Question

State the effect on the rate of reaction for each of the following: (a) decrease the concentration of a reactant (b) decrease the temperature of the reaction (c) add an acid catalyst

Short answer

(a) Decreases the rate; (b) Decreases the rate; (c) Increases the rate.

Step-by-step solution

Analyzing Concentration Effect

When the concentration of a reactant is decreased, the number of molecules or ions available to undergo reaction per unit volume is reduced. This leads to fewer collisions per second between reactant molecules, thereby decreasing the rate of reaction. Less frequent collisions result in a slower formation of products.

Evaluating Temperature Change

Decreasing the temperature of a reaction lowers the average kinetic energy of the molecules. This reduction in energy means that fewer molecules have the necessary activation energy to successfully collide and form products. Consequently, the rate of reaction decreases as temperature decreases.

Considering Catalyst Addition

Adding an acid catalyst provides an alternative reaction pathway with a lower activation energy. Catalysts increase the rate of reaction by allowing more reactant particles to achieve the activation energy needed for the reaction to occur. As a result, the reaction proceeds at a faster rate with the presence of a catalyst.

Key concepts

Concentration Effect

Imagine you're in a crowded room trying to start conversations. In a packed room, you're much more likely to bump into someone and strike up a chat. Similarly, in chemical reactions, the concentration of the reactants relates directly to how often molecules collide. When you decrease the concentration of a reactant, the number of molecules available for collisions drops.

• Fewer molecules mean fewer collisions.
• Fewer collisions lead to fewer reactions.
• Ultimately, the reaction rate slows down.

Think of it as having fewer players in a game. With fewer players, the game progresses slower because there aren't enough interactions taking place.

Temperature Effect

Temperature significantly affects how molecules move and interact. Temperature is essentially a measure of how much kinetic energy - or motion energy - the molecules have. When you decrease temperature:

• The kinetic energy of molecules decreases.
• Molecules move more slowly and collide less often.
• Less energy in collisions means fewer reactions reach the activation energy needed to proceed.

As the temperature lowers, imagine cars in a traffic jam moving at a crawl instead of cruising freely. The slower the cars move, the less distance they cover, representing fewer successful reactions occurring over time.

Catalyst Function

Catalysts play the magic trick of speeding up reactions without getting used up. This often mystifies students, but it's simpler than it appears. A catalyst works by providing an easier route for the reaction to happen, one that requires less energy to break bonds and form new ones. When you add a catalyst, such as an acid catalyst:

• An alternative pathway is introduced with lower activation energy.
• More molecules can muster enough energy to react because the energy barrier is reduced.
• This results in a quicker formation of products and an overall faster reaction rate.

Picture it like adding more lanes to a highway, allowing traffic to move swiftly even during peak times. The catalyst optimizes the reaction conditions, just like highway lanes optimize traffic flow.