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?
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 reaction pathway with a lower activation energy (Ea). It does not get consumed in the reaction and remains chemically unchanged at the end. By lowering the energy barrier, more reactant molecules can successfully convert into products per unit of time, thus speeding up the reaction. For more details, you can refer to the Chemical Kinetics Class 12 Notes.
2. What are the key characteristics of a catalyst?
A catalyst has several distinct properties that define its function in a chemical reaction. The main characteristics are:
Chemical Composition: It remains unchanged in mass and chemical composition after the reaction is complete.
Small Quantity: Only a very small amount of the catalyst is typically needed to alter the rate of reaction for large amounts of reactants.
Specificity: Catalysts are often highly specific, meaning a particular catalyst will only speed up a specific reaction. You can learn more about Catalysis here.
No Initiation: A catalyst cannot start a reaction that is not thermodynamically feasible. It can only accelerate a reaction that is already occurring, however slowly.
Equilibrium: It does not alter the position of equilibrium in a reversible reaction; it only helps in attaining the equilibrium faster.
3. What are positive and negative catalysts? Provide an example for each.
Catalysts can be classified based on whether they increase or decrease the reaction rate:
A positive catalyst is a substance that increases the rate of a chemical reaction. It works by lowering the activation energy. Example: Manganese dioxide (MnO₂) acts as a positive catalyst for the decomposition of hydrogen peroxide (H₂O₂) into water and oxygen.
A negative catalyst, also known as an inhibitor, is a substance that decreases the rate of a chemical reaction. It generally works by increasing the activation energy or by a different mechanism. Example: Acetanilide is used as a negative catalyst to slow down the decomposition of hydrogen peroxide.
4. How can the effect of a catalyst be explained using an energy profile diagram?
An energy profile diagram plots potential energy against the reaction progress. The peak of the curve represents the transition state, and the energy required to reach this peak is the activation energy (Ea). When a catalyst is introduced, it provides a new reaction mechanism. This new path has a different transition state with a significantly lower energy level. Therefore, on the diagram, the catalyzed reaction shows a lower activation energy peak compared to the uncatalyzed reaction. This lower energy barrier allows more molecules to react, increasing the reaction rate. Importantly, the initial and final energy levels of reactants and products remain unchanged, so the overall enthalpy change (ΔH) of the reaction is not affected by the catalyst. More on this can be explored under Activation Energy concepts.
5. Does a catalyst change the equilibrium position of a reversible reaction?
No, a catalyst does not change the final equilibrium position of a reversible reaction. A catalyst increases the rate of both the forward and the backward reactions to an equal extent. Since the ratio of the forward and backward rate constants determines the equilibrium constant (K), this ratio remains unchanged. The only effect of a catalyst on a reversible reaction is that it helps the system reach equilibrium much faster than it would without the catalyst. You can find more information in the Equilibrium Class 11 Notes.
6. How does a catalyst work according to the Collision Theory?
According to the Collision Theory, a reaction occurs when reactant particles collide with sufficient energy (activation energy) and in the correct orientation. A catalyst increases the reaction rate by lowering this required activation energy. By providing an alternative pathway, it ensures that a larger fraction of collisions are successful or 'effective' at any given temperature. This increases the frequency of effective collisions per unit of time, which directly translates to a faster rate of reaction.
7. What are some important industrial and biological examples of catalysts?
Catalysts are crucial in many processes. Some key examples include:
Haber Process: Finely divided iron is used as a catalyst to manufacture ammonia from nitrogen and hydrogen gases, a vital process for producing fertilisers. Learn more about the Haber Process.
Catalytic Converters: In vehicle exhausts, metals like platinum and rhodium catalyse the conversion of toxic gases (like carbon monoxide and nitrogen oxides) into less harmful substances (like carbon dioxide and nitrogen gas).
Enzymes: These are biological catalysts. For instance, the enzyme amylase in our saliva catalyses the breakdown of complex carbohydrates (starch) into simpler sugars.
8. Why are catalysts usually highly specific to the reactions they catalyse?
The high specificity of catalysts, also known as catalytic selectivity, arises because their mechanism often involves the formation of a temporary intermediate complex with the reactants. The surface of a catalyst has specific 'active sites' with a unique shape, size, and electronic structure. Only reactant molecules with a complementary shape and chemical affinity can bind effectively to these active sites, much like a lock and key. This specific interaction lowers the activation energy for only one particular reaction, making the catalyst highly selective. You can read more about the Activity and Selectivity of a Catalyst.
9. What is the main difference between a catalyst poison and a negative catalyst?
While both substances can slow down a reaction, their roles and mechanisms are different:
A negative catalyst (or inhibitor) is intentionally added to a reaction to slow it down. It works by increasing the activation energy or interfering with the reaction mechanism.
A catalyst poison is a substance that reduces or completely destroys the effectiveness of a positive catalyst. It does this by strongly adsorbing onto the active sites of the catalyst, thereby blocking them from the reactants. Poisoning is an undesirable effect that deactivates a catalyst. For example, arsenic can poison the iron catalyst used in the Haber Process.
