Question

One of the simplest metals to extract from its ores is mercury. Mercury vapor is produced by roasting cinnabar ore (HgS) in air. Alternatives to this simple roasting, designed to reduce or eliminate ${\mathrm{SO}}_2$ emissions, is to roast the ore in the presence of a second substance. For example, when cinnabar is roasted with quicklime, the products are mercury vapor and calcium sulfide and calcium sulfate. Write equations for the two reactions described here.

Short answer

The chemical equations are: $HgS+O2->Hg+SO2$ and $2HgS+2CaO->2Hg+CaS+CaSO4$

Step-by-step solution

Identify all compounds involved in reactions

In the first reaction, cinnabar (HgS) and oxygen (from air) are the reactants. The products are mercury vapor (Hg) and sulfur dioxide (SO2).\nIn the second reaction, cinnabar (HgS), quicklime (CaO) are the reactants. The products are mercury vapor (Hg), calcium sulfide (CaS) and calcium sulfate (CaSO4).

Write the unbalanced chemical equations

Based on the reactants and products identified, write the unbalanced equations for both reactions.\nFor the first reaction: $HgS+O2->Hg+SO2$\nFor the second reaction: $HgS+CaO->Hg+CaS+CaSO4$

Balance the chemical equations

Balance the equations so that there are equal numbers of each type of atom on both sides of the equation.\nFor the first reaction: $HgS+O2->Hg+SO2$ is already balanced.\nFor the second reaction: balance by putting coefficients in front of compounds as needed. The balanced equation is $2HgS+2CaO->2Hg+CaS+CaSO4$

Key concepts

Chemical Reactions

Chemical reactions are the processes by which substances combine or break apart to form new substances. Each reaction involves reactants that undergo a change to become the products. In the context of extracting mercury, two different types of reactions are mentioned: roasting cinnabar ore in air and roasting cinnabar with quicklime.

Roasting cinnabar in the presence of air is a type of oxidation reaction, where cinnabar (HgS) reacts with oxygen (O2) to produce mercury vapor (Hg) and sulfur dioxide (SO2). This reaction is significant for it not only yields mercury but also a potentially harmful byproduct, SO2, which contributes to environmental pollution.

The alternative method described involves roasting cinnabar with quicklime (CaO), which is an example of a chemical reaction that results in multiple products: mercury vapor, calcium sulfide (CaS), and calcium sulfate (CaSO4). This method is designed to reduce SO2 emissions by incorporating it into other less harmful products.

Balancing Chemical Equations

Balancing chemical equations is essential in ensuring the law of conservation of mass is upheld; the mass of the reactants must equal the mass of the products. To balance an equation, one must make sure the number of atoms of each element is the same on both sides.

In the solution provided, the first reaction, involving the roasting of cinnabar in air, was already balanced: the reactants HgS and O2 yield the products Hg and SO2 with no need for additional coefficients.

However, the second reaction required adjusting the coefficients to achieve balance. The starting, unbalanced equation for the reaction of cinnabar with quicklime was: $HgS+CaO\to Hg+CaS+CaSO4$. With the coefficients added, the balanced form became: $2HgS+2CaO\to 2Hg+CaS+CaSO4$, ensuring the same number of each type of atom on both sides of the equation. Balancing chemical equations is not only a mathematical exercise but also a fundamental principle of understanding and predicting the outcomes of chemical reactions.

Cinnabar Roasting Process

The cinnabar roasting process is a method used to extract mercury from its ore—cinnabar (HgS). Roasting refers to heating the ore in the presence of air or another substance, which triggers a chemical reaction that releases mercury as a vapor.

The traditional roasting process involves heating cinnabar in air, leading to the formation of mercury vapor and sulfur dioxide, as represented by the equation: $HgS+O2\to Hg+SO2$. This process, while straightforward, presents environmental concerns due to the emission of SO2, a noxious gas that can lead to acid rain and respiratory problems.

To mitigate these effects, an alternative method, known as the lime roasting process, incorporates quicklime (CaO) as an additional reactant. The presence of quicklime allows the capture of sulfur in the form of less harmful compounds such as calcium sulfide and calcium sulfate, as seen in the balanced reaction: $2HgS+2CaO\to 2Hg+CaS+CaSO4$. This reduces SO2 emissions, making the process more environmentally friendly. Understanding the cinnabar roasting process is crucial for both obtaining mercury and developing sustainable industrial practices.