Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

Ammonium dichromate is used in some fire works. The green coloured powder blown in the air is (a) \(\mathrm{CrO}_{3}\) (b) \(\mathrm{Cr}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{Cr}\) (d) \(\mathrm{CrO}\left(\mathrm{O}_{2}\right)\)

Step-by-Step Solution

Answer

The green powder is chromium(III) oxide, \( Cr_2O_3 \). Option (b).

Step 1: Identify the Decomposition Reaction

Ammonium dichromate (\( (NH_4)_2Cr_2O_7 \)) decomposes when heated, resulting in a characteristic green powder. The chemical reaction for the decomposition is:\[ (NH_4)_2Cr_2O_7 ightarrow Cr_2O_3 + N_2 + 4H_2O \] This equation shows the formation of chromium(III) oxide, nitrogen gas, and water vapor.

Step 2: Determine the Green Compound

The decomposition reaction of ammonium dichromate produces chromium(III) oxide (\( Cr_2O_3 \)), which is known for its green color. This is the compound often seen as the green powder in the reaction context.

Step 3: Match to Available Options

Compare the compound formed, chromium(III) oxide (\( Cr_2O_3 \)), to the given options:- (a) \( CrO_3 \) is chromium trioxide (red-orange solid).- (b) \( Cr_2O_3 \) is chromium(III) oxide (green powder).- (c) \( Cr \) is elemental chromium (grey, metallic).- (d) \( CrO(O_2) \) does not exist in typical circumstances.The green powder matches option (b).

Key Concepts

Decomposition Reactions

Decomposition reactions are a type of chemical reaction in which a single compound breaks down into two or more simpler substances. Think of it as nature's way of unboxing a package, where one substance gives rise to multiple products. A great example is the decomposition of ammonium dichromate. When this compound is heated, it explosively breaks down into chromium(III) oxide, nitrogen gas, and water vapor. The chemical equation representing this process is: \[ (NH_4)_2Cr_2O_7 \rightarrow Cr_2O_3 + N_2 + 4H_2O \]These reactions are crucial in both natural processes and industrial applications. They're important in fields ranging from metallurgy to environmental science. For instance:

• In nature, decomposition reactions help in the breakdown of dead organic matter.
• In industry, they are used to refine and purify metals.

By understanding decomposition reactions, students can better grasp how compounds interact and transform under different conditions.

Chromium Compounds

Chromium compounds are a family of substances containing the element chromium, each having unique properties and uses. The most common oxidation states of chromium in compounds are +3 and +6. Each state gives the compound different characteristics:

• Chromium(III) compounds, like chromium(III) oxide (\( Cr_2O_3 \)), are generally known for their stability and non-toxicity. The distinctive green color of chromium(III) oxide is often utilized in dyes and coatings.
• Chromium(VI) compounds are typically more reactive and hazardous. They are used in industries for things like stainless steel production as chromium trioxide (\( CrO_3 \)), which is a red-orange solid.

Beyond just colors and stability, the choice between chromium compounds is often made based on environmental and safety considerations. Understanding each compound's properties allows chemists to select the right one for their scientific or industrial needs.

Chemical Reactions

Chemical reactions involve the rearrangement of atoms to transform reactants into products. They are at the heart of chemical processes and are essential in the field of inorganic chemistry. A chemical reaction occurs when bonds between atoms are broken or formed, usually involving energy changes. In the context of the exercise, ammonium dichromate undergoes a decomposition reaction, highlighting an important class of reactions where a single substance breaks down into simpler products. The breaking of bonds in ammonium dichromate leads to the formation of new compounds, such as chromium(III) oxide, nitrogen gas, and water vapor. Some key aspects of chemical reactions include:

• Reactants and products, which are the starting and ending substances in a reaction.
• Conditions under which the reactions occur, such as temperature and pressure.
• Conservation of mass, where the mass of the products should equal the mass of the reactants.

Understanding chemical reactions is fundamental for analyzing chemical processes, predicting the outcomes of reactions, and creating new materials with desired properties.