Week 9 - Kinetics (activation energy, elementary reaction steps, reaction rate, catalysis) Flashcards
What is a catalyst? (5)
A catalyst is a substance that increases the reaction rate without itself being consumed in the reaction.
In general, a catalyst provides an alternative reaction pathway that has a lower total activation energy than the uncatalysed reaction.
A catalyst will speed up both the forward and the reverse reactions.
Added in non-stoichiometric amounts (~ 10-6
to 10-1X)
A catalyst does not affect either △H or the overall yield for a reaction.
Catalysis – why is it important?
- More than 90% of industrial processes actually use catalysts in one form or the other.
- If you are employed as a materials scientist, it’s likely that you will encounter tasks including catalysis.
Who uses catalysts? (4)
- Petroleum and energy production
- Chemicals and polymer production
- Pollution control
- Pharmaceutical and food industry
Activation Energy
energy required to overcome the reaction
barrier. Usually given a symbol Ea or ∆G^≠
- The activation energy (Ea) determines how fast a reaction occurs, the higher the activation barrier, the slower the reaction rate. The lower the activation
barrier, the faster the reaction.
Activation energy: catalyst effect
Catalyst lowers…
the activation energy for both forward and reverse reactions, and consequently the catalyst increases the rate of reaction.
Importance of catalysis: (6)
- increasingly important in synthesis
- selectivity in production of fine chemicals
- clean processes, high atom economy (bulk processes)
- production of high-tech products / materials
- mild conditions (low energy consumption)
- environmentally friendly
Catalytic reaction simple process steps:
Every catalytic reaction is a sequence of elementary steps, in which reactant molecules bind to the catalyst, where they react, after which the product detaches from the catalyst, liberating the latter for the next cycle.
Catalytic activity
is the increase in the rate of a specified chemical
reaction caused by a catalyst under specified conditions
Catalyst selectivity
the ability of the catalyst to drive the reaction
towards a particular product, as there may be more than one in typical catalytic reactions
Catalyst stability
refers to the chemical, thermal, and mechanical
stability of a catalyst, which determines its lifetime in industrial reactors.
The types of catalysts,
Classification based on the its physical state, a catalyst can be:
- gas
- liquid
- solid
Different types of catalysts,
Classification based on the substances from which a catalyst is made:
- Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.)
- Organic (organic acids, enzymes etc.)
Different types of catalysts,
Classification based on the ways catalysts work:
- Homogeneous - both catalyst and all reactants/products are in the same phase (gas or liquid)
- Heterogeneous - reaction system involves multi phase (catalysts + reactants/products)
Different types of catalysts,
Classification based on the catalysts’ action:
- Acid-base catalysts
- Enzymatic
- Photocatalysis
- Electrocatalysis, etc
Bio-catalysis
- Biocatalysis is the chemical process through which enzymes or other biological catalysts perform reactions between organic components.
- Reactants and catalyst may be in the same phase or in different phases.
- Michaelis-Menten
Biocatalysis: Pros & Cons
- Very high selectivity; stereo-selectivity, regio-selectivity (allows selective modification of a specific site in a molecule), and functional group selectivity.
- Operational advantages; reactions under mild operational conditions.
- Catalysts that are biodegradable thus typically “greener” and more sustainable.
- Often very costly
- Requires a development timeline that is too long to meet the needs of real world industrial manufacturing
Homogeneous Catalysis
A homogeneous catalyst exists in the same phase as the reactants.
- An example is the decomposition of hydrogen peroxide in aqueous solution
The Catalyzed Decomposition of H2O2:
- A small amount of NaBr is added to a solution of
H2O2 (A). - Oxygen gas forms quickly as Br− (aq) catalyzes the H2O2 decomposition; the intermediate Br2 turns the solution orange (B).
- The solution becomes colourless when the Br2 is consumed in the final step of the mechanism (C).
Homogeneous catalysts: pros and cons
- Catalytic mechanisms are relatively easy to study in homogeneous systems, using powerful methods as NMR to both assign structures
and follow reaction kinetics. - High selectivity.
- Homogeneous catalysts have the disadvantage that they can be difficult to separate from the product.
- Poor stability
Heterogeneous catalysis
Heterogeneous catalyst are catalysts for which the catalyst and the substrates for the reaction are in different phases, cf. solid catalyst
and gas-phase reactions.
- A heterogeneous catalyst is in a different phase than the reaction mixture, e.g hydrogenation, the addition of H2 to C═C bonds to form C―C bonds
Simplified process: absorption, reaction, desorption
Heterogeneous catalysis: pros and cons
- High catalyst activities
- Cheap
- Ease of preparation
- Often low selectivity
- Deactivation
Application of catalysis,
Industrial applications:
Almost all chemical industries have one or more steps employing catalysts:
Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals …
Advantages of catalytic processes
- Achieving better process economics and productivity
- Increase reaction rates - fast
- Simplify the reaction steps - low investment cost
- Carry out reaction under mild conditions (e.g. low T, p) - low energy consumption
- Reducing wastes
- Improving selectivity toward desired products - less raw materials required, less unwanted wastes
- Replacing harmful/toxic materials with readily available ones
- Producing certain products that may not be possible without catalysts
- Having better control of process (safety, flexible etc.)
- Encouraging application and advancement of new technologies and materials
And many more …
What is Chemical Kinetics:
study of reaction rates,
and the factors that affect them.
What is Reaction Rate:
the change in the concentrations of reactants or products as a function of time
Rates of reaction vary greatly for everyday processes