Question

In which of the following reaction there is change in the oxidation state of chromium? (1) \(2 \mathrm{~K}_{2} \mathrm{CrO}_{4}+\mathrm{II}_{2} \mathrm{SO}_{4} \longrightarrow \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}+\mathrm{K}_{2} \mathrm{SO}_{4}+\) \(\mathrm{II}_{2} \mathrm{O}\) (2) \(\mathrm{K}_{2} \mathrm{CrO}_{4}+\mathrm{II}_{2} \mathrm{SO}_{4}+\mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{K}_{2} \mathrm{SO}_{4}+\mathrm{CrO}_{5}+\) \(3 \mathrm{II}_{2} \mathrm{O}\) (3) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}+6 \mathrm{II}_{2} \mathrm{SO}_{4}+4 \mathrm{NaCl} \longrightarrow 2 \mathrm{KIISO}_{4}+\) \(4 \mathrm{NaIISO}_{4}+2 \mathrm{CrO}_{2} \mathrm{Cl}_{2}+3 \mathrm{II}_{2} \mathrm{O}\) (4) Nonc

Short answer

Reaction (3) has a change in the oxidation state of chromium.

Step-by-step solution

Identify oxidation states of chromium in reactants

Determine the oxidation state of chromium in the reactants for each reaction. For K2CrO4 in reactions (1) and (2), chromium is in the +6 oxidation state. For K2Cr2O7 in reaction (3), chromium is also in the +6 oxidation state.

Identify oxidation states of chromium in products

Determine the oxidation state of chromium in the products for each reaction. In K2Cr2O7 in reaction (1), the oxidation state of chromium remains +6. In CrO5 in reaction (2), the oxidation state of chromium is +6. In CrO2Cl2 in reaction (3), the oxidation state of chromium is +4.

Compare oxidation states of chromium

Compare the oxidation states of chromium in reactants and products for each reaction. A change in oxidation state indicates a redox reaction. In reaction (1), chromium's oxidation state remains +6, and in reaction (2), it also remains +6. In reaction (3), the oxidation state changes from +6 in K2Cr2O7 to +4 in CrO2Cl2.

Conclusion

From the comparison, we observe that reaction (3) shows a change in the oxidation state of chromium, where it changes from +6 to +4.

Key concepts

Redox Reactions

Redox reactions are at the heart of understanding how electrons move between elements during chemical processes.
The term 'redox' stands for reduction-oxidation, representing two key concepts:

• **Reduction**: gain of electrons
• **Oxidation**: loss of electrons

Each of these reactions involves a transfer of electrons, affecting the oxidation state of the involved elements.
When chromium changes its oxidation state, it indicates that a redox reaction has occurred.
For example, in the provided exercise, comparing the oxidation states of chromium helps determine if a redox reaction has taken place.
In reaction (3), where chromium's oxidation state changes from +6 to +4, we can conclude that a redox reaction has occurred.

Oxidation States

Oxidation states, also known as oxidation numbers, are used to keep track of electron transfers in chemical reactions.
They help us determine which element is oxidized and which one is reduced.
The oxidation state is a hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic.
Some common rules for determining oxidation states include:

• Elements in their pure form always have an oxidation state of 0.
• For monoatomic ions, the oxidation state is the same as the ion's charge.
• In compounds, hydrogen typically has an oxidation state of +1, and oxygen typically has -2.
• The sum of oxidation states in a neutral molecule must be zero; in an ion, it must equal the ion's charge.

In the exercise, identifying the oxidation states of chromium in different reactions reveals important information about the nature of each chemical process.
For instance, in reaction (3), the oxidation state of chromium changes from +6 to +4, confirming a redox reaction.

Chemical Reactions

Chemical reactions involve the transformation of reactants into products through the breaking and forming of chemical bonds.
These transformations often involve changes in the oxidation states of the elements.
Understanding chemical reactions requires identifying the reactants and products, and determining if any changes in oxidation states occur.
In the provided exercise, we compare the oxidation states of chromium in different reactions to identify redox reactions.
In reaction (1), chromium remains at an oxidation state of +6 throughout, indicating no redox reaction.
Similarly, in reaction (2), there is no change in oxidation state for chromium.
However, in reaction (3), the oxidation state of chromium changes from +6 in the reactant to +4 in one of the products, confirming the occurrence of a redox reaction. Understanding these principles helps in analyzing and predicting the outcomes of various chemical processes.