This set of Engineering Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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.
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