Which of the following molecules does not have zero entropy even at zero kelvin? `CO, CO_(2), NaCl, NO`

Options

A. `CO,CO_(2)`
B. `CO, NO`
C. `CO_(2), NaCl`
D. NaCl

Answer

To determine which of the given molecules does not have zero entropy even at absolute zero (0 Kelvin), we can refer to the third law of thermodynamics. According to this law, the entropy of a perfect crystalline substance approaches zero as the temperature approaches absolute zero. However, there are exceptions to this rule, particularly for certain molecules that do not exhibit perfect order at 0 Kelvin.

### Step-by-Step Solution:

1. **Understanding the Third Law of Thermodynamics**:
The third law states that the entropy of a perfect crystalline substance is zero at absolute zero (0 K). This means that for most substances, as temperature decreases to absolute zero, the entropy should also approach zero.

2. **Identifying the Molecules**:
We need to analyze the given molecules:
- **CO (Carbon Monoxide)**
- **CO2 (Carbon Dioxide)**
- **NaCl (Sodium Chloride)**
- **NO (Nitric Oxide)**

3. **Analyzing Each Molecule**:
- **CO**: Carbon monoxide is a diatomic molecule and does not have a perfect order at 0 K. Therefore, it retains some entropy.
- **CO2**: Carbon dioxide is a linear symmetrical molecule. At 0 K, it can achieve a perfect order, leading to zero entropy.
- **NaCl**: Sodium chloride is an ionic compound. At absolute zero, it can also achieve a perfect crystalline structure, resulting in zero entropy.
- **NO**: Nitric oxide is a diatomic molecule and does not achieve perfect order at 0 K, hence it retains some entropy.

4. **Conclusion**:
From the analysis, both CO and NO do not have zero entropy at 0 K due to their inability to achieve perfect order. However, since the question asks for which molecule does not have zero entropy, we can conclude that:
- **NO** is the molecule that does not have zero entropy even at 0 K.

### Final Answer:
**NO** does not have zero entropy even at zero Kelvin.

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