Question

Explain why (a) A reaction rate cannot be calculated from the collision frequency alone. (b) The rate of a chemical reaction may increase dramatically with temperature, whereas the collision frequency increases much more slowly. (c) The addition of a catalyst to a reaction mixture can have such a pronounced effect on the rate of a reaction, even if the temperature is held constant.

Short answer

Reaction rates cannot be calculated from collision frequency alone because not all collisions result in reactions. Only those with sufficient energy and correct orientation will be effective. Reaction rates increase dramatically with temperature not just because of increased collision frequency, but also thanks to a stark upsurge in the number of molecules with necessary activation energy. A catalyst can increase reaction rates without needing temperature fluctuation as it provides an alternate reaction pathway with a lower activation energy.

Step-by-step solution

Justifying the inadequacy of collision frequency alone to determine reaction rate

A chemical reaction occurs when reactant molecules collide with each other. However, not all collisions result in reactions. This is because, for a reaction to occur, the colliding molecules must have a certain minimum amount of energy called activation energy and must also be correctly oriented. Therefore, only those collisions which meet this criterion are said to be effective or fruitful. Hence, collision frequency alone is not sufficient to determine the rate of a reaction.

Understanding temperature's effect on reaction rate versus collision frequency

While it's true that increasing the temperature increases the collision frequency, it also escalates the proportion of molecules having energy equal to or greater than the activation energy. This dramatic increase in the number of molecules with requisite energy, as a result of temperature hike, outweighs the more modest increase in collision frequency alone, resulting in a significant increase in reaction rate.

Effect of a catalyst on reaction rate, holding temperature constant

A catalyst works by providing an alternate reaction pathway with a lower activation energy than the uncatalyzed reaction. Thus, more reactant molecules will have the necessary energy to react in the presence of a catalyst, even without a boost in temperature. As a result, the addition of a catalyst can significantly increase the rate of reaction while maintaining constant temperature.

Key concepts

Collision Frequency

The concept of collision frequency refers to the number of times reactant molecules collide in a given period. Collisions are important because they provide the necessary condition for a chemical reaction. However, not every collision results in a reaction.

For a reaction to occur, molecules must collide with sufficient energy and proper orientation. This energy threshold is called **activation energy**. Thus, collision frequency alone cannot determine the reaction rate. It's more about how **effectively** these collisions meet the energy and orientation criteria.

Important points to consider regarding collision frequency:

• High collision frequency doesn't guarantee a high reaction rate.
• Only collisions meeting certain criteria (sufficient energy and correct orientation) lead to reactions.

Activation Energy

Activation energy is the minimum energy that reactant molecules must have during collisions to react and form products. It acts as an energy barrier that molecules need to overcome.

When temperature increases, the average kinetic energy of molecules also rises. A greater proportion of molecules reach or exceed this activation energy. As a result, even a small increase in temperature can lead to a dramatic increase in reaction rate.

Some key aspects of activation energy include:

• It determines the speed of reaction; high activation energy generally corresponds to slower reactions.
• Temperature plays a crucial role in enabling more molecules to surpass the activation energy.

Catalyst

A catalyst is a substance that accelerates a reaction without being consumed in the process. It works by providing an alternative reaction pathway with a lower activation energy.

Since the activation energy is lower, more molecules have enough energy to react even at the same temperature. This leads to a faster reaction rate without needing to increase temperature.

Significant factors about catalysts include:

• Catalysts remain unchanged after the reaction.
• They allow reactions to proceed faster and sometimes make it feasible at lower temperatures.
• Use of catalysts is common in industrial processes to enhance efficiency and reduce energy consumption.