Question

Which of the following factors are responsible for the increase in the rate of a surface catalysed reaction? 1\. A catalyst provides proper orientation for the reactant molecules to react. 2\. Heat of adsorption of reactants on a catalyst helps reactant molecules to overcome activation energy. 3\. The catalyst increases the activation energy of the reaction. 4\. Adsorption increases the local concentration of reactant molecules on the surface of the catalyst. Select the correct answer using the code given below: (a) 1 and 2 (b) 1 and 3 (c) 2,3 and 4 (d) 1,2 and 4 .

Short answer

The correct answer is (d) 1, 2, and 4.

Step-by-step solution

Understanding the Role of a Catalyst

A catalyst provides an alternative reaction pathway with a lower activation energy. It may also orient reactant molecules properly to facilitate effective collisions. Thus, statement 1 is correct: 'A catalyst provides proper orientation for the reactant molecules to react.'

Considering the Heat of Adsorption

When reactants adsorb onto the surface of a catalyst, the heat released (heat of adsorption) can help overcome the activation energy barrier, facilitating the reaction. Thus, statement 2 is also correct: 'Heat of adsorption of reactants on a catalyst helps reactant molecules to overcome activation energy.'

Evaluating Effect on Activation Energy

In general, a catalyst lowers the activation energy; it does not increase it. Therefore, statement 3 is incorrect: 'The catalyst increases the activation energy of the reaction.'

Effect of Adsorption on Concentration

Adsorption of reactants increases their local concentration on the surface of a catalyst, leading to a higher rate of reaction. This makes statement 4 correct: 'Adsorption increases the local concentration of reactant molecules on the surface of the catalyst.'

Combining Correct Statements

Based on the above analysis, the correct statements are 1, 2, and 4; therefore, the correct option is (d) 1, 2, and 4.

Key concepts

Catalyst Orientation

Understanding how catalysts work helps us see why reactions occur faster on their surface. Catalysts not only lower the activation energy, but they also provide what we call 'proper orientation.' This means that they arrange the reactant molecules in such a way that favors a successful collision. This orientation is crucial because only properly aligned molecules can undergo the chemical changes needed to form the products. Thus, a catalyst helps ensure that molecules come together in just the right way to react. By doing so, the energy needed for the reaction to occur is further minimized, enhancing the reaction rate.

Heat of Adsorption

Adsorption is when molecules stick to the surface of a catalyst, and heat is released during this process, known as 'heat of adsorption.' This released energy can be very beneficial. Why? Because it helps to further lower the activation energy of the reaction. This effect allows reactants to reach the energetic state needed for a reaction to proceed. More simply put, the heat of adsorption helps the reactants to gather enough "push" to overcome energy hurdles and carry on with the reaction, thus increasing the rate of the reaction.

Activation Energy

Let's talk about activation energy. This is the energy barrier that must be surpassed for a reaction to happen. Catalysts are superheroes in the chemical reaction world because they help lower this barrier. By providing an alternative route with lower activation energy, catalysts enable reactions to take place with less input energy. It is important to understand that contrary to some misconceptions, there is a decrease, not an increase, in activation energy when a catalyst is present.

Adsorption and Reaction Rate

The role of adsorption on the catalyst surface is particularly interesting. When reactant molecules are adsorbed, they gather on the surface of the catalyst. This increases their local concentration, making it more likely that they will collide and react. Think of it like increasing the number of people in a room—when more people are packed in, bumping into each other is more common. Similarly, when more molecules are gathered on the catalyst surface, the rate at which they react can increase significantly. This local concentration boost is another reason why catalyzed reactions are faster.