reaction kinetics Flashcards
define rate of reaction
change in concentration of reactant or product with time
define rate equation
it relates the rate of the reaction to the concentration of reactants raised to the appropriate power
define order of reaction
power to which the concentration of that reactant is raised to in the rate law
define rate constant, k
k is proportionality constant in the rate equation that relates the concentration of the reactants to the rate of reaction
define half-life
it is the time taken for the reactant concentration to decrease to half its original value
define rate-determining step
it is the slowest step in the sequence of steps leading to the formation of the product
define activation energy, Ea
the minimum amount of energy that the reactant particles must possess to overcome the energy barrier in order for effective collisions to result in a chemical reaction
define catalyst
a substance that increases the rate of a chemical reaction by providing an alternative pathway with lower activation energy. it remains chemically unchanged at the end of the reaction
what does it mean when ‘equal amounts of reactants are used’ ?
concentration is halved
what are the properties of enzymes?
- they remain chemically unchanged at the end of reaction
- small qtys
- affected by pH(best 5-8), temperature(20-25C)[denatured at high temp], substrate concentration, enzyme concentration
- Highly specific in their function
Why proposed mechanism is chosen?
- Slow step - - > stoichiometric coefficients of reactants agree to the orders of reaction in the rate equation
- Sum of steps in the proposed mechanism gives the balanced equation
Factors affecting rate of reaction: presence of catalyst
- In presence of catalyst, Ea is lowered
- No. of reacting particles with E >= Ea’(cat) increases
- Frequency of effective collisions increases
- since rate of reaction is proportional to the frequency of effective collisions, rate of reaction increases
Factors affecting rate of reaction: conc/pressure
- orders of reaction wrt to (reactants name) are non-zero
- Higher means no. Of reactant particles per unit volume increases
- Frequency of effective collisions increases
- since rate of reaction is proportional to the frequency of effective collisions, rate of reaction increases
Heterogeneous catalysis
- Adsorption
a) reactant molecules diffuse towards the catalyst(s) surface and are absorbed onto the active sites at the surface
b) They become chemisorbed ie. weak chemical bonds are formed between molecules on the active sites thereby weakening the bonds in the adsorbed reactant molecules
c) The adsorption process increases the concentration of the reactant molecules on the catalyst surface - Reaction at the surface
a) Intramolecular bonds within the reactant molecules are weakened, lowering the Ea
b) Reactant molecules are brought into close contact and are correctly orientated for reaction - Desorption
a) Product molecule desorbs and diffuses away from the catalyst surface.
b) This frees up the active sites on the catalyst surface for adsorption with new reactant molecules
Factors affecting rate of reaction: temperature
- At higher temperature, total number of reactant particles remain unchanged but average KE of the reactant particles increases
- no. of reactant particles with energy >= Ea increases.
- Frequency of effective collisions increases
- since rate of reaction is proportional to the frequency of effective collisions, rate of reaction increases
suggest why step 1 is the slow step:
step1: (CH3)3CBr –> (CH3)C+ + Br-
step2: (CH3)C+ +H2O –> (CH3)CH2O+
step3: CH3)CH2O+ + Br- –> CH3)COH +HBr
-In step 1, energy is absorbed to break the C-Br bond to form a reactive intermediate that is unstable relative to (CH3)3CBr.
- Step 2 is a bond formation process which releases energy and Step 3 involves lower activation energy due to favourable bond formation between 2 oppositely charged species
-Since the activation energy is highest in Step 1, it is the slowest step
Why is this mechanism not plausible?
Step1: NO2 +O3 –> NO + 2O2 (slow)
Step2: NO2 + NO + O2 –> N2O5 (fast)
The fast elementary step involvign reaction among 3 molecules is unlikely to occur cuz the probability for 3 molecules to collide effectively with correct orientation and sufficient KE AT THE SAME TIME is low.
Effect of pH on stucture of enzyme catalase
The optimum pH at which catalase works best is __. Above and below this value, the rate of decomposition decreases as the structure of the enzyme changes leading to a decline in its performance. At extreme pH, the enzyme is denatured with loss in shape of its active sites, leading to a much lower rate of decomposition.
Account for the shape of the graph. (as [H2O2] increases, rate of reaction increases at decreasing rate until it becomes constant)
- At low [H2O2], there are plenty of active sites available on each enzyme molecule. The rate of reaction increases proportionately with increasing [H2O2] OR rate of reaction is first-order wrt H2O2.
- At intermediate [H2O2], rate of reaction is of mixed order wrt H2O2
- At high [H2O2], all the active sites are fully occupied. Increasing [H2O2] no longer increases reaction rate as there are no more active sites available for H2O2 to attach to and react. Reaction becomes zero order wrt H2O2
