Kinetics Flashcards
Rate of reaction equation and units (4)
Change in conc/time for change to occur
Mol/dm3/s
How fast a reactant is used up
How fast a product is formed
Gradient = change in y/change in x of tangent
What 3 things are required for particles to react
Collide
Enough NRG
right orientation
Steric factor
Atom group shapes influence reaction
Steric hindrance
Large atom group can get in the way of attacking species
Primary vs tertiary haloalkane reactions
SN2: primary, halogen dissociates and nuc joins
SN1: tertiary, halogen dissociates then nuc joins
Secondary: mixture
Activation energy
Minimum KE needed for particles to react/break reactant bonds
3 features of an activation energy curve
Bonds stretched
Parts have enough energy to break bonds
Separate parts can’t exists alone so form new bonds
Which states does the Maxwell Boltzmann theory apply to
Ga/liquid
Some slow
Some fast
Most in between
Maxwell Boltzmann theory
No particles have 0 KE
most have intermediate
No upper limit
Area under curve = total no. Particles
Maxwell Boltzmann graph features (5)
Fraction of particles with E NRG vs NRG E
Mode = peak
Mean = just past peak
Ea = line
X axis and line don’t touch
Name 8 methods of determine rate of reaction
Mass
Volume
Precipitate
Ph
Colorimetry
Clock reactions
Titrations
Electrical conductivity
Gas volume practical method (3)
Gas syringe, conical flask, reaction mixture, bunged delivery tube
Ideal gas equation
Change in mass practical method (2)
Digital balance, cotton wool bung (gas lost, no pressure)
Carbonate, H2 mass too small to record
Colorimetry practical method (4)
Light source, filter, sample tube, light meter
Accurate, quantitative measurement
Propanone + iodine
Brown —-> colourless
Precipitate practical method (3)
Less precise than colorimeter
Bscure black cross
Subjective
Ph practical method (2)
UI
H+ reacting/produced
Electrical conductivity practical method (2)
AC
Total no./type of ions in reaction changes
Clock reactions practical method (2)
Make oodine
Solution turns blue/black
Titrations practical method (4)
Pipette aliquots at reg intervals
Quench reaction (ice bath + carbonate)
Titrate aliquot against reac/prod
Find conc
Temperature effect on ROR
Increase temp
Increase KE
Increase speed
Increase frequency/success of collisions
Increase proportion of particles with Ea +
Increase ROR
Concentration effect on ROR
Increase conc
Increase no.particles/volume
Increase frequency/success of collisions
Increase ROR
Gas/pressure has the same effect
No change in proportion of particles in relation to Ea
Surface area effect on ROR
Olids
Increase no. Exposed particles
Increase frequency/success of collisions
Increase ROR
Catalyst effect on ROR
Increase ROR
Unchanged catalysts
Lower Ea, alternative reactant pathway
Don’t change Equm or enthalpy change
Increase proportion of particles with Ea +
Homogenous catalysts (4)
Same physical state as reactants
Catalyst + reactants —> intermediates —> products + catalyst
Ea to make intermediate < Ea to make products
Catalyst overall unchanged
Autocatalysts
Product of reaction acts as catalyst for same reaction
Increase product amount, increase ROR
Heterogeneous catalysts (5)
Diffrent physical state to the reactants
Reactants absorb onto SA of catalyst
Reaction bonds weakened/break
Radicals form —> new molecules —> desorb
Absorption —> reaction —> desorption
catalytic converters
CO, NO, Unburnt HCs
—> less harmful gases
Platinum, palladium, rhodium
Honeycomb structure, increase SA
Heterogenous catalyst poisoning
Cling to catalyst surface stronger than reactant
Prevent reaction
Catalyst economic benefits
Quicker product
Less energy than high temperatures
Direct reaction to more useful products
Rate equation
Relationship between chemical reaction rate and reactant concentration
Rate = k[A] x[B]y
K changes units
Rate: mol/dm*3/s
K size and rate relationship
K, large, fast
K, small, slow
Order 0 effect and graphs
No effect on rate by conc
Conc vs time: SL down
Rate vs conc: horizontal line
Order 1 effect and graphs
Linear effect on rate by conc
Conc vs time: curved line down
Rate vs conc: SL up
Order 2 effect and graphs
Square effect on rate by conc
Conc vs time: curved line down (steeper than order 1)
Rate vs conc: urged line up
Half life
Conc/time graph
Halve y axis increments
Order 0: decrease
Order 1: constant
Order 2: increase
Rate constant k
Constant of proportionality
Same for any reaction at a specific temp (increase/decrease)
Units vary depending on overall order of rate equation (make k subject, insert units and cancel)
Doesn’t change with conc/pressure
Changes with catalyst (increases)
Order
Power of reactant in rate equation
Overall order
Sum of powers of reactant Concs in rate equation
Identifying rate equation (3 methods)
Half lives
Rate/conc graphs (conc/time, tangents, find gradients, rate/conc, graph shape)
Initial rate (gradient of conc/time graph, all reactant Concs are known, compare initial Concs and rates)
Iodine clock equations
H2O2 + 2I- + 2H+ —> I2 + 2H2O
I2 + 2S2O32- —> 2I- + S4O62-
CH3COCH3 + I2 —> CH3COH2I + H+ + I-
Iodine clock steps
Continuous
H2SO4 vol/conc
Distilled water + starch
KI vol/conc
Na2S2O3 conc/vol
Swirl
H2O2 conc/vol
Time
Colorless —> blue/black
Repeat and vary KI vol
Quench aliquots and Titrate
Rate determining step (3)
Slowest step in reaction mechanism
Rate of overall reaction
Species in RDS are in rate equation (expect intermediates)
Reaction mechanism
Series of steps where reactants become products in chem reaction
SN2 (7)
Nucleophilic substitution
2 molecules in RDS
Primary haloalkanes
1 step mechanism
RX breaks and COH forms
Unstable carbocation (not true intermediate)
1 methyl group (low steric hindrance, attack immediately)
SN2
Nucleophilic substitution
1 molecule in RDS
Tertiary haloalkanes
2 step mechanism
RX breaks (RDS) THEN COH forms
Stable carbocation (true intermediate)
3 methyl group (high steric hindrance, attack slowly)
Secondary haloalkane rate equation
Provided
Indicates no. Molecules in RDS
Activation energy equation
K = Ae*(-Ea/RT)
LnK = lnA - Ea/RT
LnK = constant - (Ea/R) x (1/T)
K: rate constant
A: scaling factor
E: natural log
R: 8.31
T: temperature
Activation energy graph calulculations
LnK/(1/T)
SLG = -Ea/R
ROR inversely proportional to t
ROR proportional to 1/T
1/t or k can be used
Activation energy assumptions
A constant
Less valid over larger scales
More frequent collisions, insignificant effect on rate
Reversible reaction
Products of left to right can react to reform reactants
Closed system
Exchange energy with surroundings but not mass
Chemical equilibrium
Closed system where forward/backward rate are equal. No net conc/observable property change
Equilibria equations
Can be written either way, refer to substance NOT left/right
Kc
Equilibrium constant
[C]c [D]d (products)/ [A]a [B]b (reactants)
Reverse direction, 1/Kc
Units vary, cancel and simplify
0> greater left conc
0< greater right conc
Kc and states
Varying concentrations (aqueous/gases)
Solids - conc fixed by density, excluded
Liquids - fixed density/conc, excluded
Multiple liquids and Kc
Separate layers - fixed density/conc, excluded
Solution - dissolved, varied conc, included
Water - reactant + solvent, assume excess, exclude
Setting up an equilibria reaction
Specified mols
Left to achieve Equm
Find Equm mols
Use balance ratios
Use total vol —> Kc/concs
No unit cases don’t need volume, just use moles as they cancel out
Kp use
Gases
Pressure and conc are proportional
Define equilibria in terms of pressure
Partial pressure of each mixture gas
Kp expression
Ptotal = sum of all mixture gas partial pressures
F = proportion of fraction of moles of mixture gas
Pa = Ptotal x Fa
Unit = atm (100,000 Pa)
PCc PDd (products)/ PAa PBb (reactants)
Starting a Kp expression
Initial sums to mtotal
Find Equm amounts using mol ratios
Divide by mtotal
Get Px
Pressure effect
A —> 2B
(FbPtot)2 / FaPtot
Cancel
Fb2 x Ptot / Fa
Increase Ptotal, decrease fraction, decrease Fb, increase Fa, backward equilibria shift
Temperature effect
Forward Endo, increase temp, increase K, favour RHS
Forward exo, increase temp, decrease K, favour LHS
Rate effect
No relationship between Equm position and ROR
Catalyst doesn’t change Equm position, speeds reaction up
Increase both reactions equally
