6. Stoichiometry

Stoichiometry is a fundamental part of Chemistry that focuses on the quantitative study of the substances that are involved in a chemical reaction. The term 'stoichiometry' comes from the Greek words 'Stoicheion' (element) and 'Metron' (measurement). Therefore, stoichiometry is basically the measurement of chemical elements.

Stoichiometric calculations are based on the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction. This means that the total amount of each element does not change during the reaction. Stoichiometry uses the quantitative relationship between reactants and products in a chemical reaction, which is obtained from balanced chemical equations.

For example, the reaction to form water from hydrogen and oxygen is given by the balanced chemical equation: 2H₂ + O₂ -> 2H₂O. This means that for every 2 moles of hydrogen that react with 1 mole of oxygen, 2 moles of water are produced. Therefore, stoichiometry allows us to predict how much water will be produced if we know how much hydrogen and oxygen are reacting.

Stoichiometry is also useful for finding the amount of a reagent needed to react completely with a given amount of another reagent. This is called the limiting reagent calculation. The limiting reagent is the reagent that is completely consumed in a reaction and determines the maximum amount of product that can be formed.

Stoichiometric calculations can also involve determining the amount of energy involved in a chemical reaction. This is called energy stoichiometry. The amount of energy involved in a chemical reaction is given by the enthalpy change of the reaction, which is the difference between the energy of the products and the energy of the reactants. Energy stoichiometry is important in the chemical industry, where the energy efficiency of chemical reactions is an important concern.

In addition, stoichiometry is also used to determine the purity of a substance. This is done by comparing the amount of product formed in a reaction with the amount expected if the substance were pure. If the amount of product formed is less than expected, it indicates that the substance is not pure.

To perform stoichiometric calculations, it is necessary to know the molar mass of the substances involved. Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). The molar mass can be found from the periodic table by adding the molar masses of the elements that make up the substance.

In summary, stoichiometry is a powerful tool in chemistry that allows you to predict the amount of reactants and products in a chemical reaction, determine the amount of energy involved in a reaction, find the limiting reagent in a reaction, and determine the purity of a substance. Understanding stoichiometry is essential for success in the study of chemistry and is an important topic for the ENEM exam.

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