Question

Which of the following does not react with oxygen directly?
$Zn,Ti,Pt,Fe$

Answer 1

The noble metals are the metals which do not react with any chemical species directly. In the presence of air, they show no chemical reactivity with oxygen. Whereas, the transition elements which are not a noble metal, show high reactivity with oxygen and produce metal oxide when they come in contact with oxygen.

Complete step by step answer:

When the transition metals come in direct contact with oxygen, they produce metal oxides. This reaction is very rigorous and sometimes produces so much heat that the oxide can even melt to form its molten state.
In the given 4 elements, the elements which act as a transition metal are zinc, titanium and iron. Platinum acts as a noble metal and shows no reaction with oxygen. The reaction of the metals with oxygen can be shown separately.
(i) Zinc: The reaction of zinc with oxygen produces zinc oxide. The reaction is as follows:
$2Zn + {O_2} \to 2ZnO$
The zinc oxide layer formed over the zinc prevents the metal from further reaction with oxygen.
(ii) Titanium: The reaction of titanium with oxygen produces various types of oxides of titanium under numerous conditions of temperature and pressure. The most common reaction follows:
$Ti + {O_2} \to Ti{O_2}$
The titanium dioxide formed has various applications in food coloring and as a natural ore for obtaining titanium.
(iii) Iron: The various oxides of iron formed when iron reacts with oxygen are:
$2F{e^{2 + }} + {O_2} \to 2FeO$
The product is ferrous oxide and used as one of the components in thermite mixture.
$4F{e^{3 + }} + 3{O_2} \to 2F{e_2}{O_3}$
The compound formed is ferric oxide and occurs as a natural ore of iron.
(iv) Platinum: It is a noble metal and shows zero reactivity with oxygen due to its specific electronic configuration.
Thus, the correct option is Platinum ($Pt$ ).

Note:
The noble metals do not react with any type of corroding material and thus, do not undergo corrosion. When left over a long time, one finds the metals in the same state as they were left back then. Thus, they are generally used for making ornaments and jewellery.