Problem 15 - Chemistry: The Central Science in SI Units, Global Edition

Question

Write balanced chemical equations corresponding to each of the following descriptions: (a) Potassium cyanide reacts with an aqueous solution of sulfuric acid to form hydrogen cyanide gas. (b) When an aqueous solution of ammonium nitrite \(\left(\mathrm{NH}_{4} \mathrm{NO}_{2}\right)\) reacts with an aqueous solution of potassium hydroxide, ammonia gas, water and metal nitrate is formed. (c) When hydrogen gas is passed over solid hot iron(III) oxide, the resulting reaction produces iron and gaseous water. (d) When liquid ethanoic acid \(\left(\mathrm{CH}_{3} \mathrm{COOH}\right)\) is combusted, carbon dioxide and water are formed.

Step-by-Step Solution

Answer

(a) \(KCN + H_2SO_4 -> HCN + KHSO_4\) (b) \(NH_4NO_2 + KOH -> NH_3 + H_2O + KNO_3\) (c) \(3H_2 + Fe_2O_3 -> 2Fe + 3H_2O\) (d) \(CH_3COOH + 2O_2 -> 2CO_2 + 2H_2O\)

Step 1: Reaction (a) - Potassium cyanide with sulfuric acid

Firstly, let's write down the chemical formulas of the reactants and products: Reactants: Potassium cyanide (KCN) and sulfuric acid (H2SO4) Product: Hydrogen cyanide gas (HCN) Now let's write the unbalanced chemical equation: KCN + H2SO4 -> HCN Balance the equation by ensuring the same number of each type of atom appears on both sides of the equation: KCN + H2SO4 -> HCN + KHSO4 Our balanced equation is: \(KCN + H_2SO_4 -> HCN + KHSO_4\)

Step 2: Reaction (b) - Ammonium nitrite with potassium hydroxide

Write down the chemical formulas of the reactants and products: Reactants: Ammonium nitrite (NH4NO2) and potassium hydroxide (KOH) Products: Ammonia gas (NH3), water (H2O), and potassium nitrate (KNO3) Now let's write the unbalanced chemical equation: NH4NO2 + KOH -> NH3 + H2O + KNO3 Balance the equation by adjusting the coefficients: NH4NO2 + KOH -> NH3 + H2O + KNO3 Our balanced equation is: \(NH_4NO_2 + KOH -> NH_3 + H_2O + KNO_3\)

Step 3: Reaction (c) - Hydrogen gas with iron(III) oxide

Write down the chemical formulas of the reactants and products: Reactants: Hydrogen gas (H2) and iron(III) oxide (Fe2O3) Products: Iron (Fe) and water (H2O) Now let's write the unbalanced chemical equation: H2 + Fe2O3 -> Fe + H2O Balance the equation by adjusting the coefficients: 3H2 + Fe2O3 -> 2Fe + 3H2O Our balanced equation is: \(3H_2 + Fe_2O_3 -> 2Fe + 3H_2O\)

Step 4: Reaction (d) - Combustion of ethanoic acid

Write down the chemical formulas of the reactants and products: Reactants: Ethanoic acid (CH3COOH) and oxygen gas (O2) Products: Carbon dioxide (CO2) and water (H2O) Now let's write the unbalanced chemical equation: CH3COOH + O2 -> CO2 + H2O Balance the equation by adjusting the coefficients: CH3COOH + 2O2 -> 2CO2 + 2H2O Our balanced equation is: \(CH_3COOH + 2O_2 -> 2CO_2 + 2H_2O\)

Key Concepts

Reaction Balancing

Balancing chemical equations is a fundamental concept to ensure that a chemical reaction obeys the law of conservation of mass. This law states that matter cannot be created or destroyed in an isolated system, meaning the total mass of reactants must equal the total mass of products. Thus, in any chemical equation, the number of each type of atom on the reactant side must match the number on the product side.

To balance an equation, we typically:

• Write down the unbalanced equation using the correct chemical formulas for reactants and products.
• Count the number of atoms of each element present in reactants and products.
• Add coefficients (small whole numbers in front of the formulas) to balance each element one at a time.
• Re-check to ensure all elements are balanced, adjusting coefficients as necessary.

This process requires careful adjustment and often some trial and error to get the equation balanced correctly without altering the chemical formulas of the substances involved.

Chemical Formulas

Chemical formulas are symbolic representations of chemical compounds. They show the elements present and their ratios in that compound, serving as the language of chemistry.

There are different types of chemical formulas:

• Empirical formulas show the simplest integer ratio of atoms in a compound. For example, the formula for hydrogen peroxide is HO.
• Molecular formulas depict the exact number of atoms of each element in a molecule. In the case of hydrogen peroxide, it is H₂O₂.
• Structural formulas illustrate the arrangement of atoms in a molecule, providing more detail about the connections and arrangements.

Understanding these formulas is crucial for writing chemical equations, as they allow chemists to communicate complex chemical reactions succinctly and accurately. Chemical formulas are the foundation upon which reaction balancing is performed, as seen in the exercise solutions where each reaction begins with identifying the formulas for reactants and products.

Chemical Reactions

Chemical reactions describe the process where reactants are transformed into products. These transformations involve making and breaking chemical bonds which involve changes in energy.

There are several types of chemical reactions:

• Synthesis reactions involve combining simpler substances to form more complex compounds.
• Decomposition reactions involve breaking down complex molecules into simpler ones.
• Single replacement reactions are reactions where one element replaces another in a compound.
• Double replacement reactions involve two compounds swapping components forming two new compounds.
• Combustion reactions involve oxygen and a fuel (often a hydrocarbon) that produces carbon dioxide, water, and energy.

The provided exercise features various types of reactions, such as synthesis following the interaction of potassium cyanide and sulfuric acid, and combustion during the burning of ethanoic acid. Recognizing these reaction types helps identify the processes at play and aids in anticipating the products formed.

Stoichiometry

Stoichiometry is the calculation aspect of chemistry that deals with the relationships between quantities of reactants and products in a chemical reaction. It is based on the balanced chemical equations.

Key aspects of stoichiometry include:

• Mole ratio: This is derived from the coefficients of a balanced equation and is used to convert between moles of reactants and products.
• Limiting reactant: The reactant that gets consumed first, limiting the amount of product formed.
• Theoretical yield: This is the maximum amount of product that can be produced from the given amounts of reactants.
• Percent yield: Calculates the efficiency of a reaction by comparing the actual yield to the theoretical yield.

Understanding stoichiometry allows chemists to predict how much of each reactant is needed and how much product will be formed. It is fundamental in scaling reactions for practical applications, enabling efficient use of chemicals while minimizing waste.