Chapter 6-Chemical kinetics Flashcards
What factors need to happen in order for a reaction to occur?
+ The collision must hv. sufficient kinetic energy to overcome repulsion between the particles electric clouds when they approach e.o
+ Particles must collide with the correct geometrical alignment with the reacting parts colliding with e.o (steric factor)
+ The collisions needs to have sufficient energy that is above the Ea energy to be successful
Factors affecting collision rate (2)
1) Concentration/pressue
2) Surface area
Factors mainly affecting the proportion of particles with required Ea?
1) Temperature
2) Catalyst
What are the axis on the Maxwell-Boltzmann distribution curve labelled?
x-axis: Energy
y-axis: Probability of this energy
What does the area under a Maxwell-Boltzmann distribution curve representative of?
The total # of particles
→ in a closed system the area MUST NOT change
What does increasing the temperature affect?
+ Increases the freq. of collisions
BUT
+ The increase of the proportion of molecules with
E > Ea is more important!
Catalyst
A substance that can increase the rate of a chemical reaction without undergoing any overall change
Rate of reaction
The change in the conc. of product divided by the change in time
→ which is equal to the rate at which the reactants = consumed
Rate = Δ[P]/Δt = Δ[R]/Δt
How does catalyst work?
Speed up a reaction by providing an alternative reaction mechanism/pathway with LOWER Ea!
→ a greater proportion of collisions will hv. the required Ea for the reaction by the new mechanism = rate increases
The effect of light on reactions
+ reactant particles absorb light energy = brings abt:
1) excitation of one of the reactants (as in the case with silver halide [black and white film photography])
OR
2) Breaking of a bond (s.a halogen-halogen bond in halogenation of alkanes = which initiates the reaction)
How is the rate expression usually measured? (As it can ONLY be determined experimentally!)
By measuring the reaction rate whilst varying the conc. of one species at the same time as the conc. of another species being held constant.
→ Isolating variables
What is NOT involved in the rate eq?
Solids and Liquids!
Gradient (m) of the graph of conc. against time gives?
Rate of reaction!
Pseudo First Order reactions?
Reaction keeping 1 conc. much smaller than the others = basis for v. powerful technique to simplify rate equations.
1st order reactions undergo what? (like radioactive decay)
Exponential decay with a half life
