Question

Write a balanced equation to show (a) the formation of gas bubbles when cobalt reacts with hydrochloric acid. (b) the reaction of copper with nitric acid. (c) the reduction half-reaction of dichromate ion to \(\mathrm{Cr}^{3+}\) in acid solution.

Short answer

a) Cobalt reacting with hydrochloric acid. b) Copper reacting with nitric acid. c) Reduction half-reaction of dichromate ion to Cr3+ in an acid solution. Answer: a) Co + 2HCl → CoCl2 + H2 b) 3Cu + 8HNO3 → 3Cu(NO3)2 + 4H2O + 2NO2 c) Cr2O7^(2-) + 14H^+ → 2Cr^(3+) + 7H2O

Step-by-step solution

Reaction (a): Cobalt reacting with hydrochloric acid

In this reaction, cobalt (Co) reacts with hydrochloric acid (HCl) to produce cobalt(II) chloride (CoCl2), hydrogen gas (H2), and gas bubbles. The unbalanced equation can be written as: Co + HCl \(\rightarrow\) CoCl2 + H2 To balance the equation, we need to make sure that the number of atoms on both sides of the equation is equal. In this case, we can simply add a 2 in front of HCl to balance the chlorines and hydrogens: Co + 2HCl \(\rightarrow\) CoCl2 + H2 So, the balanced equation for reaction (a) is: Co + 2HCl \(\rightarrow\) CoCl2 + H2

Reaction (b): Copper reacting with nitric acid

In this reaction, copper (Cu) reacts with nitric acid (HNO3) to produce copper(II) nitrate (Cu(NO3)2), water (H2O), and nitrogen dioxide (NO2). The unbalanced equation can be written as: Cu + HNO3 \(\rightarrow\) Cu(NO3)2 + H2O + NO2 To balance the equation, we need to make sure that the number of atoms on both sides of the equation is equal. In this case, first, we need to put a 2 in front of HNO3, followed by adjusting the coefficients for H2O and NO2 to match the number of atoms: 3Cu + 8HNO3 \(\rightarrow\) 3Cu(NO3)2 + 4H2O + 2NO2 So, the balanced equation for reaction (b) is: 3Cu + 8HNO3 \(\rightarrow\) 3Cu(NO3)2 + 4H2O + 2NO2

Reaction (c): Reduction half-reaction of dichromate ion in acid solution

In this half-reaction, dichromate ion (Cr2O7\(^{2-}\)) is reduced to chromium(III) ion (Cr\(^{3+}\)) in an acidic solution. As it is a half-reaction, we need to balance the number of atoms and charges using the half-reaction balancing method. The unbalanced reduction half-reaction is: Cr2O7\(^{2-}\) \(\rightarrow\) Cr\(^{3+}\). To balance the atoms, first balance the chromium and oxygen atoms: 1. Add 2Cr\(^{3+}\) on the right side for balancing the chromium atoms. 2. To balance the oxygen atoms, add 7H2O on the right side. Cr2O7\(^{2-}\) \(\rightarrow\) 2Cr\(^{3+}\) + 7H2O Next, to balance the charges, add 14H\(^+\) ions on the left side since we have 2 extra negative charges on the left side due to Cr2O7\(^{2-}\): Cr2O7\(^{2-}\) + 14H\(^+\) \(\rightarrow\) 2Cr\(^{3+}\) + 7H2O Now, the reduction half-reaction of dichromate ion to chromium(III) ion is balanced: Cr2O7\(^{2-}\) + 14H\(^+\) \(\rightarrow\) 2Cr\(^{3+}\) + 7H2O

Key concepts

Balancing Equations

Balancing chemical equations is a fundamental skill in chemistry that ensures that the same number of each type of atom appears on both sides of the equation. This is vital because it reflects the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. To balance an equation, follow these simple steps:

• Write down the unbalanced equation.
• List all elements involved in the reaction.
• Count the number of atoms of each element on both sides of the equation.
• Add coefficients (whole numbers) in front of compounds to balance the atoms of each element on both sides. Start with the element that appears in the least amount of compounds.
• Repeat the process until all elements are balanced.
• Finally, check your work by counting the atoms for each element to ensure they are equal on both sides.

For example, in the reaction of cobalt with hydrochloric acid, cobalt and hydrogen atoms must be counted and adjusted so that both sides have an equal number of each atom. The balanced equation ensures that two HCl molecules provide enough chlorine to match one CoCl2 molecule while producing hydrogen gas.

Reduction Half-Reaction

Understanding reduction half-reactions is key to mastering redox reactions in chemistry. A redox reaction is a chemical reaction where oxidation and reduction processes occur simultaneously. In a reduction half-reaction, one species gains electrons, resulting in a decrease in oxidation state. This half-reaction focuses only on the reduction process, separating it from the oxidation.To balance a reduction half-reaction:

• Start with the unbalanced reduction half-reaction and identify the species being reduced.
• Balance the atoms of the species being reduced, except for oxygen and hydrogen.
• Balance the oxygen atoms by adding water (H2O) molecules as needed.
• Balance the hydrogen atoms by adding hydrogen ions (H\(^+\)) if in acidic solution or hydroxide ions (OH\(^-\)) in a basic solution.
• Finally, balance the charges by adding electrons (\(\text{e}^-\)) to the more positive side.

An example of a reduction half-reaction is the conversion of dichromate ions (Cr2O7\(^{2-}\)) to chromium(III) ions (Cr\(^{3+}\)) in an acidic solution. Balancing such reactions involves aligning chromium, oxygen, and adjusting for charges by introducing electrons.

Metal-Acid Reactions

Metal-acid reactions are a type of chemical reaction where a metal reacts with an acid to produce salt and hydrogen gas. These reactions are classic examples of single displacement reactions and often produce visible gas bubbles. The general form of this reaction is:\[ \text{Metal} + \text{Acid} \rightarrow \text{Salt} + \text{Hydrogen gas} \]Here is how these reactions typically work:

• The metal loses electrons and gets oxidized, turning into metal ions.
• The acid provides H\(^+\) ions, which gain electrons to form hydrogen gas (H2), resulting in a reduction process.
• The product includes a metal salt, formed by the metal's cation and the acid's anion.

For example, when cobalt reacts with hydrochloric acid, the cobalt loses electrons to become Co\(^{2+}\) ions. The HCl provides H\(^+\) ions, which gain electrons and evolve as hydrogen gas, leading to the formation of cobalt(II) chloride and gas bubbles.