Engineering Chemistry MCQ (Quantum Efficiency)

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Engineering Chemistry MCQ (Quantum Efficiency)

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Quantum Efficiency

1. To express the relationship between the number of molecules reacting with the number of photons absorbed, the concept of _____ was introduced.
a) quantum efficiency
b) photon analysis
c) quantum mechanism
d) photon degradation
Answer: a
Explanation: The concept of quantum efficiency was introduced to express the
relationship between a number of molecules reacting with the number of photons
absorbed. It is defined as the number of molecules of the substance undergoing photochemical change per quantum of radiation absorbed.

2. Classification of photochemical reaction on the basis of quantum efficiency can be done into ______ categories.
a) 3
b) 2
c) 4
d) 5
Answer: a
Explanation: Photochemical reactions can be divided into three categories on the
basis of quantum efficiency i.e., the reactions in which quantum yield is “a small
integer like 1, 2” OR “less than 1” OR “a very high value”.

3. Quantum efficiency is given by _______
a) nNhc/qλ
b) nq/Nhcλ
c) qλ/nNhc
d) ncλ/Nhq
Answer: a
Explanation: Quantum efficiency=Number of molecules reacting in a given
time/Number of quanta of light absorbed in the same time. Hence, Quantum
efficiency is given by nNhc/qλ.

4. If a reaction obeys Einstein law, quantum yield is _______
a) 1
b) more than 1
c) less than 1
d) 0
Answer: a
Explanation: If a reaction obeys Einstein law, one molecule is decomposed per
photon. Quantum efficiency=Number of molecules reacting in a given time/Number
of quanta of light absorbed in the same time. So, quantum yield is 1.

5. When two or more molecules are decomposed per photon, the reaction has ______ quantum yield.
a) high
b) low
c) unpredictable
d) variable
Answer: a
Explanation: When two or more molecules are decomposed per photon, the
reaction has greater than one quantum yield. So, the reaction has high quantum yield.
Quantum efficiency = Number of molecules reacting in a given time/Number of quanta of light absorbed in the same time.

6. When the number of molecules decomposed is less than one per photon, the reaction has ______ quantum yield.
a) low
b) high
c) unpredictable
d) variable
Answer: a
Explanation: When the number of molecules decomposed is less than one per
photon, the reaction has less than one quantum yield. So, the reaction has low quantum yield. Quantum efficiency=Number of molecules reacting in a given time/Number of quanta of light absorbed in the same time.

7. Decomposition of HI has quantum yield = _______
a) 1
b) 2
c) 0
d) 4
Answer: b
Explanation: HI + hv→ H + I ……………….. Primary reaction H + HI → H2 + I
I + I → I2 …………….. Secondary reaction
Overall reaction: 2HI + hv → H2 + I2 the overall reaction shows that 2 HI molecules are decomposed by 1 photon. So, quantum yield=2.

8. Which of the following is the condition for high and low quantum yield?
a) The reactivity of the molecules should be temperature dependent
b) All the reactant molecules should be in the different energy state
c) The molecules in the activated state should be largely unstable and decompose to
form the products
d) quantum field=1
Answer: c
Explanation: The molecules in the activated state should be largely unstable and
decompose to form the products. The reactivity of the molecules should not be
independent on temperature. All the reactant molecules should be initially in the
same energy state.