5. Chemical Reactions

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The fifth module of our Chemistry course for the ENEM exam is dedicated to one of the most fascinating and fundamental topics in Chemistry: Chemical Reactions. In this section, we dive into the concepts, principles, and practical applications of chemical reactions, giving you a solid and comprehensive understanding of the subject.

The chemical reactions are processes that transform substances into other substances. These transformations are fundamental to life as we know it. From photosynthesis in plants to the digestion of food in our bodies, chemical reactions are constantly taking place around us and within us.

To understand chemical reactions, it is necessary to know the concepts of reagents and products. Reactants are the substances that react with each other in a chemical reaction, while products are the substances that are formed as a result of the reaction. In a chemical equation, reactants are written on the left, products on the right, and an arrow between them indicates the direction of the reaction.

For example, in the methane (CH4) combustion reaction, methane and oxygen (O2) are the reactants, and carbon dioxide (CO2) and water (H2O) are the products. The chemical equation for this reaction is CH4 + 2O2 -> CO2 + 2H2O.

Another fundamental concept in chemical reactions is that of mass conservation. According to the Law of Conservation of Mass, the total mass of the reactants in a chemical reaction equals the total mass of the products. This means that matter cannot be created or destroyed in a chemical reaction, only transformed.

Chemical reactions can be classified into several types, depending on how they occur. Some of the most common types of chemical reactions include synthesis, decomposition, displacement, and neutralization reactions.

The synthesis reactions are those in which two or more substances combine to form a new substance. For example, the reaction to form water from hydrogen and oxygen is a synthesis reaction: 2H2 + O2 -> 2H2O.

The decomposition reactions are the opposite of synthesis reactions. In them, a substance is broken down into two or more substances. For example, the decomposition of water into hydrogen and oxygen is a decomposition reaction: 2H2O -> 2H2 + O2.

The displacement reactions are those in which one element in a substance is replaced by another element. For example, the displacement reaction of iron in copper sulfate by zinc is a displacement reaction: Zn + CuSO4 -> ZnSO4 + Cu.

The neutralization reactions are those in which an acid and a base react to form a salt and water. For example, the neutralization reaction of hydrochloric acid and sodium hydroxide is a neutralization reaction: HCl + NaOH -> NaCl + H2O.

Understanding chemical reactions and being able to predict the products of a reaction is an essential skill in chemistry. This module will equip you with the necessary knowledge and tools to master this important topic, effectively preparing you for the chemistry section of the ENEM test.

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546. Stoichiometry