Metals and Metallurgy - SS2 Chemistry Past Questions and Answers - page 4

Alloys are solid solutions composed of two or more elements, with at least one being a metal. They are significant in materials science and engineering due to the following reasons:

a.    Enhanced Mechanical Properties: Alloys are designed to exhibit improved mechanical properties compared to pure metals. By combining different elements, alloys can achieve desirable characteristics such as increased strength, hardness, toughness, and wear resistance. This makes alloys suitable for applications that require high-performance materials, such as automotive components, structural materials, and tooling.

b.    Tailored Physical and Chemical Properties: Alloying allows the modification of various physical and chemical properties of metals. For example, alloys can be engineered to have specific electrical conductivity, thermal conductivity, magnetic properties, corrosion resistance, or even superconductivity. This versatility enables alloys to be used in a wide range of applications, from electrical wiring and electronic devices to chemical processing and medical implants.

c.     Improved Alloy Processing: Alloys often exhibit improved processability compared to pure metals. The addition of certain elements can lower the melting point, increase ductility, or improve casting and forging characteristics. This facilitates the manufacturing and shaping of alloys into complex forms, contributing to their widespread use in industries such as aerospace, automotive, and construction.

Examples of commonly used alloys include:

●     Steel: An alloy of iron and carbon, steel exhibits high strength, durability, and resistance to corrosion. It finds applications in construction, infrastructure, tools, machinery, and the automotive industry.

●     Brass: A copper-zinc alloy, brass combines the attractive golden appearance of copper with enhanced mechanical properties. It is used in musical instruments, plumbing fittings, decorative items, and electrical connectors.

●     Aluminum Alloys: Various aluminium alloys, such as 6061 and 7075, offer improved strength, lightweight properties, and corrosion resistance. They are extensively used in aerospace, automotive, and structural applications.

●     Stainless Steel: A steel alloy containing chromium, nickel, and other elements, stainless steel possesses high corrosion resistance and excellent mechanical properties. It is widely used in kitchen utensils, medical equipment, cutlery, and industrial applications.

The significance of alloys lies in their ability to provide materials with tailored properties, enabling the development of materials suitable for specific applications and advancing technological advancements in numerous industries.

Alloying plays a crucial role in improving the properties of metals by introducing specific elements or additives. These elements can significantly influence the properties of alloys in the following ways:

a.    Strengthening: Alloying elements can contribute to solid solution strengthening or precipitation strengthening. For example, adding small amounts of carbon to iron forms the alloy steel, which exhibits increased strength due to the formation of a harder phase called cementite. Similarly, adding elements like chromium, molybdenum, or tungsten to steel can enhance its strength and wear resistance.

b.    Corrosion Resistance: Alloying elements can improve the corrosion resistance of metals. For instance, the addition of chromium to iron forms stainless steel, which creates a passive oxide layer that protects against corrosion. Aluminium alloys are often alloyed with small amounts of copper, magnesium, or zinc to enhance their resistance to corrosion.

c.     Heat Resistance: Alloying elements can improve the heat resistance of metals, enabling their use in high-temperature applications. Nickel-based superalloys, alloyed with elements such as chromium, cobalt, and tungsten, exhibit excellent heat resistance and mechanical strength, making them suitable for gas turbine engines and aerospace components.

d.    Electrical Conductivity: Alloying elements can influence the electrical conductivity of metals. For example, copper alloys, such as bronze and brass, contain different proportions of zinc and tin, which modify the electrical conductivity while retaining desirable properties like malleability and corrosion resistance.

e.    Magnetic Properties: Alloying elements can affect the magnetic properties of metals. The addition of elements like iron, cobalt, or nickel to various base metals can create alloys with magnetic properties, such as Alnico magnets (aluminium, nickel, and cobalt) used in electric motors and magnetic sensors.

Examples of these effects can be seen in the alloying of titanium with aluminium and vanadium to produce a lightweight and high-strength alloy known as Ti-6Al-4V, used in aerospace applications. The addition of silicon to aluminium forms aluminium-silicon alloys (e.g., 6061), which exhibit excellent castability, good strength, and corrosion resistance. Alloying provides the means to tailor the properties of metals to meet specific requirements, expanding their range of applications and enhancing their overall performance.

The reactivity series is a list of metals arranged in order of their relative reactivity towards displacement reactions. In this series, metals are placed in descending order, starting from the most reactive metal to the least reactive metal. The reactivity series helps predict the outcome of displacement reactions between metals and metal ions in aqueous solutions.

Trends observed in the reactivity series:

●     Most Reactive Metals: Metals such as potassium, sodium, and calcium are at the top of the reactivity series and are highly reactive. They readily react with water or acids to release hydrogen gas and form metal hydroxides or metal salts.

●     Less Reactive Metals: Metals like iron, zinc, and aluminium are less reactive. They react with acids but not with water. They displace hydrogen from acids but not from water.

●     Least Reactive Metals: Metals such as copper, silver, and gold are at the bottom of the reactivity series and are the least reactive. They do not readily react with water or acids and do not displace hydrogen from acids.

The reactivity series is significant in predicting displacement reactions. A metal higher in the reactivity series can displace a metal lower in the series from its salt solution. This displacement reaction occurs because a more reactive metal has a greater tendency to lose electrons and form positive ions, which can replace less reactive metals in their compounds.

For example, according to the reactivity series, zinc (Zn) is more reactive than copper (Cu). Therefore, when zinc is added to a copper sulphate (CuSO4) solution, a displacement reaction occurs:

Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)

In this reaction, zinc displaces copper from the copper sulphate solution, resulting in the formation of zinc sulphate and copper metal.

The reactivity series helps in understanding the relative reactivity of metals and their ability to displace other metals in aqueous solutions, which has implications in various fields such as metallurgy, corrosion prevention, and the extraction of metals.