Rates of reaction Flashcards
(23 cards)
Define reaction rates
how written
6.1
How fast a reaction proceeds
not constant
can be written as amount of product made or reactant consumed over time
Rate = -Δ[R]/ΔT
Rate = Δ[P]/ΔT
(g/s, mol/hr, ml/min, mol/L/s)
avg reaction rate vs inst reaction rate
6.1
avg- rate between 2 points , get secant, get slope
inst - get tangent, get slop of tangent
(C2 – C1)/(t2 – t1) = slope
Methods of measuring rates of reaction
6, when to use
6.1
volume of gas - (when one prod is gas )
colour change - spectrophotometer or colorimeter
pH - to measure H+ ions
conductivity - when ions are being made or used
change in pressure - when gases being made or used
mass - also gas?
Define collision theory
2 things needed
6.2
things must collide to have reaction
not all collisions cause reaction
Depends on
1. orientation of react
2. must have enough energy
Define activation eng
6.2
minimum eng required between 2 reactants for it to occur
unique to each reaction
difference between reactants and peak of pot eng graph
changes if the reaction of reversed
how it enthalphy measured on pot eng graph
6.2
difference between react and prods on graph
is reversed if the reaction is reversed
Define activation complex, transition state
6.2
chem species temporarly made from reactants, not final prod, does not make sense
aka transition state, the peak of the graph
Define catalyst (hetero and homo)
6.2
allows for a different pathway for the reaction
lowers act eng
het - different state than reactants
hom - same state as reactants
How to interpret maxwell-boltzmann graph
6.2
y axis is num of particles
X axis is eng
activation eng is along x axis, area under graph after activ eng line is amt of particls with enough eng
can be changed by different things
What are the 6 factors affectin reaction rate
and effect on boltzman and pot eng graph
6.2
1.Concentration of a solution - increases area under boltz graph
2.Surface area of the particles of a solid reactant - show diagram of large shape vs smaller shape, more collisions
3.Temperature - lowers peak of graph, moves it farther down x axis, more with enough eng, changes K
4.State/Phase - homogenous state of react is faster
5.Presence of a catalyst - lower act eng on graph, moves act eng up on boltz, Changes K
6.Nature of the reactants - gases are faster than solid, single bonds are faster,
Homogenous vs heterogenous catalysts
6.2
hom - catalyst in same phase as reactants
het - catalyst is diff phase than react
What are ways catalysts can help
6.2
*bend or stretch bonds to make them easier to break / react
*bring two reactants close together
*provide a microenvironment for reactions
What is Rate law
6.3
Equation that connects the rate of a reaction with the concentration of its reactants at a given temperature and pressure
What is the initial rate of reaction, how is it useful
how to show on conct vs time graph
6.3
the best data is collected at t= 0
the presence of product will effect the rate
draw tangent from t = 0
What does the magnitude of the rate constant and activation energy tell you about a reaction
6.3
k = constant, specific to reaction at certain temp, high mag means faster
smal act eng means reaction happens faster
How to determine units of k
6.3
Take the overall order and subtract one. Use this order for L and mol
1st order : unit of k is s-1 or 1/s
2nd order : unit of k is L/mol.s
3rd order : unit of k is L^2/mol2.s
What is an elementry step
3 things it can involve
6.3
many react have multi activated complex (multi stepped)
each indiv step is called elementry step
best guesses
can involve
1.Formation of different molecules or ions
2.A change in energy or geometry of starting materials
3.Cannot be broken down into smaller steps
what is a rate determining step
how to determine
6.3
Slowest step in complex reaction
look at molecularity, lower better
amt of molec
is it dependant on anything
What are reaction intermediates
6.3
Thing made and used up in elementry step, does not need to be imediate
How do you determine if reaction mech is possible
6.3
Each step must be elementary, involving no more than three reactants (and usually only one or two) molecules;
The slowest or rate-determining step must be consistent with the rate equation
The elementary steps must add up to the overall equation
How to tell if complex or simple
6.3
simple - one step
complex - multi step
Look at molecularity - more than 2 is complex usually
if molecularity and reaction order are same then simple
What is the molecularity of
2NO (g) -> N2(g) + O2(g)
6.3
bimolecular
What does the Arrhenius equation show
6.4
k = Ae^(-actEng/RT)
R is gas constant
T is in Kelvin
Temperature changes T
Catalyst changes act Eng
used to determine effect of change in temp or catalyst
calc before and after change, divide to see how many times faster
