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Questions and Answers
What influences slow reactions in gas/porous catalyst systems?
What influences slow reactions in gas/porous catalyst systems?
- Surface Kinetics (correct)
- Film Mass Transfer
- Particle Temperature Gradients
- Pore Diffusion
What is the order in which effects intrude in liquid systems?
What is the order in which effects intrude in liquid systems?
- 1, 2, 3, and 4
- 1, 2, 5, and rarely 3 and/or 4 (correct)
- 1, 4, 2, and 3
- 1, 3, 2, and 4
What is adsorption?
What is adsorption?
- A process by which a chemical component is dissolved in a liquid
- A process by which a chemical component is transformed into a gas
- A process by which a chemical component is released from a solid phase
- A process by which a chemical component from a fluid phase is removed by attachment to a solid phase (correct)
What is the primary factor limiting the overall rate in faster reactions?
What is the primary factor limiting the overall rate in faster reactions?
In which type of systems does Surface Kinetics dominate in slow reactions?
In which type of systems does Surface Kinetics dominate in slow reactions?
What is the difference between adsorption and absorption?
What is the difference between adsorption and absorption?
In heterogeneous catalytic reactions, what is the role of the catalyst?
In heterogeneous catalytic reactions, what is the role of the catalyst?
What is the name of the phenomenon where a chemical component from a fluid phase is removed by attachment to a solid phase?
What is the name of the phenomenon where a chemical component from a fluid phase is removed by attachment to a solid phase?
What type of heterogeneous reactions involve the interaction between a gas and a solid?
What type of heterogeneous reactions involve the interaction between a gas and a solid?
What must be considered when writing rate expressions for heterogeneous reactions?
What must be considered when writing rate expressions for heterogeneous reactions?
How many types of heterogeneous reactions are mentioned in the text?
How many types of heterogeneous reactions are mentioned in the text?
What is the result of the burning of a carbon particle in air?
What is the result of the burning of a carbon particle in air?
How many rate steps are involved in aerobic fermentation?
How many rate steps are involved in aerobic fermentation?
Why is it important to consider mass transfer terms in heterogeneous reactions?
Why is it important to consider mass transfer terms in heterogeneous reactions?
What is the unit of the rate of reaction in the equation dNA / dt?
What is the unit of the rate of reaction in the equation dNA / dt?
What is the function of a catalyst in a chemical reaction?
What is the function of a catalyst in a chemical reaction?
What is the term for the ability of a catalyst to promote a specific reaction?
What is the term for the ability of a catalyst to promote a specific reaction?
What type of catalyst is typically used in biochemical systems?
What type of catalyst is typically used in biochemical systems?
What is the process by which a reactant attaches to the surface of a catalyst?
What is the process by which a reactant attaches to the surface of a catalyst?
What is the purpose of a support in a supported catalyst?
What is the purpose of a support in a supported catalyst?
What is the characteristic of a catalyst that allows it to emerge from a reaction unchanged?
What is the characteristic of a catalyst that allows it to emerge from a reaction unchanged?
What is the type of catalyst that is typically encountered in processes where pressure drop and heat removal are major considerations?
What is the type of catalyst that is typically encountered in processes where pressure drop and heat removal are major considerations?
What is the term for the phenomenon where a reactant attaches to the surface of a catalyst, but does not react?
What is the term for the phenomenon where a reactant attaches to the surface of a catalyst, but does not react?
What is the type of catalyst that is composed of minute particles of active material dispersed over a less active substance?
What is the type of catalyst that is composed of minute particles of active material dispersed over a less active substance?
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Study Notes
L/S Reaction Rate Expression
- The reaction rate expression for a liquid-solid (L/S) reaction is derived from the flux of A to the surface and the reaction rate at the surface.
- The flux of A to the surface is given by:
𝑟𝐴1 = -𝐷𝐶𝐴𝑙 - 𝐶𝐴𝑠 / ∆𝑥 - The reaction rate at the surface is given by:
𝑟𝐴2 = 𝑘″𝐶𝐴 - At steady state, the two rates are equal:
𝑟𝐴1 = 𝑟𝐴2
Rate Expression Rearrangement
- Rearranging the rate expression equations, we get:
𝑘𝑙𝐶𝐴𝑙 - 𝐶𝐴𝑠 = 𝑘″𝐶𝐴 - Solving for
𝐶𝐴𝑠, we get:𝐶𝐴𝑠 = 𝑘𝑙 / (𝑘𝑙 + 𝑘″) 𝐶𝐴𝑙
Overall Rate Expression
- The overall rate expression is:
𝑟𝐴 = 𝑘𝑜𝑣𝑒𝑟𝑎𝑙𝑙 𝐶𝐴𝑙 / (1 + 𝑘𝑙 / 𝑘″)
Catalysts
- A catalyst is a substance that speeds up a reaction without being consumed by the reaction.
- There are two broad classes of catalysts: homogeneous and heterogeneous.
- Enzymes are a type of catalyst that operate at close to ambient temperature with biochemical systems.
Types of Catalysts
- Porous catalysts have a large surface area and are used in processes where pressure drop and heat removal are major considerations.
- Monolithic catalysts are normally encountered in processes where pressure drop and heat removal are major considerations.
- Supported catalysts consist of minute particles of an active material dispersed over a less active substance called support.
- Molecular sieves are a type of catalyst that allows selective reactions to occur.
Factors Influencing Reaction Rate
- Film diffusion resistance or concentration gradients across the gas film surrounding the particle can influence the reaction rate.
- Pore diffusion can slow the reaction rate in faster reactions.
- Surface kinetics alone can influence the reaction rate in slow reactions.
- Particle ΔT and/or Film ΔT can enter the picture in faster reactions.
Adsorption on Solid Surfaces
- Adsorption is a process by which a chemical component from a fluid phase is removed by attachment to a solid phase.
- Adsorption is an important step in heterogeneous catalytic reactions.
Heterogeneous Reactions
- Types of heterogeneous reactions include gas-solid, liquid-solid, and solid-solid reactions.
- Rate expressions for heterogeneous reactions must consider mass transfer terms in addition to chemical kinetics terms.
- No single rate expression has general application to all heterogeneous reactions.
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