Reaction Kinetics

How does concentration affect rate of reaction?

Particles come closely together → Frequency of effective collisions increases → Increase in rate of reaction

How does partial pressure affect rate of reaction?

Partial pressure increases → Concentration of reactants increases → Higher frequency of effective collisions → Rate or reaction increases

How does temperature affect rate of reaction?

Increases rate of reaction

What is a catalyst?

A substance that increases rate of reaction by providing an alternative reaction pathway for lower activation energy without itself undergoing any permanent change

How does physical state of reactants affect rate of reaction?

• Finely divided → Greater surface area per unit volume → More contact made with other reactant → Faster reaction

• Solution form (rather than solid) → Reaction occurs

• Molecule or ion form

  • Molecule → Requires heating to cleave covalent bonds

  • Ions → No covalent bonds broken, dissociation occurs to form ions that have a natural tendency to attract and form H₂O

What is instantaneous rate?

Rate at a particular time during the reaction

What is initial rate?

Instantaneous rate at t=0

What is average rate?

Change in concentration of a reactant / product over that time interval

• Small time interval that starts at t=0 → Approximate initial rate

What is the expression for rate if A is a reactant and B is a product?

rate = -1/a d[A]/dt = 1/b d[B]/dt

What is rate equation and what does it mean?

rate = k [A]^m [B]ⁿ

Exact dependence of reaction rate on concentration of all reactants

What does rate constant mean?

Constant of proportionality in the rate equation

• Larger → Faster reaction

• Constant for a given reaction at a particular temperature; increases with increasing temperature or presence of catalyst

What is order of reaction with respect to a reactant?

Power to which concentration of the reactant is raised in the rate equation → Exact dependence of rate on concentration of a given reactant

What is overall order of a reaction?

Sum of powers of the concentration terms in the rate equation

What is the continuous method in measuring reaction rate?

Monitors concentration / volume / mass of reactant / product continuously over time

What is the clock method in measuring reaction rate?

Measuring the time taken for a visible change to occur

What are the rate equation and units of zero order reactions?

• rate = k

• mol dm^-3 s^-1

What are the rate equation and units for first order reactions?

• rate = k [A]

• s^-1

What are the reaction equation and units for second order reactions?

• rate = k [A]²

• mol^-1 dm³ s^-1

How is half-life in zero order, first order and second order reactions?

• zero → not constant

• first → constant

• second → not constant

What is the equation for half life of a first order reaction?

t_1/2 = ln2 / k

When is a reaction pseudo-order?

• Concentration of reactant not in rate equation is effectively constant due to it being in large excess

  • Water, the solvent, tends to not be in the rate equation

• Catalyst is not in the rate equation as it is regenerated and its concentration is effectively constant

What is the reaction mechanism?

Collection of elementary steps in the proper sequence showing how reactant particles are converted into products

What is an elementary step and what is molecularity of a single step?

• A single molecular event that involves making or breaking bonds

• Molecularity = Number of reactant particles taking part in that step

What is an intermediate?

A species formed in one step of a reaction mechanism and consumed in a subsequent step

What is the rate-determining step?

• Slowest step in the reaction mechanism f a multi-step reaction

• Highest activation energy

What is the collision theory?

Reactant particles must collide in a favorable orientation and with a certain minimum amount of energy to react

What is the transition state theory?

Given a sufficiently energetic collision and correct molecular orientation, the reactant species become an unstable transition state that exists at a potential energy maximum

What happens to the rate of reaction (using Maxwell-Boltzmann distribution curve) when temperature increases from T₁ to T₂ K?

1. Average kinetic energy of reactant particles increases

2. More reactant particles have energy greater than or equal to activation energy

3. Increase in effective collision frequency and hence rate of reaction

4. Larger rate constant k

What happens to rate of reaction (using Maxwell-Boltzmann distribution curve) when a catalyst is added?

1. A catalyst increases the rate of reaction by providing an alternative reaction pathway with lower activation energy

2. More reactant molecules have energy greater than or equal to activation energy

3. Increase in effective collision frequency and hence rate of reaction

4. Larger rate constant k

What are homogeneous and heterogeneous catalysts?

Homogeneous → Catalyst and reactants are in the same phase (either liquid or gaseous)

Heterogeneous → Catalyst and reactants are in different phases

How does homogeneous catalysis work?

Larger activation energy for the uncatalysed reaction is replaced by 2 smaller activation energies for the catalyzed reaction

How does heterogeneous catalysis work?

1. Diffusion: Reactant molecules diffuse towards the catalyst surface

2. Adsorption: Reactant molecules are adsorbed onto active sites of the catalyst surface → Increases the concentration of reactants at the catalyst surface + Weakens the covalent bonds in the molecules

3. Reaction: Adjacent reactant molecules react to form products at a lower activation energy

4. Desorption: Product molecule eventually breaks free from the catalyst surface

5. Diffusion: Product molecules diffuse away from the surface → Vacant active sites are now available for adsorbing other reactant molecules

What is autocatalysis?

Type of catalytic action whereby the product of a reaction acts as a catalyst for the reaction

Explain the graph for autocatalysis

1. Initially, the reaction is slow since it has high activation energy

2. As products are produced, they increase the rate of reaction by acting as an autocatalyst

3. Towards the end of the reaction, concentration of reactants has fallen to low level so the rate of reaction decreases even though there is an adequate supply of catalyst

What are enzymes?

Biological catalysts with high specificity

What is the lock and key model for enzymatic action?

• Substrate and enzyme molecules are thought to have complementary shapes

• When enzyme and substrate molecules collide in the correct orientation → The substrate fits the active site and is bound by attractive forces

• ES complex is formed, weakening the bonding within the substrate → Substrate can react via an alternative reaction pathway of lower activation energy

• ES complex breaks apart when the product formed is no longer of the right shape to fit into the active site → Enzyme is free to bind to another substrate

How is enzymatic action affected by temperature?

1. Average kinetic energy of enzyme and substrate molecules increases

2. More molecules have energy greater than or equal to activation energy required

3. Increase in frequency of effective collisions between substrate and enzyme molecules at active sites

4. Rate of formation of ES complexes increases

How is enzymatic action affected by pH?

• Rate of reaction is maximum at optimal pH

• Deviation from optimal pH lowers rate of reaction

What is half life?

Time taken for concentration of reactant to decrease to half its original value