Question
The role of a catalyst is to change:
The correct answer is
Activation energy of reaction
Understanding the Role of a Catalyst in Chemical Reactions
A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. Catalysts work by providing an alternative reaction pathway with a lower energy barrier.What is Activation Energy?
Every chemical reaction requires a certain minimum amount of energy to occur. This energy is needed to break existing bonds in the reactants and form new bonds in the products. This minimum energy barrier is known as the activation energy (\( E_a \)). Think of it like pushing a ball over a hill; you need a certain amount of energy to get it over the top before it can roll down the other side. The top of the hill represents the transition state, which is a high-energy intermediate state between reactants and products.How Catalysts Affect Reactions
Catalysts speed up reactions because they lower this activation energy barrier. They do this by forming temporary bonds with the reactants, guiding them through a different pathway that requires less energy to reach the transition state. This means more reactant molecules have enough energy to overcome the barrier at any given temperature, leading to a faster reaction rate. Let's look at the options provided and how a catalyst affects each one:- Gibbs energy of reaction (\( \Delta G \)): The Gibbs energy change of a reaction depends only on the initial and final states of the system (reactants and products). A catalyst affects the path or mechanism of the reaction, not the starting or ending points. Therefore, a catalyst does not change the Gibbs energy of the overall reaction. \( \Delta G \) determines the spontaneity and equilibrium constant, which are not altered by a catalyst.
- Enthalpy of reaction (\( \Delta H \)): Similar to Gibbs energy, the enthalpy change (\( \Delta H \)) is also a state function, depending only on the difference in enthalpy between products and reactants. The catalyst only provides a new reaction pathway; it does not change the inherent energy content of the reactants or products. Thus, the enthalpy of reaction remains unchanged.
- Activation energy of reaction (\( E_a \)): This is the energy barrier that reactants must overcome to form products. A catalyst works precisely by lowering this barrier, offering an alternative reaction mechanism with a lower activation energy. This is the primary way a catalyst speeds up a reaction.
- Equilibrium constant (\( K \)): The equilibrium constant (\( K \)) is related to the Gibbs energy change (\( \Delta G \)) by the equation \( \Delta G^\circ = -RT \ln K \). Since a catalyst does not change \( \Delta G \), it also does not change the equilibrium constant. A catalyst speeds up both the forward and reverse reactions equally, helping the system reach equilibrium faster, but it does not shift the position of the equilibrium. It only affects the rate at which equilibrium is achieved.
Summary Table: Catalyst Effects
| Property | Affected by Catalyst? | Explanation |
|---|---|---|
| Rate of reaction | Yes | Increases by lowering activation energy. |
| Activation energy (\( E_a \)) | Yes | Lowers the energy barrier. |
| Gibbs energy of reaction (\( \Delta G \)) | No | Depends on initial/final states, not pathway. |
| Enthalpy of reaction (\( \Delta H \)) | No | Depends on initial/final states, not pathway. |
| Equilibrium constant (\( K \)) | No | Related to \( \Delta G \); equilibrium position is unchanged. |
| Time to reach equilibrium | Yes | Decreases as rate increases. |
Revision Table: Key Concepts about Catalysts
| Concept | Description | Catalyst Effect |
|---|---|---|
| Catalyst | Substance speeding up reaction without being consumed. | Provides alternative pathway. |
| Activation Energy (\( E_a \)) | Minimum energy for reaction to occur. | Lowered by catalyst. |
| Reaction Rate | Speed at which reactants are converted to products. | Increased by catalyst. |
| Equilibrium | State where forward and reverse rates are equal. | Reached faster with catalyst. |
| Thermodynamic Properties (\( \Delta G \), \( \Delta H \)) | Properties of initial and final states. | Unaffected by catalyst. |
| Equilibrium Constant (\( K \)) | Ratio of products to reactants at equilibrium. | Unaffected by catalyst. |
Additional Information: Types of Catalysts
Catalysts can be classified into different types:- Homogeneous Catalysis: The catalyst is in the same phase as the reactants (e.g., all are liquids or gases).
- Heterogeneous Catalysis: The catalyst is in a different phase from the reactants (e.g., a solid catalyst used for a gas or liquid reaction). Heterogeneous catalysis often involves adsorption of reactants onto the catalyst surface.
- Enzyme Catalysis: Biological catalysts (enzymes) which are proteins that speed up biochemical reactions in living organisms. They are highly specific.
- Autocatalysis: Where one of the products of the reaction acts as a catalyst for the reaction itself.
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Important Questions from Electrochemistry
Identify transition metal complexes which are not octahedral in shape.
(A) [Co(NH₃)₆]³⁺
(B) [Ni(CO)₄]
(C) [CoCl(NH₃)₅]²⁺
(D) [CoCl₂(NH₃)₄]⁺
(E) [PtCl₄]²⁻
Choose the correct answer from the options given below:
The product of complete hydrolysis of XeF₆ in the following reaction is:
XeF₆ + H₂O → ? HF
In a reaction A and B react to form product. The initial rate of reaction (ro) was determined using different initial concentrations of A and B as shown below:
A/mol L-1 B/mol L-1 ro/mol L-1 s-1 0.10 0.30 6.81 × 10-4 0.10 0.10 2.27 × 10-4 0.20 0.30 13.62 × 10-4 What is the initial rate of reaction (ro) when the critical concentration of A and B is 0.50 mol/L and 0.50 mol/L, respectively?
In which of the following actinoid elements 6d subshell is vacant?
Which of the following shows both, Frenkel and Schottky defect?
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