chemistry : class12 Reaction temperature control : Preparation of phosphine : p-Block Elements MCQ Questions and Answers
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Reaction temperature control
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MCQ Questions
1. During laboratory preparation of phosphine by warming white phosphorus with concentrated sodium hydroxide solution, why is gentle and controlled warming preferred over vigorous heating?
A.
It prevents the complete conversion of phosphine into hypophosphite ions
B.
It limits rapid phosphine evolution and reduces the risk associated with its ignition-prone impurities
C.
It ensures that sodium hydroxide remains completely undissociated
D.
It prevents water from participating in the reaction
Correct Answer:
It limits rapid phosphine evolution and reduces the risk associated with its ignition-prone impurities
Explanation:
Controlled warming provides a steady evolution of phosphine. Vigorous heating can cause excessively rapid gas liberation and increase the hazard from traces of diphosphine, which may ignite readily in air.
2. If the reaction mixture containing white phosphorus and concentrated sodium hydroxide is heated too strongly during phosphine preparation, which undesirable result is most directly associated with temperature control?
A.
Phosphine may decompose at the hot region, giving phosphorus and hydrogen
B.
Sodium hypophosphite changes quantitatively into phosphoric acid
C.
White phosphorus becomes completely inert toward alkali
D.
Water stops acting as a reactant in the disproportionation
Correct Answer:
Phosphine may decompose at the hot region, giving phosphorus and hydrogen
Explanation:
Excessive heating is undesirable because phosphine is thermally unstable at sufficiently high temperature and can decompose into phosphorus and hydrogen. Hence the generator should be warmed only as necessary.
3. In a controlled phosphine generator based on the reaction P4 + 3NaOH + 3H2O → PH3 + 3NaH2PO2, what is the most appropriate heating practice?
A.
Heat strongly from the beginning to maximize the reaction rate
B.
Maintain gentle warming sufficient to sustain steady gas evolution
C.
Keep the mixture at its boiling point throughout the preparation
D.
Apply intermittent red heat to the phosphorus before adding alkali
Correct Answer:
Maintain gentle warming sufficient to sustain steady gas evolution
Explanation:
The reaction is normally conducted with controlled, gentle warming. This balances the need for phosphine evolution against the risks of violent gas release, overheating, and possible phosphine decomposition.
4. A student observes that phosphine evolution becomes extremely rapid when the preparation flask is heated strongly. Which immediate adjustment best follows the principle of reaction-temperature control?
A.
Increase the flame further to complete the reaction quickly
B.
Remove the heat source and allow the mixture to cool until gas evolution becomes steady
C.
Add a reducing agent to suppress phosphine formation
D.
Expose the flask opening to air to dilute the phosphine
Correct Answer:
Remove the heat source and allow the mixture to cool until gas evolution becomes steady
Explanation:
Rapid evolution indicates excessive heating or an uncontrolled reaction rate. Removing the heat and restoring gentle conditions helps prevent dangerous pressure buildup and minimizes the risk from rapidly released phosphine.
5. Why should a phosphine-generation apparatus not be heated with an open flame when the gas is being evolved?
A.
The flame converts sodium hydroxide into sodium peroxide
B.
Phosphine or phosphorus-containing impurities may ignite on contact with the heated flame or air
C.
The flame prevents hypophosphite formation by removing water
D.
The flame causes white phosphorus to become soluble in water
Correct Answer:
Phosphine or phosphorus-containing impurities may ignite on contact with the heated flame or air
Explanation:
Phosphine prepared from phosphorus and alkali can contain traces of diphosphine, which is especially liable to ignite. Avoiding an open flame and using controlled indirect warming reduces ignition risk.
6. In the preparation of phosphine by warming white phosphorus with concentrated sodium hydroxide, which observation most strongly indicates that the temperature is being controlled appropriately?
A.
A steady stream of gas is evolved without violent bumping or sudden frothing
B.
The reaction mixture glows red while gas evolution stops
C.
The gas evolution occurs only after the flask is heated to red heat
D.
The solution ejects droplets through the delivery tube intermittently
Correct Answer:
A steady stream of gas is evolved without violent bumping or sudden frothing
Explanation:
Appropriate temperature control gives smooth, moderate gas evolution. Violent bumping, frothing, or ejection of liquid indicates excessive heating and an unsafe reaction rate.
7. Why is cooling particularly important if phosphine evolution continues after the external heater has been removed?
A.
The exothermic continuation can maintain rapid gas evolution and cause pressure or entrainment hazards
B.
Cooling converts all sodium hypophosphite back into white phosphorus
C.
Cooling prevents water from hydrolysing calcium phosphide
D.
The reaction requires the solution to freeze before phosphine can be collected
Correct Answer:
The exothermic continuation can maintain rapid gas evolution and cause pressure or entrainment hazards
Explanation:
A hot reaction mixture can continue producing gas after heating stops. Cooling moderates the reaction rate, preventing sudden gas surges and possible carry-over of alkaline liquid into the delivery system.
8. A phosphine generator is heated too intensely, and the delivery tube becomes hot near the generator. Which explanation best connects this observation with temperature control?
A.
The hot zone can promote phosphine decomposition and make the collected gas composition less reliable
B.
The hot zone converts all phosphine directly into sodium hypophosphite in the delivery tube
C.
The hot zone prevents phosphorus from undergoing disproportionation
D.
The hot zone causes hydrogen to react quantitatively with nitrogen from air
Correct Answer:
The hot zone can promote phosphine decomposition and make the collected gas composition less reliable
Explanation:
Excessive local heating can thermally decompose phosphine into phosphorus and hydrogen. Therefore, heating should be confined to gentle warming of the reaction mixture, not intense heating of the gas pathway.
9. Which change would most likely make phosphine preparation unsafe even if the correct reactants are used?
A.
Using a temperature regulator to maintain slow, continuous warming
B.
Heating the reaction mixture strongly while keeping the outlet partially restricted
C.
Allowing the apparatus to cool before stopping gas collection
D.
Using indirect heating instead of a direct flame
Correct Answer:
Heating the reaction mixture strongly while keeping the outlet partially restricted
Explanation:
Strong heating increases the rate of phosphine production, while a restricted outlet impedes gas escape. The combination can produce pressure buildup and dangerous ejection or rupture of the apparatus.
10. Which statement best describes the temperature-related balance required when generating phosphine from white phosphorus and aqueous sodium hydroxide?
A.
The mixture must be kept cold because the reaction cannot occur at room temperature
B.
The mixture should be warmed enough to sustain the reaction but not overheated, since rapid evolution and thermal decomposition are undesirable
C.
The mixture must be heated to red heat so that hypophosphite is formed instead of phosphine
D.
The mixture should be boiled vigorously because phosphine is stable at all temperatures
Correct Answer:
The mixture should be warmed enough to sustain the reaction but not overheated, since rapid evolution and thermal decomposition are undesirable
Explanation:
Preparation requires warming to obtain a useful and steady rate of phosphine evolution. Excessive temperature is avoided because it can cause violent gas release, ignition hazards, and decomposition of phosphine.
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