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

Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.
$(a)$ Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.
$(b)$ Ethanol is burnt in air to form carbon dioxide,water and releases heat.

The reaction which is used in black and white photography is:

$A, B$ and $C$ are three elements which undergo chemical reactions according to the following equations:
$A_{2}O_{3} + 2B \rightarrow B_{2}O_{3} + 2A$
$3CSO_{4} + 2B \rightarrow B_{2}(SO_{4})_{3} + 3C$
$3CO + 2A \rightarrow A_{2}O_{3} + 3C$
Answer the following questions with reasons:
$(a)$ Which element is the most reactive?
$(b)$ Which element is the least reactive?
$(c)$ What is the type of reactions listed above?

Giving chemical equations,answer the following:
$(a)$ What happens when copper is heated in air?
$(b)$ What happens when the product obtained in the above reaction is heated in hydrogen?

Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is