Question

Describe the reason why a reaction between two substances comes to an end.

Short answer

A reaction between two substances comes to an end primarily due to the full consumption of the limiting reactant, which stops the formation of products. Other factors like formation of an equilibrium or changes in temperature and pressure may also contribute to the end of a reaction.

Step-by-step solution

Introduction to Chemical Reactions

In a chemical reaction, substances (reactants) are transformed into new substances (products). The reaction comes to an end when there is no more reactant left to be converted into products.

Role of Limiting Reactants

In a reaction, the limiting reactant is the substance that is completely consumed by the reaction process. When the limiting reactant is used up, the reaction stops as there is no more reactant available to form products. The other reactants that are left unreacted are called excess reactants.

Factors Affecting the Reaction Process

Several factors can cause a reaction to come to an end. Some of these factors include: 1. Limited availability of reactants: If there are insufficient quantities of one or more reactants, the reaction will come to an end as soon as the available reactant(s) is used up. 2. Formation of an equilibrium: Some reactions are reversible and reach an equilibrium point, meaning that the rate of the forward reaction becomes equal to the rate of the reverse reaction. In this case, the reaction stops due to the balance between the reactants and products. 3. Temperature and pressure: Some reactions are highly sensitive to external conditions like temperature and pressure. A change in these conditions may inhibit the reaction and cause it to come to a halt.

Summary

A reaction between two substances comes to an end primarily due to the full consumption of the limiting reactant, which stops the formation of products. Other factors like formation of an equilibrium or changes in temperature and pressure may also contribute to the end of a reaction.

Key concepts

Limiting Reactants

In the context of chemical reactions, the concept of limiting reactants is fundamental to understanding why a reaction may cease. Imagine you're baking cookies and you've run out of sugar; no matter how much flour you have left, you can't make more cookies without sugar. Similarly, in a chemical reaction, the limiting reactant is the ingredient that determines how much product can be formed. It's the reactant that will be completely used up first, bringing the reaction to a stop because there's simply no more of it left to react.

The other reactants present, which have not been completely used up, are referred to as the excess reactants. They remain because there was more of them than was necessary to completely react with the limiting reactant. This idea is crucial not only in laboratory settings but also in industrial processes where maximizing efficiency and predicting product yield are pivotal.

Chemical Equilibrium

Another important concept is chemical equilibrium, which occurs in reversible reactions—those where the products can react to form the original reactants. Unlike in irreversible reactions, where reactants are permanently converted to products, reversible reactions can reach a state of balance. At this point, the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products over time.

Think of it as a crowded room where people are just as likely to enter as they are to leave; the overall number inside the room doesn't change because the in-flow and out-flow are balanced. In a chemical system at equilibrium, this balance means that the reaction appears to stop, but in reality, the reactants and products are still continuously converting back and forth. Factors like temperature and pressure can shift this equilibrium, thereby affecting the reaction's progression and product amounts.

Reaction Completion

Finally, the reaction completion is a stage that signifies the end of the chemical reaction. But how do we know a reaction has completed? In the case of a reaction that goes to completion, this is generally obvious because the limiting reactant has run out, just like when the sugar is gone in our cookie analogy. With no more limiting reactant to fuel the reaction, no additional product can be formed.

However, in the case of a reaction that reaches chemical equilibrium, completion is not so straightforward. Even though the reaction appears to have stopped, with no observable changes in the concentrations of reactants and products, on the molecular level the reaction is still in motion. The term 'completion' in this context is a bit of a misnomer because the dynamic process of reactants and products converting into each other at equal rates continues indefinitely, unless some external condition changes to disturb the equilibrium state.