3.2.2 Reaction rates Flashcards
activation energy definition
minimum energy particles need to collide with for a reaction to proceed
effect of concentration on rate of reaction
higher conc
more particles per unit volume
particles collide with greater frequency
higher frequency of successful collisions
rate of reaction definition
change in concentration of a substance in unit time
what does the gradient of a line represent on a conc time graph
rate of reaction
initial rate
where reaction is fastest
how do you calculate the rate from a graph
draw a tangent to the curve then change in y/ change in x
catalyst definition
substance that increases the rate of reaction without getting used up, provide an alternative route or mechanism with an alternative pathway with a lower activation energy so more molecules can collide successfully
2 types of catalysts
heterogeneous
homogeneous
heterogeneous catalysis
in a different phase to the reactants
usually solids, reaction occurs at the surface of the catalyst
homogeneous catalysis
where the reactants are in the same phase as the catalyst, reaction proceeds through an intermediate
benefits of catalysts
speed up rate without getting used up
can lower temperatures and pressures used which can save energy costs and less CO2 burned
can enable different reactions to be used with better atom economy and reduced waste/less undesired products
often enzymes and generate very specific products
what are the different ways to measure the rate of a reaction
change in volume of gas change in mass titrate sample mixtures with acid, alkali sodium thiosulphate colorimetry change in electrical conductivity
measuring change in volume of gas
this works if there is a change in the number of moles of gas in an equation
can use a gas syringe to measure
measuring change in mass
works if there is a gas that is allowed to escape
works better with heavy gases such as CO2
titrating samples of reaction mixture with acid,alkali, sodium thiosulphate etc
- small samples are removed from the reaction mixture
- quench to stop reaction by diluting with water/cooling/adding a reagent that reacts with one of the reactants
- titrate with a suitable reagent
colorimetry
if one of the reactants or products is coloured then colorimetry can be used to measure the change in colours in the reaction mixtures
measuring change in electrical conductivity
can be used if there is a change in the number of ions in an equation
describe the Maxwell-boltzmann distribution curve
x axis: collision energy then has Ea drawn as a line from the curve down to the x-axis
y axis: fraction of molecules with energy
curve goes from 0 up to a peak then gradually flattens back down
NEVER TOUCHES 0 ON THE X AXIS
what doesthe area under the maxwell-botlzmann curve represent
the number of particles present
what must the maxwell-boltzmann curve never do
touch the x axis other than at the beginning
how can a reaction go to completion if only few particles have energy greater than Ea
particles gain energy through collisions
increasing temperature effect on the maxwell boltzmann curve
distribution shifts to have more molecules with higher energies
molecules have a wider range of energies
peak is lower
end bit is higher
area under the curve should remain constant
explain the increasing temperature effect on the maxwell boltzmann curve
at higher temperatures the energy of the particles increases
collide more frequently and more often with energy greater than the activation energy, more collisions results in a reaction
as temperature increases the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy so the frequency of collisions increases
effect of increasing surface area
increasing surface area will cause collisions to occur more frequently between the reactant particles and this increases the rate of reaction
effect of catalysts
if the activation energy is lowered then more particles will have greater energy than the activation energy so there will be a higher frequency of effective collisions, reaction will be faster
need to draw the Ea line further up the curve
