How Catalysts Increase Reaction Rate Activation Energy And Reaction Mechanism Explained
What is a Catalyst? What is Catalysis?
Catalysts are substances that modify the reaction rate and themselves remain chemically and quantitatively unchanged after the reaction. The mechanism by which a catalyst increases the reaction rate is referred to as catalysis.
In this article, we will study what is the meaning of catalyst, how does a catalyst affect the rate of chemical reactions, and also the type of catalysis.
What is a Catalyst in a Chemical Reaction?
Let’s try to understand this with the help of an example- When potassium chlorate is heated, it readily decomposes to give dioxygen. This decomposition occurs at high temperatures- 653-873K.
2KClO3 → 2KCl + 3O2
When manganese dioxide is added, decomposition takes place at a lower temperature and a much faster rate. The catalyst, manganese dioxide thus accelerates the chemical reaction while itself remaining unchanged throughout the reaction. Here manganese oxide acts as a catalyst.
Hence, this reaction is known as a catalytic reaction.
The mechanism followed by the catalyst is catalysis. There are two types of catalysis- heterogeneous and homogeneous catalysis.
What is Homogeneous Catalysis?
If the reactant and catalysts are in the same phase, they are said to be homogeneous catalysis.
The oxidation of sulfur dioxide with dioxygen into sulfur trioxide in the presence of nitrogen oxides as the catalyst.
2SO2(g) + O2(g) → 2SO3(g)
In the given reaction, both the reactant and catalyst are in the same phase i.e gaseous phase.
What is Heterogeneous Catalysis?
If the reactant and the catalysts are in different phases, they are said to be heterogeneous catalysis.
Sulfur dioxide oxidized to sulfur trioxide in the presence of Pt.
2SO2(g) → 2SO3
Here the catalyst is in a solid phase while the reactant in the gaseous phase.
In the presence of nickel as a catalyst, hydrogenation of vegetable oils
Vegetable oils(l) + H2(g) → Vegetable ghee(s)
Here one reactant is in the liquid phase while the catalyst is in a solid phase.
How Does a Catalyst Affect the Rate of Chemical Reactions?
The reactant molecules must have threshold energy for reactants to react and give a product, and the number of molecules with this energy should also be above the threshold value. Activation Energy is the name of this basic energy. Only those reactant molecules would be able to form products that have energy above the energy of activation.
Now the question is - ”What do catalyst do?”
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Catalyst adjusts this activation energy or has a different mechanism of reaction that needs lower activation energy to form products. In intermediate-complex theory, the role of a catalyst in chemical reactions is best explained.
It brings down the activation energy for a reaction, according to intermediate-complex theory, or offers a separate reaction pathway where activation energy is lower.
To form an intermediate complex, it makes temporary bonds with the reactant molecules. To provide the components and the catalyst, this intermediate complex then decomposes. Prior to and after the reaction, the catalyst remains the same. No (chemical) changes are found in them.
A catalyst can only catalyze spontaneous reactions, since it can not modify the Gibbs Free Energy, G, and can thus not catalyze a non-spontaneous reaction.
It has been noted that for a reaction, a catalyst does not alter the equilibrium constant but rather accelerates backwards as well as the forward reaction to rapidly reach equilibrium. A catalyst catalyzes both the forward and the backward response to the same degree, so the point of equilibrium stays the same and is easily reached compared to the reaction without it.
Do you know?
Even our body has different kinds of catalysts, which are called enzymes, which play an important role in chemical reactions that occur within our body.
Enzymes are complex organic nitrogenous compounds that are provided by plants and animals. They are the protein of high molecular mass molecules and form colloidal solutions in water.
They are very powerful catalysts; they catalyze various reactions, particularly numerous reactions. To continue the life process, the bodies of animals and plants are catalyzed by enzymes. The enzymes are thus called biochemical enzymes. The phenomenon and catalysts are known as biochemical catalysis.
FAQs on How Does A Catalyst Affect The Rate Of Chemical Reactions In Chemistry
1. How does a catalyst affect the rate of a chemical reaction?
A catalyst increases the rate of a chemical reaction by providing an alternative pathway with lower activation energy. This means more reactant particles have enough energy to collide successfully per unit time.
- It lowers the activation energy barrier.
- It increases the frequency of effective collisions.
- It remains chemically unchanged at the end of the reaction.
- It does not alter the overall enthalpy change (ΔH) of the reaction.
2. What is a catalyst in chemistry?
A catalyst is a substance that increases the rate of a chemical reaction without being permanently consumed or chemically changed. It participates in intermediate steps but is regenerated at the end of the reaction.
- Lowers activation energy.
- Provides an alternative reaction mechanism.
- Can be solid, liquid, or gas.
- Does not change the final equilibrium position.
3. Why does a catalyst lower activation energy?
A catalyst lowers activation energy by offering an alternative reaction pathway with a lower energy barrier. This pathway may involve the formation of intermediate species or adsorption of reactants on a surface.
- It stabilizes the transition state.
- It weakens bonds in reactant molecules.
- It forms temporary catalyst–reactant complexes.
4. Does a catalyst change the equilibrium of a reaction?
A catalyst does not change the position of equilibrium or the equilibrium constant (K) of a reaction. It speeds up both the forward and reverse reactions equally.
- Equilibrium is reached faster.
- The value of K remains constant at a given temperature.
- The final concentrations of reactants and products are unchanged.
5. What are the types of catalysts in chemistry?
The main types of catalysts are homogeneous, heterogeneous, and enzymatic catalysts. They are classified based on their physical state relative to the reactants.
- Homogeneous catalyst: Same phase as reactants (e.g., H+(aq) in esterification).
- Heterogeneous catalyst: Different phase, often solid with gaseous or liquid reactants (e.g., Fe(s) in the Haber process).
- Enzyme catalyst: Biological protein catalysts (e.g., catalase decomposing H2O2).
6. What is an example of a catalytic reaction?
A common example of a catalytic reaction is the Haber process for ammonia synthesis. In this reaction, iron acts as a heterogeneous catalyst.
- Balanced equation: N2(g) + 3H2(g) ⇌ 2NH3(g)
- Catalyst: Fe(s)
- Conditions: High pressure and moderate temperature
7. How does a catalyst work according to collision theory?
According to collision theory, a catalyst increases reaction rate by increasing the number of effective collisions per second. It does this by lowering the activation energy required for successful collisions.
- More particles have sufficient kinetic energy.
- A greater fraction of collisions exceed activation energy.
- The frequency of successful collisions increases.
8. Does a catalyst get used up in a chemical reaction?
A catalyst is not used up in a chemical reaction and is regenerated at the end of the mechanism. Although it may form temporary intermediates, its chemical composition remains unchanged overall.
- It participates in reaction steps.
- It is reformed in the final step.
- Its mass remains constant after the reaction.
9. What is the difference between a catalyst and an inhibitor?
The key difference is that a catalyst increases reaction rate, while an inhibitor decreases reaction rate. They have opposite effects on the speed of a chemical reaction.
- Catalyst: Lowers activation energy and speeds up reaction.
- Inhibitor: Raises effective activation energy or blocks reactive sites.
- Catalysts do not shift equilibrium position.
- Inhibitors slow down both forward and reverse reactions.
10. How does a heterogeneous catalyst increase reaction rate?
A heterogeneous catalyst increases reaction rate by adsorbing reactant molecules onto its surface, weakening their bonds and bringing them closer together. This surface mechanism lowers activation energy.
- Step 1: Reactants adsorb onto catalyst surface.
- Step 2: Bonds weaken and new bonds form.
- Step 3: Products desorb from the surface.
