Can oxidation or reduction take place alone? Why or why not? What are such reactions called?

If copper metal is heated over a flame, it develops a coating. What is the colour and composition of this coating?

Which is the reducing agent in the following reaction?
$MnO_{2}(s) + 4HCl(aq) \longrightarrow MnCl_{2}(s) + 2H_{2}O(l) + Cl_{2}(g)$

Identify the reducing agent in the following reactions:
$(a)$ $4 NH_{3} + 5 O_{2} \longrightarrow 4 NO + 6 H_{2}O$
$(b)$ $H_{2}O + F_{2} \longrightarrow HF + HOF$
$(c)$ $Fe_{2}O_{3} + 3 CO \longrightarrow 2 Fe + 3 CO_{2}$
$(d)$ $2 H_{2} + O_{2} \longrightarrow 2 H_{2}O$

$A$ dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?

Give reason why:
$(i)$ Sodium acts as a reducing agent while chlorine acts as an oxidising agent in the following reaction:
$2 Na (s) + Cl_2 (g) \rightarrow 2 NaCl (s)$