06 Reaction Kinetics Flashcards
Rate of Reaction, ROR
Definition
The change in concentration of a particular reactant, rxt, or product, pdt, [X] per unit time.
Common Units in order:
1. mol dm-3 s-1
2. mol dm-3 min-1
3. mol dm-3 h-1
Rate of Reaction, ROR
Formula
[X]/ t
[X] = concentration of rxt/ pdt
t = time taken
Expressing ROR
- [X]-t graph
a. rxt-t graph
b. pdt-t graph
Derivations from ROR graphs
- Instantaneous Rate
- For rxt-t graph, -gradient of tangent at tx
- For pdt-t graph, gradient of tangent at tx - Initial Rate
- For rxt-t graph, -gradient of tangent at t0
- For pdt-t graph, gradient of tangent at t0 - Average Rate
- Average Rate = Initial Rate if time interval, Δt is small enough & Δt starts from 0.
Initial Rate
Definition
Instantaneous Rate when t = 0
Instantaneous Rate
Definition
The rate at a particular time.
Average Rate
Definition
The change in the concentration of a reactant or product over a time interval.
Experimental Techniques to determine ROR
- Continuous Method
a. Sampling & Titration
b. Measuring Colour Intensity at Regular Time Intervals
c. Measuring Electrical Conductivity at Regular Time Intervals
d. Measuring Volume of Gas Produced at Regular Time Intervals
e. Measuring Mass of Reaction Mixture at Regular Time Intervals
f. Measuring Pressure at Regular Time Intervals - ‘Clock’ Method aka Method of Initial Rates
a. Reaction between Thiosulfate, S2O3 and Hydrogen Ions H+
b. Reaction between Hydrogen Peroxide, H2O2 and Iodide Ions, I-
For reaction 2b, Initial Rate is inversely proportional to the time taken for the blue-black colour to appear aka Initial Rate ∝ 1/t
Rate of Reaction, ROR
Factor 1: Physical State of Rxts
- (s) or (aq) form?
- (s) particles in finely divided (s) form?
- (aq) particles are molecules or ions?
Rate of Reaction, ROR
Factor 2: Concentration of Rxts
- Concentration and the Collision Theory, CT
a. Based on CT, rxt particles must collide for rxn to occur.
b. [X] increases
c. Particles are closer together
d. Frequency of collisions increase
e. Frequency of effective collisions increase
f. ROR increases - Partial Pressures of (g) reactants (only affects (g) systems)
a. Partial Pressure = [X]
b. Increase in pressure = increase in [X]
(steps c-f)
Rate of Reaction, ROR
Factor 4: Catalyst
- Alternative pathway with lower EA than the uncatalysed reaction
- More rxt particles have EK > EA for catalysed reaction.
- Frequency of collisions increase
- Frequency of effective collisions increase
- Increase in ROR
Rate constant, K increases as well
Rate of Reaction, ROR
Factor 3: Temperature, T
- T increases
- Average kinetic energy, EK, of rxt particles increases
- More particles have EK > activation energy, EA, of reaction
- Frequency of collisions increase
- Frequency of effective collisions increase
- Increase in ROR
Rate constant, K increases as well
Rate Equation
Rate = k[A]m[B]n
m/n = order of reaction with respect to, w.r.t, A/B
Units:
- Zero-order reaction: mol dm-3 s-1
- First-order reaction: s-1
- Second-order reaction: mol-1 dm3 s-1
Interpretation of Rate Equation, rate = k[A]m[B]n
Given A + B –> C + D
1. m & n = 0 aka zero-order reaction
ROR is independent w.r.t [A] & [B]
2. m = 1, n = 0 aka first-order reaction
ROR is first-order w.r.t [A] but is independent of [B]
3. m & n = 1 aka second-order reaction
ROR is first-order w.r.t to both [A] & [B]
4. n = 2 aka second-order reaction
ROR is second-order w.r.t [B]
Half-Life, t1/2
Applicable to first-order reactions only
t1/2 = ln 2/ k
k = rate constant
Pseudo-order Reactions
Conditions
- Presence of large excess of a rxt
- Rxt is solvent
- Presence of catalyst
Determining order of reaction w.r.t to rxt
Given Initial Rate Data
- Inspection
[rxt] x 2, rate x 2/ 4
- Rate x 2, rate ∝ [rxt]
- Order of reaction w.r.t to [rxt] is 1
- Rate x 4, rate ∝ [rxt]2
- Order of reaction w.r.t to [rxt] is 2 - Mathematical
Rate of experiment 1/ ‘’ experiment 2 = k[A1]m[B1]n/ k[A2]m[B2]n
- Form simultaneous equations to solve for m & n
- Cancel out common terms to solve for m/ n
M1 can be used when only 1 change occurs. Otherwise, M2 has to be used.
Determining order of reaction w.r.t to rxt
Given concentration-time graphs
- Independent Rxt
Constant t1/2? - Other Rxts
- Inspection Method
- Mathematical Method
Reaction Mechanism, RM
Definition
A collection of elementary steps in the proper sequence showing how reactant particles are converted into products.
Elementary Step
Definition
A distinct step in a reaction mechanism which describes a single molecular event that involes breaking and/ or making bonds.
Elementary steps cannot be broken down into simpler steps.
Elementary Steps
Molecularity
Unimolecular: involves 1 rxt particle
Bimolecular: involves 2 rxt particles
Termolecular: involves 3 rxt particles
Termolecular Elementary Steps are rare as probability of 3 particles colliding simultaneously with the require Ea and correct orientation is low.
Single-step Reactions
aka Elementary Reactions
RM:
- consists of 1 elementary step
- is identical to the stoichiometric equation for the rxn
- rate equation can be deduced directly from the stoichiometric equation
Multi-step Reaction
RM:
- consists ≥ 2 elementary steps
- rate equation is determined by the rate-determining step
Rate-determining step, RDS
aka Slow Step
The slowest step in the reaction mechanism of a multi-step reaction and is has the highest activation energy.
Writing a Reaction Mechanism
| Criterias
- Must agree with stoichiometric equation
- Must be consistent with reaction kinetics
- Must include intermediates detected during the rxn
The Collision Theory
Rxt particles must collide:
- in order for reaction to occur
- only Effective Collisions result in reaction occurring
- in a favourable orientation for reaction to occur
- specific bonds must be broken so that specific bonds can be formed
- with a certain minimum amount of energy (i.e. Activation Energy, EA)
- brings about necessary reorganisation of bonds in colliding particles
Activation Energy, EA
Definition
The minimum amount of energy that the reactant particles must possess before their collisions can result in a reaction.
Catalyst
Definition
A substance which increases the rate of reaction without itself undergoing any permanent chemical change.
Catalysts participate in the rxn and are regenerated.
Inhibitor
Definition
A substance which decreases the rate of a chemical reaction.
Homogenous Catalysis
Definition
Catalyst and rxts are in the same phase, typically the liquid or gaseous phase.
Heterogenous Catalysis
Definition
Catalyst and rxts are in different phases
Heterogenous Catalysis
Mechanism
- Adsorption weakens the covalent bonds within the rxt molecules. Thus, lowering EA.
- Adsorption increases the concentration of rxt molecules at the catalyst surface and allows the rxt molecules to come into close contact with proper orientation for reaction.
Adsorption
Definition
The formation of weak bonds between reactant molecules and catalyst surface.
Autocatalysis
Definition
The product of a reaction acts as a catalyst for the reaction.
Enzymes
Definition
aka Biological Catalysts
Proteins that catalyse chemical reactions in living systems
Enzymes
Properties
- Nature & Size
Globular proteins with active sites contained in their 3-dimensional structure - High Efficiency
Small amounts of the enzyme is needed as they are extremely effective catalysts - Specificity
An enzyme will catalyse only 1 reaction - Sensitivity to Temperature
Operate most effectively at body temperature, around 37°C - Sensitivity to pH
Enzyme-catalysed Reactions
Factor(s)
- Temperature
- pH
- Concentration of enzyme
