Problem 72 - Chemistry: The Central Science

Question

Based on the chemistry described in this chapter and others, propose balanced chemical equations for the following sequence of reactions involving nickel: (a) The ore millerite, which contains NiS, is roasted in an atmosphere of oxygen to produce an oxide. (b) The oxide is reduced to the metal, using coke. (c) Dissolving the metal in hydrochloric acid produces a green solution. (d) Adding excess sodium hydroxide to the solution causes a gelatinous green material to precipitate. (e) Upon heating, the green material loses water and yields a green powder.

Step-by-Step Solution

Answer

The balanced chemical equations for the sequence of reactions involving nickel are: a) \(2NiS + 3O_2 \rightarrow 2NiO + 2SO_2\) b) \(NiO + C \rightarrow Ni + CO\) c) \(Ni + 2HCl \rightarrow NiCl2 + H2\) d) \(NiCl2 + 2NaOH \rightarrow Ni(OH)2 + 2NaCl\) e) \(Ni(OH)2 \rightarrow NiO + H2O\)

Step 1: Roasting of millerite in oxygen

Roasting of millerite (NiS) in the presence of oxygen produces nickel oxide (NiO) and sulfur dioxide (SO2). The balanced chemical equation for this reaction is: \[2NiS + 3O_2 \rightarrow 2NiO + 2SO_2\]

Step 2: Reduction of nickel oxide using coke

The next step is to reduce the nickel oxide (NiO) to metallic nickel (Ni) using coke (which is primarily carbon). The balanced chemical equation for the reduction process is: \[NiO + C \rightarrow Ni + CO\]

Step 3: Dissolving metallic nickel in hydrochloric acid

Now, we will dissolve the produced metallic nickel in hydrochloric acid (HCl) to produce a green solution. This reaction forms nickel chloride (NiCl2) and hydrogen gas (H2). The balanced chemical equation for this reaction is: \[Ni + 2HCl \rightarrow NiCl2 + H2\]

Step 4: Precipitation of the green gelatinous material with sodium hydroxide

When we add sodium hydroxide (NaOH) to the green solution, a gelatinous green material precipitates. This is because nickel hydroxide (Ni(OH)2) is formed, which is insoluble in water. The balanced chemical equation for this reaction is: \[NiCl2 + 2NaOH \rightarrow Ni(OH)2 + 2NaCl\]

Step 5: Heating the green material to yield a green powder

Finally, heating the green gelatinous material (nickel hydroxide) causes it to lose water and yield a green powder (nickel oxide). The balanced chemical equation for this reaction is: \[Ni(OH)2 \rightarrow NiO + H2O\] These balanced chemical equations represent the sequence of reactions involving nickel as requested in the exercise.

Key Concepts

Chemical Equations

Understanding chemical equations is a fundamental part of studying chemistry, especially when it concerns reactions with metals such as nickel. A chemical equation is a concise representation of a chemical reaction using the symbols and formulas of the substances involved. For example, during the roasting of nickel sulfide (millerite), the equation

\(2NiS + 3O_2 \rightarrow 2NiO + 2SO_2\)

shows how nickel sulfide (NiS) and oxygen (O2) react to form nickel oxide (NiO) and sulfur dioxide (SO2). Each substance in the equation is balanced, meaning the number of atoms of each element is the same on both sides of the reaction arrow, satisfying the Law of Conservation of Mass. Recognizing and balancing chemical equations are skills that are valuable for analyzing and predicting the outcome of chemical reactions.

Roasting of Ore

The roasting of ore is a step in the metallurgy of nickel where compounds like nickel sulfide are heated in the presence of oxygen. As demonstrated in the solution, millerite (NiS) is roasted to produce nickel oxide.

Significance of Roasting

Roasting helps in removing volatile impurities as gases and converts sulfides into oxides which are easier to reduce. Through the equation
\(2NiS + 3O_2 \rightarrow 2NiO + 2SO_2\),

we observe how sulfur compounds are transformed into sulfur dioxide, a gas that escapes, and nickel oxide, a compound ready for the next stage, reduction. This transformation is crucial as it prepares the nickel for the reduction process that follows.

Reduction Process

The reduction process in metallurgy refers to the conversion of metal oxides into the pure metal. This step comes after roasting and in the case of nickel, involves the reaction of nickel oxide with coke to produce elemental nickel.

Using the chemical equation
\(NiO + C \rightarrow Ni + CO\),

we see that carbon from coke reduces the nickel oxide to pure nickel, with carbon monoxide as a byproduct. This step is a crucial part of extracting metals from their ores as it isolates the pure metal, which is of practical use in various applications, by removing the oxygen bonded to the metal.

Precipitation Reaction

A precipitation reaction occurs when two aqueous solutions react to form an insoluble solid, called the precipitate. In our nickel chemistry example, when sodium hydroxide is added to the solution of nickel chloride, nickel hydroxide precipitates out due to its low solubility in water.

The equation
\(NiCl2 + 2NaOH \rightarrow Ni(OH)2 + 2NaCl\)

describes this precisely. Precipitation reactions are not only essential in chemistry for isolating and purifying substances but also serve as a foundation for understanding other processes in nature and industry, such as water treatment and qualitative analysis.

Chemical Reactions in Metallurgy

The series of chemical reactions in metallurgy that involve nickel highlight the intricate process of metal extraction from ores. Each step, from roasting to reduction, and through to precipitation, represents a specific reaction with designated reactants and products. The metamorphosis of nickel from its sulfide ore to a pure metal involves complex reactions that carefully remove other elements like sulfur and oxygen.

Understanding the sequence of these reactions, for example, helps in predicting the physical and chemical behavior of the metal at each stage. As each reaction is tied to the next, the quality of the final product is dependent on these chemical transformations, making them a critical aspect of metallurgy. In the nickel example, the process concludes with the conversion of nickel hydroxide back into nickel oxide upon heating. This cyclical process showcases the repetitive nature of some chemical reactions in metallurgy.