topic 9 Flashcards
activation energy
the minimum energy which particles need to collide to start a reaction
Maxwell-Boltzmann distribution curve
- Emp : most probable energy (peak)
- mean energy : below the peak of the curve
- energy distribution must go through the origin as there are no molecules with no energy
- area under the curve is the total number of particles present
effect of temperature increase on maxwell curve
distribution will shift towards having more molecules with higher energies.
at higher energies both the Emp and mean energy shift to higher energy values - number of molecules with those energies decrease.
rate of reaction
change in concentration of a substance in unit time
measuring rate of reaction
the gradient of a concentration time graph - draw a tangent to the curve
effect of increasing concentration and pressure
at higher conc there are more particles per unit volume hence the particles collide with greater frequency
doubling concentration and rate?
if conc/rate is doubled then the number of particles is doubled per unit volume and the frequency of effective collisions is doubled
increased concentration - effect on graph
shape of energy distribution curve doesn’t change - peak is at the same energy - curve is higher and area under curve is greater because there are more particles
effect of increasing temperature
the energy of the particles increases - they collide more frequently and more often with energy greater than the activation energy
peak is lower and curve is more to the right
effect of increasing surface area
results in successful collisions to occur more frequently between the reactant particles and hence increases the rate of the reaction
catalyst
catalysts increase the rate of reaction without getting used up
effect on Maxwell curve of using a catalyst
lowers the activation energy therefore more particles will have energy that is greater than the activation energy so there will be a higher frequency of effective collisions
heterogeneous catalyst
usually solids and they are in a different phase from the reactants
effect of pressure on heterogeneous catalysis
increasing pressure has no effect
why does pressure change have no effect on heterogeneous catalysis
because the reaction takes place on the surface of the catalyst so the active site on the catalyst surface is already saturated with reactant molecules
adsorption
the process by which a solid holds molecules of a gas or liquid or solute as a thin film
how does adsorption at the active site result is catalytic action
the active site is where reactants adsorb on to the surface of the catalyst. This results in the bonds within the reactant molecules to become weaker. There will also be a higher concentration of reactants at the solid surface - leading to higher collision frequency
environmental benefits of catalysts
- catalysed reactions can occur at lower temperature so less fuel needed and fewer emissions from fuels
- enables use of an alternative process with higher atom economy hence fewer raw materials needed and less waste products produced
how can a reaction go to completion if few particles have energy that is greater than activation energy
particles can gain energy through collisions
effect of industrial catalysts
- speed up the rate
- hence allow lower temperatures to be used hence lower costs
- no effect on equilibrium
