The transition metal elements are widely distributed in the Earth’s crust. Many of these elements fi — Physical Chemistry Chemistry Question
Applications of Transition Metals
The transition metal elements are widely distributed in the Earth’s crust. Many of these elements find uses in everyday objects: iron pipes, copper wiring, chromium auto parts, etc.
Part 1: Properties of Chromium
Chromium is a silvery–white, lustrous metal, whose name (from the Greek chroma, meaning color) alludes to its many colorful compounds. The bright colors of chromium (VI) compounds lead to their use in pigments for artists paints and ceramic glazes.
In an acidic solution, the yellow chromate ion (CrO4 2– ) changes into the orange dichromate ion (Cr2O7 2– ). Write the equation for the reaction.
Model Answer
2 CrO4 2– (aq) + 2 H+ (aq) Cr2O7 2– (aq) + H2O(l)
What is the oxidation state of each metal center in the chromate ion and the dichromate ion?
Model Answer
CrO4 2–: VI, Cr2O7 2–: VI.
Is this a redox reaction? Explain.
Model Answer
This is not a redox reaction because the oxidation state in each metal centre does not change.
What is the main factor that controls the equilibrium position?
Model Answer
Hydrogen ion concentration is the main factor to control the equilibrium position.
Draw the three–dimensional structures of CrO4 2– and Cr2O7 2– .
Part 2: Uses of Chromium
An antique automobile bumper is to be chrome plated. The bumper is dipped into an acidic Cr2O7 2– solution where it serves as the cathode of an electrolytic cell. (The relative atomic mass of Cr is 51.996; 1 F = 96485 C.)
Given that oxidation of H2O occurs at the anode, write equation for the two half–reactions and the overall cell reaction.
Model Answer
Cathode: Cr2O7 2– + 14 H+ + 12 e– 2 Cr + 7 H2O
Anode: 2 H2O O2 + 4 H+ + 4 e–
Overall: Cr2O7 2– (aq) + 2 H+ (aq) 2 Cr(s) + H2O(l) + 3 O2 (g)
How many moles of oxygen gas will be evolved for every 52.0 g of chromium deposited?
Model Answer
52 g Cr × (1 mol / 52 g) × (3 mol O2 / 2 mol Cr) = 1.5 mol O2
1.5 mol of oxygen gas will evolve.
If the current is 10.0 amperes, how long will it take to deposit 52.0 g of chromium onto the bumper?
Model Answer
52 g Cr × (1 mol Cr / 52 g) × (6 F / 1 mol Cr) × (96485 C / 1 F) × (1 sec / 10 C) × (1 min / 60 sec) × (1 h / 60 min) = 16 h
What is the chemical reason that chromium is so useful for decorative plating on metals?
Model Answer
Chromium readily forms a thin, adherent, transparent coating of Cr2O3 in air, making the metal extremely useful as an attractive protective coating on easily corroded metals.