Question

Nickel sulfate, NiSO_ \(_{4}(a q),\) is a green solution. Nickel chloride, \(\mathrm{NiCl}_{2}(a q),\) is a yellow solution. And hydrochloric acid, HCI \((a q),\) is a clear, colorless solution. If you add nickel, Ni(s), to hydrochloric acid, HClaq), what color solution do you expect to form? Explain your reasoning.

Short answer

The solution will turn yellow because nickel chloride forms.

Step-by-step solution

Understanding the Reagents

We begin by identifying the components involved in the reaction. Nickel sulfate and nickel chloride each form colored solutions due to the presence of nickel ions. Nickel sulfate forms a green solution, while nickel chloride forms a yellow solution. Pure hydrochloric acid, comprised of hydrogen and chloride ions, is clear and colorless.

Anticipating the Reaction

Next, we consider the possible chemical reaction that could occur: when nickel (Ni) metal is added to hydrochloric acid (HCl), a reaction might produce nickel ions in solution. The potential reaction is given by \( \text{Ni(s) + 2HCl(aq) \rightarrow NiCl_2(aq) + H_2(g)} \). During this reaction, the nickel dissolves and forms nickel ions that result in nickel chloride.

Analyzing the Resulting Solution Color

Nickel chloride is known to form a yellow solution when dissolved in water. In the anticipated reaction, nickel ions combine with chloride ions to produce \( \text{NiCl}_2 \), resulting in a yellow solution. Thus, the resulting solution color is expected to match that of nickel chloride, which is yellow.

Key concepts

Chemical Reactions

When we talk about chemical reactions, we are describing a process where substances (reactants) undergo a transformation to become new substances (products). Understanding these reactions is key to predicting what happens when different chemicals come into contact. For example, when nickel metal is added to hydrochloric acid, a chemical reaction occurs that transforms the metal into nickel chloride and hydrogen gas.
This is an example of a single displacement reaction, where nickel takes the place of hydrogen in the acid:

• Reactants: Nickel metal and hydrochloric acid.
• Products: Nickel chloride and hydrogen gas.

This reaction demonstrates how metal atoms in a solid state can interchangeably react with ions in a solution, resulting in new bonds and, hence, a new set of compounds. Understanding these reactions helps us anticipate things like solution color, effervescence from gas formation, or even precipitate formation when studying chemistry.

Solution Color

The color of a solution is largely determined by the ions present within it. Nickel compounds, for instance, often have distinct colors depending on the ions they form. Coloring in a solution can tell us a lot about the chemical composition and reactions that have taken place.
For nickel compounds:

• Nickel sulfate solution appears green.
• Nickel chloride solution appears yellow.

This happens because metal ions absorb certain wavelengths of light. The color that we perceive is the combination of wavelengths that are not absorbed. In general, understanding the color of a solution helps in identifying substances and their concentrations. If you see a green solution, you might consider the presence of nickel sulfate, while a yellow solution might indicate the presence of nickel chloride.

Nickel Ion Reactions

Nickel ions play a significant role in defining the outcome of their reactions in solutions. When nickel metal interacts with hydrochloric acid, it forms nickel ions through a displacement reaction, notably: \[ \text{Ni(s) + 2HCl(aq) \rightarrow NiCl_2(aq) + H_2(g)} \] In this example, the nickel ions form a complex with chloride ions, resulting in a yellow nickel chloride solution.
These reactions are influenced by electron transitions that occur between energy levels of nickel ions, which contribute to the observable color changes. Nickel ion reactions are central in determining their practical applications, both in chemical synthesis and in industrial processes. Recognizing these ion reactions helps chemists predict product formation beyond just colors, influencing choices concerning reaction conditions and expected product yields.