Science [4 marks Questions]

$Mg (s)+ H _{2} SO _{4}(a q) \rightarrow MgSO _{4}(a q)+ H _{2}(g)$

$2 Al ( s )+3 H _{2} SO _{4}(a q) \rightarrow Al _{2}\left( SO _{4}\right)_{3}(a q)+3 H _{2}(g)$

$Zn (s)+ H _{2} SO _{4}(a q) \rightarrow ZnSO S _{4}(a q)+ H _{2}(g)$

$Fe (s)+ H _{2} SO _{4}(a q) \rightarrow FeSO (a q)+ H _{2}(g)$

The metals in the middle of the reactivity series such as iron, lead, zinc, copper are moderately reactive.

These metals are usually present as sulphides or carbonates ore in nature.

It is easier to obtain a metal from Its oxide as compared to its sulphides and carbonates. Therefore, prior to reduction, the metal sulphides and carbonates are converted into metal oxides.

The suiphide ores are converted into oxides by heating strongly in the presence of excess air. This process is called roasting.

The carbonate ores are changed into oxides by heating strongly in limited air. This process is called calcination.

The chemical reactions that take place during roasting and calcination of zinc ores are as follows:

Roasting:

$2 ZnS (s)+3 O _{2}(g) \rightarrow 2 ZnO (s)+2 SO _{2}(g)$

Calcinatlon:

$ZnCO _{3}(s) \rightarrow ZnO (s)+ CO _{2}(g)$

The metal oxides are then reduced to the corresponding metals by heating with reducing agent like carbon.

$Z n O(s)+C(s) \rightarrow Z n(s)+C O(g)$

Besides using carbon, the highly reactive metals such as sodium, calcium, aluminium, etc. are used as reducing agent, because they can displace metals of lower reactivity from their compounds.

For example, when manganese dioxide Is heated with aluminium powder, it is reduced to $Mn$ as follows:

$3 M n O_{2}(s)+4 A l(s) \rightarrow 3 M n(l)+2 Al _{2} O_{3}(s)+H e a t$

Alloying is a method of improving the properties of metal and get desired properties in metal. For example, iron is the most widely used metal; but it is never used in its pure state; because pure iron is very soft and stretches easily when it is hot. But, if it is mixed with a small amount of carbon (about $0.05 \%$ ), it becomes hard and strong.

When iron is mixed with nickel and chromium, then we get stainless steel which is hard and does not rust.

When iron is mixed with some other substances (metals) its properties change. In fact, the properties of any metal can be changed if it Is mixed with some other substance.

The substance added may be a metal or a non-metal.

An alloy is a homogenous mixture of two or more metals or a metal and a non-metal.

It Is prepared by first melting the primary metal and then dissolving the other elements In It in definite proportions. It is then cooled to room temperature.

In an alloy, if one of the metal is mercury, then the alloy is known as an amalgam. For example, $Zn - Hg$ (Zinc amalgam).

The electrical conductivity and melting point of an alloy are less than that of pure metals. For example, brass is an ahoy of copper $( Cu )$ and zinc $( Zn )$; while, bronze Is an alloy of copper $( Cu )$ and tin $( Sn )$. Bronze is not good conductor of electricity but copper $( Cu )$ and tin $( Sn )$ possess good electrical conductivity. Similarly, solder, an alloy of lead and tin ( $Pb$ and $Sn$ ), has a low melting point and is used for welding electrical wires together.

Sulphide form - copper pyrite $\left(\mathrm{CuFeS}_{2}\right)$

Oxide form - Bauxite $\left(\mathrm{Al}_{2} \mathrm{O}_{3} 2 \mathrm{H}_{2} \mathrm{O}\right)$

Carbonate form - Calamine $\left(\mathrm{ZnCO}_{3}\right)$

Active metals generally interact with dil $\mathrm{HCl}$ or dil $\mathrm{H}_{2} \mathrm{SO}_{4}$ to evolve hydrogen gas. For eg-

$\mathrm{Zn}(\mathrm{s})+2 \mathrm{Hcl}(a q) \longrightarrow \mathrm{Zncl}_{2}(a q)+\mathrm{H}_{2}(g)$

$M g(s)+2 \mathrm{Hcl}_{(a q)} \longrightarrow \operatorname{Mgcl}_{2}(a q)+\mathrm{H}_{2}(g)$

The metals which lie below hydrogen is the activity series do not interact with dilute acid.

$(i)$ Mercury

$(ii)$ Graphite

$(iii)$ Aluminum

$(iv)$ A pure metal is always deposited at the cathode

$(v)$ Zinc oxide $(\mathrm{ZnO})$ is an amphoteric oxide.

$(1)$ All metals except mercury are solids at room temperature, while mercury is a liquid at room temperature.

$(2)$ Generally, metals have high melting points but Galium and Caesium have very low melting points.

$(3)$ Alkali metals (Lithium, sodium, potassium) are very soft hence, they can be cut with a knife, while other metals cannot be cut with a knife.

Exceptions of non-metallic elements:

$(1)$ Generally, non-metals exist as solids or gases at room temperature while bromine Is a Liquid.

$(2)$ Non-metals does not possess lustre, but iodine is the only non-metal which possesses lustre.

$(3)$ Non-metals have low melting points, but the diamond, an allotrope of carbon possesses very high melting point.

$(4)$ Non-metals are generally non-conductors of electricity, but the graphite, an allotrope of carbon is the only non-metal which conducts electric current.

• The metals obtained from their ores are not very pure.
• Hence, it is essential to purify the metals.
• The most widely used method for refining impure metals is electrolytic refining.
• [Electrolytic refining of copper]
• Metals such as copper, zinc, tin, nickel, silver and gold are refined by electrolytic process.
• In this process, the impure metal is made the anode and a thin strip of pure metal is made the cathode.
• A solution of the metal salt is used as an electrolyte.
• The apparatus is set up as shown in the figure
• On passing the electric current through the electrolyte, the pure metal from the anode dissolves into electrolyte.
• An equivalent amount of pure metal from the electrolyte is deposited on the cathode.
• For example,
• At anode (+): $Cu ( s ) \rightarrow Cu ^{2+}( aq )+2 e ^{-}$ (Impure)
• At cathode (-): $Cu ^{2+}( aq )+2 e ^{-} \rightarrow Cu ( s )$ (Pure)
• Net reaction: $Cu ( s ) \rightarrow Cu ( s )$ (Impure) (Pure)
• The soluble impurities pass into the solution while the insoluble impurities such as gold, silver, platinum are settled at the bottom of the anode and are called anode-mud.