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    Oxidation Number - Practice Questions & MCQ

    Edited By admin | Updated on Sep 25, 2023 25:23 PM | #NEET

    Quick Facts

    • Oxidation Number and Oxidation State is considered one of the most asked concept.

    • 63 Questions around this concept.

    Solve by difficulty

    In which compound does Vanadium have an oxidation number of +4?

    Which of the following is a redox reaction?

    Which of the following chemical reactions depict the oxidising behaviour of \mathrm{ H_{2}SO_{4}} ?

    The oxidation number of the underlined atom in the following species

    Identify the incorrect option.

     The correct order of N-compounds in its decreasing order of oxidation states is:

    In which of the following compounds, nitrogen exhibits the highest oxidation state?

    In acidic medium, H2O2 changes \text{Cr}_{2}\text{O}_{7}^{-2} to CrO_5  which has two (-O-O-) bonds. Oxidation state of Cr in CrO_5 is:

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    Which of the following processes does not involve oxidation of iron ?

    For an ideal gas, number of moles per litre in terms of its pressure P, gas constant R and temperature  T is

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    In acidic medium, H_{2}O_{2} changes Cr_{2}O_{7}^{-2} to CrO_{5} which has two (-O-O) bonds. Oxidation state of Cr in CrO_{5} is:-

    Concepts Covered - 1

    Oxidation Number and Oxidation State

    It is a process that involves the loss of electrons by the atoms or ions.
    It is a process that involves the gain of electrons by the atoms or ions.

    Any reaction, in which the electrons are exchanged between atoms or ions, represents a simultaneous process of oxidation and reduction and is called as a Redox Reaction.

    In a Redox Reaction, the species that loses electron (i.e., gets oxidised) is known as reducing agent or reductant, (since it causes reduction of other species), the species which accepts electrons from reductant (i.e., gets reduced) is known as oxidising agent or oxidant (as it causes oxidation of other species).

    Oxidation Number (O.N.): It refers to the total charge on all atoms of same kind in a compound.
    Oxidation State (O.S.): It refers to the charge per atom of all atoms of same kind in a compound.

    Oxidation state, many times, is also referred to as Oxidation Number.
    This means oxidation number of an element in a compound is equal to the oxidation state of that element multiplied by total atoms of that element in particular compound.
    (i) In ionic compounds, it is simply the charge on corresponding cation and anion which is expressed as oxidation state of that partiular element. For example, oxidation state of potassium and chlorine in potassium chloride (KCl) is simply +1 and –1 respectively as KCl is treated as K+Cl.

    Refer to the following examples where oxidation states are written above the atoms:

    +2-1 +2-2 +3-1 +1+6-2
    MgCl2 CaS AlCl3 K2SO4

    NOTE: (a) In MgCl2 and AlCl3, -1 is the oxidation state of Cl atom and its oxidation number is -2 and -3 respectively.
               (b) In each of the cases, the sum of oxidation number of all atoms of all kinds is equal to zero since the compound is neutral.

    (ii) In Covalent Compounds, it is not so easy to assign oxidation state of an atom. In order to simplify the concept, we are going to define a set of rules which would enable us to assign oxidation state to every element in any compound.

    Rules for Assigning Oxidation State (O.S.) and Oxidation Number (O.N.):

    • Any element in free state is assigned an oxidation state of zero. For example: O.S. of H, P, S, O in H2, P4, S8, O2 respectively is zero.
    • The oxidation state of any cation or anion (of form A+ or B-) is equal to the magnitude of its charge. For example: O.S of Ca in Ca2+ = +2 and O.S of Al in Al3+ = +3.
    • The algebraic sum of the oxidation number of all atoms in a neutral compound is equal to 0. The algebraic sum of the oxidation numbers of all atoms in an ion (like PO43-) is equal to charge on the ion.

    • The oxidation states of Alkali Metals (Group IA) is +1 in all of their compounds and that of Alkaline
      Earth elements (Group IIA) is +2 in all of their compounds.

    • Hydrogen in almost all of its compounds is assigned an oxidation state of +1. The exception occurs when hydrogen forms compounds with strong metals like KH, NaH, MgH2, CaH2, etc. In all of these, the oxidation state of hydrogen is -1.

    • Oxygen in almost all of its compounds is assigned an oxidation state of -2. But in certain compounds like Peroxides(H2O2), the oxidation state of oxygen is -1. Another exception is OF2, where O.S. is +2. O2F2, where O.S. is +1 and KO2 in which O.S. is -1/2.

    • Fluorine is most electronegative element and is assigned an O.S. of -1, in all its compounds. For other halogens, O.S. is generally -1 except when they are bonded to a more electronegative halogen or oxygen. O.S. of iodine in IF7 is +7, O.S. of chlorine in KClO3 is +5.

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    Reference Books

    Oxidation Number and Oxidation State

    Chemistry Part II Textbook for Class XI

    Page No. : 268

    Line : 34

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