Zinc sulfide reacts with oxygen according to the reaction: $$ 2 \mathrm{ZnS}(s)+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{ZnO}(s)+2 \mathrm{SO}_{2}(g) $$ A reaction mixture initially contains \(4.2 \mathrm{~mol} \mathrm{ZnS}\) and \(6.8 \mathrm{~mol}\) \(\mathrm{O}_{2} .\) Once the reaction has occurred as completely as possible, what amount (in moles) of the excess reactant remains?

The branch of chemistry known as stoichiometry is vital for understanding quantitative aspects of chemical reactions. It involves calculations based on the relationships between reactants and products in a chemical reaction. These relationships are determined by the coefficients in a balanced chemical equation, which expresses the conservation of mass and the fixed proportions in which chemicals react.

For instance, if we consider the reaction of zinc sulfide with oxygen, stoichiometry allows us to predict how much product will form given certain amounts of reactants. By applying stoichiometric principles, we can determine the limiting reactant, the excess reactant, and the amounts of each substance involved.

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions are characterized by the rearrangement of atoms and involve breaking bonds within reactants and forming new bonds to create the products.

The reaction between zinc sulfide and oxygen is an example of a chemical reaction where zinc sulfide (ZnS) and oxygen (O₂) are converted to zinc oxide (ZnO) and sulfur dioxide (SO₂). This particular reaction is a combustion reaction, where a substance combines with oxygen to release energy.

A balanced chemical equation represents a chemical reaction with the same number of atoms for each element in the reactants and products. Balancing an equation is crucial because it reflects the Law of Conservation of Mass, which states that mass is neither created nor destroyed in a chemical reaction.

In the equation given for the zinc sulfide reaction, the coefficients (2, 3, and 2) ensure that there are equal numbers of zinc, sulfur, and oxygen atoms on both sides of the equation. This balance is necessary for accurate stoichiometric calculations.

The mole ratio is derived from the coefficients of a balanced equation and represents the proportional relationship between the amounts of reactants and products in a chemical reaction. In stoichiometry, the mole ratio is used to convert between moles of one substance and moles of another.

In our example, the mole ratio of ZnS to O₂ is 2 to 3, as indicated by their balanced equation coefficients. This ratio tells us that two moles of zinc sulfide react with three moles of oxygen. By using these ratios, we can calculate how many moles of one reactant are required to completely react with a certain amount of another reactant.

The excess reactant is the substance in a chemical reaction that remains after all of the limiting reactant has been consumed. Identifying the excess reactant is vital in order to calculate the remaining quantity left over after the reaction has occurred.

In the provided example, by determining that zinc sulfide (ZnS) is the limiting reactant, we can establish that oxygen (O₂) is in excess. After the reaction has completed, we can calculate that 0.5 moles of oxygen remain unreacted. This excess reactant can be crucial for understanding the efficiency and completion of a chemical process.