Question

In Alka-Seltzer antacid tablet bubbles vigorously in room-temperature water but only slowly in a \(50: 50 \mathrm{mix}\) of alcohol and water also at room temperature. Propose an explanation involving the relationship between reaction speed and frequency of molecular collisions.

Short answer

Alcohol reduces collision frequency, slowing reactions compared to water.

Step-by-step solution

Understand the Reaction Speed

The speed of a chemical reaction is influenced by how frequently the reactant molecules collide with each other. For a reaction to occur, the molecules must collide with enough energy and proper orientation. This is known as the frequency of effective collisions.

Analyze Room-Temperature Water

In room-temperature water, the molecules are spaced in such a way that they can move freely and collide with each other often. The water molecules can facilitate more collisions with the antacid tablet, leading to a high frequency of collisions and a faster reaction speed.

Examine the Alcohol-Water Mixture

In a 50:50 alcohol-water mixture at room temperature, alcohol molecules introduce a different environment for molecular movement and spacing. Alcohol molecules are generally larger than water molecules, which reduces the frequency of collisions between the antacid molecules and the liquid molecules.

Relate Collision Frequency with Reaction Speed

The reduced frequency of collisions in the alcohol-water mixture results in a slower reaction. The presence of alcohol reduces the effective collision rate, leading to fewer successful interactions between reactant molecules, hence the reaction (bubbling) occurs more slowly.

Key concepts

Molecular Collisions

Molecules are constantly moving and interacting with each other through collisions. Picture it like an energetic dance where each molecule is a dancer on a crowded floor. When molecules collide with one another, several things can occur. They may simply bounce off, or they may result in a chemical reaction. The frequency and nature of these collisions play a vital role in determining the speed of chemical reactions.

Factors such as temperature, concentration, and molecular size impact molecular collisions. More collisions occur in a space where there are numerous molecules and they can move rapidly. In water, for example, the small size of water molecules allows them to move freely and frequently bump into other particles. This free movement in turn causes numerous collisions, an essential factor in speeding up reactions.

Effective Collisions

Not all molecular collisions lead to a chemical reaction. For a collision to be effective—resulting in a reaction—certain conditions must be met. Two key factors make a collision effective: the energy with which molecules collide and the orientation during the collision.

When molecules collide with sufficient energy, they overcome what's called the activation energy barrier. This barrier is the minimum energy needed for a reaction to occur. Additionally, reacting molecules must collide in the right orientation to allow new bonds to form while existing bonds break. Therefore, while the number of collisions is crucial, the quality of these collisions—energy and orientation—is what really determines if they will be effective.

Chemical Reactions

Chemical reactions happen when molecules undergo transformations during collisions. This transformation creates new chemical bonds and is often accompanied by visual cues like bubbling or color changes. Take, for instance, the Alka-Seltzer tablet reacting with water. As the molecules collide effectively, carbon dioxide is released, causing visible bubbles.

Several factors can affect the speed of a chemical reaction, or reaction rate. Increased temperature and concentration of reactants often lead to more frequent and energetic collisions, thus speeding up the reaction. On the flip side, any factor that decreases the frequency or energy of collisions can slow down the reaction speed.

Alcohol-Water Mixture

An alcohol-water mixture creates a unique environment for molecular collisions. In this scenario, alcohol molecules introduce changes in how substances interact at a molecular level. Alcohol molecules are generally larger than water molecules, affecting how often and how effectively molecules collide.

In a 50:50 alcohol-water mix, the larger alcohol molecules interfere with the free movement of smaller water molecules. This reduced movement results in fewer collisions between the reactant molecules and the solution itself. As a result, the reaction speed slows down because there are not enough effective collisions to drive the reaction as quickly as in pure water. Hence, the Alka-Seltzer bubbles less vigorously in this mixture.