Question

The text mentions that scandium metal is obtained from its molten chloride by electrolysis, and that titanium is obtained from its chloride by reduction with magnesium. Why are these metals not obtained by the reduction of their oxides with carbon (coke), as are metals such as zinc and iron?

Short answer

Scandium and titanium are not obtained by the reduction of their oxides with carbon, as it often leads to the formation of unwanted metal carbides due to high reactivity of these metals compared to carbon. Instead, scandium is obtained from its molten chloride by electrolysis and titanium is obtained from its chloride by reduction with magnesium - both processes do not involve carbon, thereby avoiding the creation of unwanted by-products.

Step-by-step solution

- Understanding Reduction with Coke

Reduction with coke, or carbon, is a method typically used to extract metals. This process usually involves heating the metal oxide with carbon, which acts as the reducing agent. Carbon grabs the oxygen atoms, creating carbon dioxide, and leaves the metal behind. An important element to note here is that this method works best when the metal oxide reacts with carbon to release a metal and when the metal is less reactive than carbon.

- Reactivity of Scandium and Titanium

Scandium and titanium are transition metals. Compared to carbon, these metals are more reactive. As such, in the reduction process, these metals will likely react with carbon to form carbides instead of freeing the metal. The production of unwanted metal carbides complicates the extraction process and defeats the purpose of extracting pure metal in the first place.

- Advantage of Using Chlorides

The extraction methods of scandium from its molten chloride by electrolysis and titanium from its chloride by reduction with magnesium represent alternatives to the traditional coke reduction. Both processes do not involve carbon and so, avoid the formation of useless metal carbides. Electrolysis involves the use of an electric current to encourage a non-spontaneous reaction. This type of extraction allows for more control over the process and avoids the formation of unwanted products. Likewise, the reaction of titanium chloride with magnesium is a more efficient and controlled method, and avoids carbon impurities.

Key concepts

Reduction with Coke

Let's dive into the method known as reduction with coke. In the metallurgical field, 'coke' refers to a high-carbon material derived from coal that serves as a reducing agent; a substance that donates electrons to another compound. When a metal oxide is heated in the presence of coke, the carbon in the coke bonds with the oxygen from the metal oxide, culminating in carbon dioxide and leaving the pure metal behind. This procedure is especially suited for less reactive metals, such as iron and zinc, which do not form stable compounds with carbon.

However, not all metals can be extracted efficiently this way due to the reactivity of certain metals. Highly reactive metals would rather bond with carbon, forming stable carbides, which are difficult to break apart. Consequently, we must seek alternative methods of extraction for those metals where coke reduction is not suitable, to prevent contaminating the metal with carbon and to ensure energy and cost-effectiveness.

Reactivity of Transition Metals

The term transition metals refers to the elements found in the center of the periodic table that are known for their ability to form compounds with a wide range of oxidation states. The reactivity of transition metals is a key concept to understand because it influences how these metals can be extracted from their compounds. Transition metals like scandium and titanium are more reactive when compared to metals like iron; they can readily form carbides when exposed to carbon at high temperatures.

Because of this propensity to form strong bonds with non-metal elements such as carbon and oxygen, alternative extraction methods are preferred. These methods avoid using reducing agents such as coke that will result in undesirable carbides, which are not only difficult to decompose but also impair the quality and purity of the extracted metal.

Electrolysis of Molten Chlorides

The process of electrolysis of molten chlorides is a sophisticated method used for extracting metals, particularly those that form undesirable compounds when processed with more traditional methods. In this technique, the ionic compound of the metal, typically a chloride, is melted. Applying a direct current causes the ions in the molten state to move towards their respective electrodes, where they gain or lose electrons (are reduced or oxidized).

For instance, the metal scandium is obtained via the electrolysis of its molten chloride. This ensures that the resulting metal is free from carbon contaminates since no carbon is involved in the process. Moreover, the high level of control over the variables of the process, like temperature and voltage, enables the production of very pure metal.

Magnesium Reduction Process

The magnesium reduction process, also known as the Kroll process, is particularly notable for its role in obtaining titanium. Here, instead of carbon, magnesium is used as the reducing agent. Magnesium has the ability to reduce titanium tetrachloride to produce metallic titanium and magnesium chloride. This happens at temperatures significantly lower than the melting point of titanium.

When magnesium is used, we avoid the pitfalls of titanium carbide formation seen with more reactive carbon. After the reaction, the magnesium chloride can be easily separated from the titanium through techniques like distillation or chemical treatments. This process allows for the extraction of high-purity titanium, which is a valuable material for a vast array of applications due to its strength, light weight, and resistance to corrosion.