Problem 84 - Advanced Problems in Physical Chemistry for Competitive Examinations

Question

At a certain temperature, the reaction between \(\mathrm{NO}\) and \(\mathrm{O}_{2}\) to form \(\mathrm{NO}_{2}\) is fast, while that between \(\mathrm{CO}\) and \(\mathrm{O}_{2}\) is slow. It may be concluded that (a) \(\mathrm{NO}\) is more reactive than \(\mathrm{CO}\). (b) CO is smaller in size than NO. (c) activation energy for the reaction: \(2 \mathrm{NO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{NO}_{2}\) is less. (d) activation energy for the reaction: \(2 \mathrm{NO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{NO}_{2}\) is high.

Step-by-Step Solution

Answer

The correct conclusions are: (a) \(\mathrm{NO}\) is more reactive than \(\mathrm{CO}\) and (c) activation energy for the reaction \(2 \mathrm{NO} + \mathrm{O}_{2} \rightarrow 2 \mathrm{NO}_{2}\) is less.

Step 1: Understanding Chemical Reactivity

The reactivity of a chemical species in a reaction is often reflected by the speed or rate of the reaction. A faster reaction typically indicates a more reactive species or lower activation energy.

Step 2: Evaluating the Given Information

From the given information, we know that the reaction between \(\mathrm{NO}\) and \(\mathrm{O}_{2}\) to form \(\mathrm{NO}_{2}\) is fast, which suggests higher reactivity or lower activation energy for this reaction compared to the reaction between \(\mathrm{CO}\) and \(\mathrm{O}_{2}\), which is slow.

Step 3: Analyzing Choice (a)

Considering the rate of reaction is a valid indicator of reactivity, able to say that \(\mathrm{NO}\) is more reactive than \(\mathrm{CO}\) because the reaction involving \(\mathrm{NO}\) is faster.

Step 4: Analyzing Choice (b)

The size of the molecules \(\mathrm{CO}\) and \(\mathrm{NO}\) is not provided, and molecule size is not directly related to speed of reaction. Therefore, this choice cannot be concluded based on the given information.

Step 5: Analyzing Choice (c)

A faster reaction rate usually suggests that the activation energy required for the reaction to proceed is lower. Therefore, it could be considered that the activation energy for the reaction \(2 \mathrm{NO} + \mathrm{O}_{2} \rightarrow 2 \mathrm{NO}_{2}\) is less.

Step 6: Analyzing Choice (d)

Given that the reaction between \(\mathrm{NO}\) and \(\mathrm{O}_{2}\) is fast, it would be incorrect to conclude that the activation energy for the reaction is high. A high activation energy would result in a slower reaction rate.

Key Concepts

Rate of Chemical Reactions

When we talk about chemical reactions, one of the first things that might come to mind is how quickly they occur. This speed, or rate of chemical reactions, is a measure of the change in concentration of reactants or products over time. Factors like temperature, pressure, concentration, and the presence of catalysts can influence these rates.

Picture a busy kitchen during a morning rush; the quick preparation of dishes can be akin to a fast chemical reaction, while a slower pace when the kitchen is less bustling is like a slower reaction. In our exercise scenario, the rapid formation of 2 from and 2 gives us a hint that we're dealing with a 'busy kitchen' scenario, where the reactivity of is higher than in the presence of 2.

Activation Energy in Reactions

Imagine you're at the bottom of a hill and your favorite ice cream stand is on the other side. To enjoy the treat, you must first climb up and over the hill. In chemical terms, you just encountered what's known as activation energy, the minimum energy needed by reacting particles to form a product.

This energetic 'hill' must be overcome for reactants to transform into products. Catalysts are like a tunnel through our metaphorical hill, reducing the energy required to reach the other side, thus accelerating the reaction. Reflecting on the original exercise, we can see the rapid reaction of and 2 to produce 2 likely has a smaller 'hill' to climb, indicating a lower activation energy compared to the slow reaction involving and 2.

Comparative Reactivity of Chemical Species

Chemical species have varying reactivity, much like students in a class who learn at different paces. Reactivity is influenced by several factors, including atomic or molecular size, electron configurations, and the energy difference between reactants and products.

In the context of the exercise, we observe that nitrogen monoxide () reacts more readily with oxygen (2) than carbon monoxide () does. It's not a direct measure of molecular size (which isn't a focus in this scenario) but rather how willing and able a molecule is to engage in a chemical reaction. Just as some students are quicker to raise their hands to answer questions, is quicker to react than , showcasing its higher reactivity.