Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Which one of the following need NOT be conserved in a biochemical reaction?
Options:
(A) Total mass
(B) Total moles
(C) Number of atoms of each element
(D) Total energy
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2019 Q.28)**
**1. Governing Concept**:
- **Topic**: Chemical Reaction Engineering & Mass and Energy Balances
- Fundamental conservation laws in reactive closed and open systems:
- **Mass conservation**: Total mass is conserved ($\sum m_{\text{in}} = \sum m_{\text{out}}$).
- **Elemental conservation**: Every atomic element (C, H, O, N) must be strictly conserved across chemical reactions.
- **Energy conservation**: First Law of Thermodynamics guarantees conservation of total energy.
- **Total number of moles (mole balance)**: Chemical reactions involve changes in stoichiometry (e.g., $1\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$, where $4\text{ moles} \rightarrow 2\text{ moles}$). Hence, **total moles need NOT be conserved**.
**2. Option Analysis**:
- **Option (A) [Total mass] is INCORRECT**: Strictly conserved in non-nuclear reactions.
- **Option (B) [Total energy] is INCORRECT**: Strictly conserved by First Law of Thermodynamics.
- **Option (C) [Total atoms] is INCORRECT**: Atomic elements are strictly conserved.
- **Option (D) [Total moles] is CORRECT**: Chemical reactions frequently alter total molar quantity; moles are not a conserved quantity.
**3. Final Answer**:
- **Correct Choice**: **D**
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