Problem 6 - Chemistry: Atoms First

Question

Write a balanced equation describing each of the following chemical reactions. (a) Solid potassium chlorate, \(\mathrm{KClO}_{3}\), decomposes to form solid potassium chloride and diatomic oxygen gas. (b) Solid aluminum metal reacts with solid diatomic iodine to form solid \(\mathrm{Al}_{2} \mathrm{I}_{6}\) (c) When solid sodium chloride is added to aqueous sulfuric acid, hydrogen chloride gas and aqueous sodium sulfate are produced. (d) Aqueous solutions of phosphoric acid and potassium hydroxide react to produce aqueous potassium dihydrogen phosphate and liquid water.

Step-by-Step Solution

Answer

(a) \(2\mathrm{KClO}_3(s) \rightarrow 2\mathrm{KCl}(s) + 3\mathrm{O}_2(g)\) (b) \(2\mathrm{Al}(s) + 3\mathrm{I}_2(s) \rightarrow \mathrm{Al}_2\mathrm{I}_6(s)\) (c) \(2\mathrm{NaCl}(s) + \mathrm{H}_2\mathrm{SO}_4(aq) \rightarrow 2\mathrm{HCl}(g) + \mathrm{Na}_2\mathrm{SO}_4(aq)\) (d) \(\mathrm{H}_3\mathrm{PO}_4(aq) + 3\mathrm{KOH}(aq) \rightarrow \mathrm{KH}_2\mathrm{PO}_4(aq) + 3\mathrm{H}_2\mathrm{O}(l)\)

Step 1: Write the formula for potassium chlorate decomposing

The chemical formula for solid potassium chlorate is \(\mathrm{KClO}_3\). When it decomposes, it forms solid potassium chloride (\(\mathrm{KCl}\)) and diatomic oxygen gas (\(\mathrm{O}_2\)). The chemical equation before balancing is \(\mathrm{KClO}_3(s) \rightarrow \mathrm{KCl}(s) + \mathrm{O}_2(g)\).

Step 2: Balance the oxygen atoms in potassium chlorate decomposition

There are 3 oxygen atoms on the reactant side and 2 oxygen atoms in diatomic oxygen on the product side. To balance them, place a coefficient of 2 in front of \(\mathrm{KClO}_3\) for 6 oxygens on the reactant side and a coefficient of 3 in front of \(\mathrm{O}_2\) for 6 oxygens on the product side. The balanced chemical equation is \({2\mathrm{KClO}_3(s) \rightarrow 2\mathrm{KCl}(s) + 3\mathrm{O}_2(g)}\).

Step 3: Write the formula for the reaction between aluminum and iodine

Solid aluminum metal has the formula \(\mathrm{Al}\), and solid diatomic iodine has the formula \(\mathrm{I}_2\). They react to form solid \(\mathrm{Al}_2\mathrm{I}_6\). The chemical equation before balancing is \(\mathrm{Al}(s) + \mathrm{I}_2(s) \rightarrow \mathrm{Al}_2\mathrm{I}_6(s)\).

Step 4: Balance the reaction between aluminum and iodine

To balance the aluminum atoms, place a coefficient of 2 in front of the \(\mathrm{Al}\). To balance the iodine atoms, place a coefficient of 3 in front of \(\mathrm{I}_2\). The balanced chemical equation is \(2\mathrm{Al}(s) + 3\mathrm{I}_2(s) \rightarrow \mathrm{Al}_2\mathrm{I}_6(s)\).

Step 5: Write the formula for sodium chloride reacting with sulfuric acid

The formula for solid sodium chloride is \(\mathrm{NaCl}\), and the formula for aqueous sulfuric acid is \(\mathrm{H}_2\mathrm{SO}_4\). They react to form hydrogen chloride gas (\(\mathrm{HCl}\)) and aqueous sodium sulfate (\(\mathrm{Na}_2\mathrm{SO}_4\)). The chemical equation before balancing is \(\mathrm{NaCl}(s) + \mathrm{H}_2\mathrm{SO}_4(aq) \rightarrow \mathrm{HCl}(g) + \mathrm{Na}_2\mathrm{SO}_4(aq)\).

Step 6: Balance the chemical equation for sodium chloride and sulfuric acid

To balance the equation, note that there are 2 sodium atoms on the product side, so place a coefficient of 2 in front of \(\mathrm{NaCl}\) on the reactant side. This will also balance chlorine atoms. Since sulfur and hydrogen are already balanced, the balanced chemical equation is \(2\mathrm{NaCl}(s) + \mathrm{H}_2\mathrm{SO}_4(aq) \rightarrow 2\mathrm{HCl}(g) + \mathrm{Na}_2\mathrm{SO}_4(aq)\).

Step 7: Write the formula for the reaction between phosphoric acid and potassium hydroxide

The formula for phosphoric acid is \(\mathrm{H}_3\mathrm{PO}_4\) and potassium hydroxide is \(\mathrm{KOH}\). They react to form potassium dihydrogen phosphate (\(\mathrm{KH}_2\mathrm{PO}_4\)) and liquid water (\(\mathrm{H}_2\mathrm{O}\)). The chemical equation before balancing is \(\mathrm{H}_3\mathrm{PO}_4(aq) + \mathrm{KOH}(aq) \rightarrow \mathrm{KH}_2\mathrm{PO}_4(aq) + \mathrm{H}_2\mathrm{O}(l)\).

Step 8: Balance the reaction between phosphoric acid and potassium hydroxide

To balance the potassium atoms, place a coefficient of 3 in front of \(\mathrm{KOH}\). To balance the hydrogens and oxygens, place a coefficient of 3 in front of \(\mathrm{H}_2\mathrm{O}\). The hydrogen atoms in \(\mathrm{KH}_2\mathrm{PO}_4\) and \(\mathrm{H}_2\mathrm{O}\) will now sum up to match those in \(\mathrm{H}_3\mathrm{PO}_4\). The balanced chemical equation is \(\mathrm{H}_3\mathrm{PO}_4(aq) + 3\mathrm{KOH}(aq) \rightarrow \mathrm{KH}_2\mathrm{PO}_4(aq) + 3\mathrm{H}_2\mathrm{O}(l)\).

Key Concepts

Stoichiometry

Stoichiometry is the cornerstone of chemical equations, as it allows us to quantify the relationships between reactants and products in a chemical reaction. It involves calculations that rely on the mole concept and the law of conservation of mass to ensure that atoms are neither created nor destroyed in reactions.

Thinking of stoichiometry as a recipe is helpful; just as a recipe specifies the exact amount of each ingredient needed, stoichiometry provides the precise measure of reactants required to produce a set amount of product. In the exercise, this is seen when balancing oxygen atoms by using coefficients that maintain the ratio of elements according to their mole ratios.

By using stoichiometry, one can predict the amount of products formed from given reactants or determine the amount of reactants needed to create a certain amount of products. This is not only essential in academic problems but also vital in real-world applications such as pharmaceuticals, materials science, and environmental engineering where precise chemical formulations are crucial.

Chemical Reactions

Chemical reactions are processes where reactants transform into products through the breaking and forming of chemical bonds. There are many types of chemical reactions, including synthesis, decomposition, single replacement, and double replacement reactions.

In our exercise, for example, we have a decomposition reaction where potassium chlorate breaks down into potassium chloride and oxygen gas. It is essential to identify the type of reaction to balance the equation correctly, as each type follows different patterns.

Chemical reactions are depicted through chemical equations that need to be balanced to reflect the conservation of mass. When balancing equations, it’s important to adjust the coefficients, the numbers before the chemical formulas, to ensure that the same number of each type of atom appears on both sides of the equation. This reflects the physical reality that in a closed system, atoms are not lost or gained—only rearranged.

Mole Concept

The mole concept is a fundamental idea in chemistry that relates the mass of substances to the number of particles or entities they contain. One mole corresponds to Avogadro's number, which is approximately \(6.022 \times 10^{23}\) entities, whether they are atoms, molecules, ions, or electrons.

This concept allows chemists to