternative reaction route with a lower activation energy
- a greater proportion of particles will have energy greater than the new lower EA
- a greater proportion of particles will have enough energy to react when they collide and a greater proportion of collisions will be succesful
homogenous catalyst
have the same physical state as the reactants
homogenous catalysts in esterification reactions
sulphuric acid
heterogenous catalysts
have different physical state to the reactants (usually solid with gaseous reactants)
example of a heterogenous catalyst
- iron in haber process
- v2o5 in contact process
- zieglar-natta catalyst (ticl4 and al2(ch3)6 in addition polymerisation
how do heterogenous catalysts work
- reactant gas adsorbs on the catalyst surface
- bonds in the reactant molecules are weakened, lowering the EA
- reaction occurs, reactant bonds break and product bonds form
- product molecules desorb from the catalyst surface
benefits of catalysts
- allow lower temp use while still achieving same rate of reaction
- reduce cost of reaction
- reduce the demand for non renewable fossil fuels
- reduce the amount of co2 through combustion of fossil fuels
- are not used up and so require only small amount
- allow a wider choice of reaction routes, possibly permitting use of a higher atom economy choice
equilibrium state
- can be approached from either direction
- dynamic so both reactions occur at same time
- consistency of macroscopic properties eg temp pressure
- only exists in a closed system
dynamic equilibrium
rate of forwards reaction is equal to rate of backwards reaction and concentration of all reactants remain constant
le chateliers principle
the position of equilibrium will shift so as to minimise the effect of any change in conditions
if more products are formed
the equilibrium has shifted to the right
if more reactants are formed
the equilibrium has shifted to the left
the contact process
2so2 + o2 ⇆ 2so3
increasing temp contact process
- position of equilibrium will changein order to minimise the effect by absorbing energy and therefore decreasing the temperature
- endothermic reaction favoured so backwards reaction
and equilibrium will shift to left
increasing pressure contact process
- will change in order to minimise the effect by reducing the number of gas molecules present
- therefore will favour the forwards reaction and will shift to the right due to fewer moles of product
increasing concentration contact process
- will change in order to decrease the concentration of that species so the reaction where the species is a reactant will be favoured
- eg if so2 is increased the forwards reaction will be favoured and equilibrium position will shift to the right
catalyst equlibrium
will not affect the position of equilibrium but does increase the rate of reactions equally
if forward reaction if exothermic and temp increases
- shifts to LHS and more reactants made
if forward reaction is exothermic and temp decreases
shifts to rhs and more products made
if forwards reaction is endothermic and temp increases
shifts to rhs and more products made
if forwards reaction is endothermic and temp decreases
shifts to lhs and more reactants made
haber process
materials: hydrogen (CH4 + h2O -> CO2 + 4H2)
N2 (fractional distillation of liquid air)
n2 + 3H2 ⇆ 2NH3
haber process conditions
450˚c 150 atm finely divided iron
temperature haber process
- forwards reaction is exothermic therefore favoured by low temperatures
- low temps result in low rate of reaction
- compromise temperature chosen that gives a reasonable yield at a reasonable rate
pressure haber process
- forwards reaction produces fewer moles of gas therefore favoured by higher pressures
- this gives a faster rate of reaction
- high pressure is expensive to produce and are dangerous
- compromise pressure used that gives a good yield and rate at a sensible cost
catalyst haber process
increases rate of both reactions equally. permits use of lower pressure and temperature whilst still achieving overall rate
increases yield
what is the equilibrium constant Kc
a mathematical expression for the relationship between concentrations of the substances present once a reaction has reached equilibrium
if Kc is large
equilibrium is to the right so lots of products and high yield
if Kc is small
equilibrium is to the left so lots of reactants and low yield
