Reaction, \(2 \mathrm{Br}^{-}{ }_{(\mathrm{aq})}+\mathrm{Cl}_{2}(\mathrm{aq}) \longrightarrow 2 \mathrm{Cl}^{-}_{(\mathrm{aq} .)}+\mathrm{Br}_{2(\mathrm{aq} .)}\), is used for commercial preparation of bromine from its salts. Suppose we have \(50 \mathrm{~mL}\) of a \(0.060 \mathrm{M}\) solution of NaBr. What volume of a \(0.050 \mathrm{M}\) solution of \(\mathrm{Cl}_{2}\) is needed to react completely with the Br?

Balancing chemical equations is a fundamental skill in chemistry, ensuring that matter is conserved. Every chemical equation represents a recipe for a reaction, spelling out the exact amount of reactants that combine to form products.

For instance, in the reaction of converting bromide ions to bromine, the equation \[2 \mathrm{Br}^{-}_{(\mathrm{aq})} + \mathrm{Cl}_{2(\mathrm{aq})} \longrightarrow 2 \mathrm{Cl}^{-}_{(\mathrm{aq})} + \mathrm{Br}_{2(\mathrm{aq})}\]is already balanced, meaning both sides of the equation have equal numbers of each type of atom. This is crucial for predicting how much of each reactant is necessary and what amount of product will be formed.

Balancing Tips:

• Start by counting and comparing the number of atoms of each element on both sides.
• Adjust coefficients, the numbers before each compound, to get the same number of atoms on both sides.
• Remember to balance polyatomic ions as a whole when they appear on both sides of the equation.

Balancing is not only important for understanding the reaction, it's also the first step in performing calculations, such as those involved in stoichiometry.

The mole concept is a bridge between the micro world of atoms and molecules and the macro world we observe. One mole corresponds to Avogadro's number, \(6.022 \times 10^{23}\), of particles, be they atoms, ions, or molecules.

Understanding the moles of a substance is integral to stoichiometry, which involves calculating the amounts of reactants and products in a chemical reaction. In our example, we have a 0.060 M solution of sodium bromide (NaBr), which means every liter of solution contains 0.060 moles of NaBr. When the volume is 50 mL, or 0.050 L, the moles of NaBr can be found using the equation: \[\text{moles} = \text{concentration (M)} \times \text{volume (L)}\].

Moles allow us to quantify substances and relate them to chemical equations for predicting and analyzing reactions. It's this concept that lets us apply the stoichiometry to find out how much chlorine gas is required to react with the given amount of bromide ions.

Concentration indicates how much of a substance is dissolved in a certain volume of solution and is often expressed in molarity (M), which is moles per liter. Molarity calculations are involved when you need to prepare solutions or understand reaction mixtures.

For example, to know how much chlorine gas is needed to react with the bromide ions from NaBr, you would calculate the molarity of the reacting solutions. The volume of a 0.050 M Cl2 solution that contains the required moles of Cl2 can be calculated using: \[\text{volume} = \frac{\text{moles}}{\text{concentration}}\].

Once you have the moles of Cl2 needed, derived from stoichiometry, you can determine the necessary volume of the chlorine gas solution to react completely with the bromide ions. Knowing how to calculate concentrations ensures you can control the conditions of a reaction effectively, which is essential in any practical chemical work.
The crossover of these concepts in chemistry problems highlights the interconnectedness of stoichiometry, including balanced equations, the mole concept, and concentration calculations in predicting and analyzing the outcomes of chemical reactions.