Definition factors affecting activity and selectivity of a catalyst with examples
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 in Chemical Reactions
1. What is the activity of a catalyst in chemistry?
The activity of a catalyst is a measure of how fast it increases the rate of a chemical reaction. It indicates the catalyst’s ability to convert reactants into products per unit time under given conditions.
Key points about catalyst activity:
- It depends on the number of active sites available on the catalyst surface.
- Higher activity means a higher reaction rate at the same temperature and pressure.
- It is often expressed as turnover frequency (TOF), which is the number of molecules converted per active site per second.
- Example: In the hydrogenation of ethene, C2H4(g) + H2(g) → C2H6(g), a more active nickel catalyst increases the reaction rate.
2. What is the selectivity of a catalyst?
The selectivity of a catalyst is its ability to favor the formation of a specific product over other possible products in a reaction. It determines how efficiently a desired product is formed when multiple reactions are possible.
Important aspects of selectivity:
- High selectivity minimizes side reactions and unwanted by-products.
- It is usually expressed as a percentage of the desired product formed.
- It is crucial in industrial processes to reduce waste and purification costs.
- Example: In partial oxidation of hydrocarbons, a selective catalyst produces desired products like aldehydes instead of complete oxidation to CO2 and H2O.
3. What is the difference between catalyst activity and selectivity?
The key difference is that activity measures how fast a catalyst works, while selectivity measures how specifically it forms the desired product. Both properties are essential in catalysis but describe different performance aspects.
Comparison:
- Activity: Related to reaction rate and conversion speed.
- Selectivity: Related to product distribution and preference.
- A catalyst can be highly active but poorly selective, producing many by-products.
- An ideal industrial catalyst is both highly active and highly selective.
4. How do you measure the activity of a catalyst?
The activity of a catalyst is measured by determining the reaction rate or conversion of reactants per unit time under controlled conditions. It is often quantified using turnover frequency or rate constants.
Common methods:
- Measure the change in concentration of reactants or products over time.
- Calculate reaction rate using rate laws.
- Determine turnover frequency (TOF) = moles of product per mole of active sites per second.
- Compare performance at constant temperature, pressure, and catalyst mass.
5. How is catalyst selectivity calculated?
The selectivity of a catalyst is calculated as the fraction or percentage of the desired product formed relative to the total products formed. It shows how efficiently a catalyst directs a reaction toward a specific product.
General formula:
- Selectivity (%) = (moles of desired product / total moles of products) × 100
- It can also be expressed relative to reactant consumed.
- Higher percentage means better selectivity toward the target product.
6. Why is catalyst selectivity important in industrial chemistry?
Catalyst selectivity is important in industrial chemistry because it reduces unwanted by-products and increases yield of the desired product. High selectivity improves economic efficiency and minimizes environmental impact.
Industrial significance:
- Reduces cost of separation and purification.
- Decreases waste generation and energy consumption.
- Improves process sustainability.
- Example: In ammonia synthesis, N2(g) + 3H2(g) → 2NH3(g), iron catalysts are designed to favor ammonia formation efficiently.
7. What factors affect the activity of a catalyst?
The activity of a catalyst is affected by factors such as surface area, temperature, pressure, and the nature of the reactants. These factors influence how effectively reactant molecules interact with active sites.
Main factors:
- Surface area: Higher surface area provides more active sites.
- Temperature: Increasing temperature generally increases reaction rate.
- Pressure: Important for gaseous reactions.
- Poisoning: Impurities can block active sites and reduce activity.
8. What factors influence the selectivity of a catalyst?
Catalyst selectivity is influenced by the catalyst’s structure, reaction conditions, and the mechanism of the reaction. These factors determine which reaction pathway is favored.
Key influences:
- Catalyst composition and electronic properties.
- Pore size and shape in heterogeneous catalysts.
- Temperature and pressure, which can shift product distribution.
- Nature of reactants and intermediates.
9. Can a catalyst be highly active but poorly selective?
Yes, a catalyst can be highly active but poorly selective if it speeds up multiple competing reactions equally. In such cases, although the reaction rate is high, many undesired products may form.
Explanation:
- High activity means fast conversion of reactants.
- Low selectivity means poor control over product formation.
- This often leads to complex product mixtures.
- Industrial catalyst design aims to balance both properties.
10. What is an example of catalyst activity and selectivity in a chemical reaction?
An example of catalyst activity and selectivity is the hydrogenation of alkynes, where a poisoned palladium catalyst selectively forms alkenes. The catalyst controls both reaction rate and product type.
Example reaction:
- C2H2(g) + H2(g) → C2H4(g) (using Lindlar catalyst)
- The catalyst is active enough to hydrogenate the alkyne.
- It is selective because it stops at the alkene and prevents further hydrogenation to C2H6.
