How Turnover Number and Frequency Define Catalyst Efficiency
Catalyst is those chemicals that change the rate of a chemical reaction but do not undergo any change itself in the process. The function of a catalyst has several implications. It has specific roles in several biological processes, like in growth, immunity, and general metabolism. Biological catalysts are called enzymes. The efficiency of a catalyst depends on the selectivity and activity of the catalyst.
In nature, the catalyst remains inactive until a reactant binds to it. The catalyst is highly specific for its reactants, and that is why some catalysts can change the reaction rate for some chemical reactions, while are rendered ineffective for other chemical reaction. Some catalyst can enhance the reaction rate for some reaction and can also inhibit other reactions.
Activity and selectivity of catalyst determine the efficiency of the catalyst.
Activity of catalyst
As per the catalytic activity definition, the ability of a catalyst to increase the rate of a chemical reaction is called its activity. The catalytic activity meaning is described by the physical interaction of the reactants with the catalyst by chemisorption. The bond formed between the catalyst and reactant determines the reaction rate. The strength of such bonds has to be optimal for the catalyst to perform. Therefore, the reaction rate or the activation energy measures for the catalytic activity.
However, the other two parameters can be used to measure catalytic activity.
Turnover Number
The turnover number is defined by the number of catalytic cycles that can be performed by the catalyst before it deteriorates. Commonly used industrial catalysts have a turnover number ranging from 10 to 105.
Turnover Frequency
The turnover frequency is the number of times the reaction takes place per catalyst per unit time.
The activity of a biological catalyst is determined by the physical fit of the reactant on the catalyst molecule. The lock and key hypothesis best describes the model, where the reactant binds to the groove of the catalyst in the form of a key fitting into the lock. However, the current scenario is not the same. Enzymes, the biological catalyst, do not have a groove where the reactant molecule perfectly fit. Rather the reactant intermediately fits into the groove of the catalyst and induces conformational changes in the structure of the catalyst. These conformational changes restructure the groove for a perfect fit with the reactant molecule. The activity of the catalyst increases with the perfect fit with the reactant.
Selectivity of catalyst
Another important property that determines the efficiency of a catalyst is its selectivity. A catalyst is highly selective for the reactant on which it acts. Moreover, the type of catalyst acting on the reactant determines the product thus formed. Like the activity of a catalyst, selectivity is also determined by the fit of the catalyst with the reactant. The reaction cannot proceed if the reactant does not fit properly into the groove of the catalyst. Moreover, the groove of different may be similar and, therefore, can bind with the same reactant. Still, the conformational changes induced may be different, which is the reason why different products can be generated from the same reactant source by different catalysts.
For example, Carbon monoxide reacts with Hydrogen in the presence of Nickel to form methane and water. However, they form methanol in the presence of chromium oxide and zinc oxide. However, it forms formaldehyde in the presence of Copper.
The catalytic properties of transition elements, especially the activity, are high. Mainly the oxides of these transition elements have high catalytic activity. Vanadium pentaoxide, Platinum, Nickel are all good catalysts. The high catalytic activity of transition elements are mainly due to
Presence of vacant d-orbitals.
Exhibits variable oxidation states.
Can form reaction intermediates.
FAQs on Activity and Selectivity of a Catalyst: Complete Guide
1. What are the activity and selectivity of a catalyst as per the Class 12 Chemistry syllabus?
In chemistry, the activity of a catalyst refers to its ability to increase the rate of a chemical reaction. A highly active catalyst can cause a significant increase in reaction speed. On the other hand, selectivity refers to the catalyst's ability to direct a reaction to yield a specific product, especially when multiple products are possible from the same set of reactants.
2. What is the importance of chemisorption in determining catalyst activity?
The activity of a solid catalyst is largely dependent on chemisorption, which is the chemical adsorption of reactants onto the catalyst's surface. For a catalyst to be active, the adsorption must be reasonably strong but not so strong that the reactant molecules become immobilised, leaving no further room for other reactants to adsorb and react. An optimal bond strength ensures a high reaction rate.
3. How can different catalysts lead to different products from the same reactants? Provide an example.
This phenomenon is a direct result of the high selectivity of catalysts. A catalyst provides a specific reaction pathway that leads to a particular product. By changing the catalyst, you change the pathway, thus forming a different product. For example, using carbon monoxide (CO) and hydrogen (H₂) as reactants:
- With a Nickel (Ni) catalyst, they produce Methane (CH₄).
- With a Copper/Zinc oxide-Chromium oxide (Cu/ZnO-Cr₂O₃) catalyst, they produce Methanol (CH₃OH).
- With a Copper (Cu) catalyst, they produce Formaldehyde (HCHO).
4. What is the main difference between the selectivity and specificity of a catalyst?
While related, these terms have different focuses. Selectivity describes a catalyst's ability to favour one reaction pathway over others to produce a desired product from a set of reactants that could form multiple products. Specificity is a more absolute term, often used for enzymes (biocatalysts), implying that the catalyst is effective for only one particular substrate or type of reaction, much like a lock and key.
5. If a catalyst is highly active, does it mean it gets consumed or used up during the reaction?
No, a fundamental property of a catalyst is that it remains chemically unchanged in mass and composition at the end of the reaction. High activity means it is very efficient at speeding up the reaction by providing an alternative, lower-energy pathway. It participates in the reaction mechanism but is regenerated in its original form, so it is not consumed.
6. Why is catalyst selectivity crucial for industrial chemical processes?
Catalyst selectivity is vital for industrial applications primarily for economic reasons. A highly selective catalyst maximises the yield of the desired product while minimising the formation of unwanted by-products. This increases the overall efficiency, reduces the cost and complexity of separating and purifying the final product, and minimises waste, making the process more sustainable and profitable.
7. How would you decide whether to prioritise a catalyst's activity or its selectivity for a new chemical process?
The decision depends on the process goals.
- Prioritise high selectivity when the desired product is one of many possible outcomes and unwanted by-products are costly to separate or hazardous. This ensures a high yield of the target substance.
- Prioritise high activity when the reaction is very slow and the primary goal is to accelerate production, especially if by-products are minimal or easily managed.
