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ii. Obtain the relatioship between ΔH and ΔU for gas phase reactions.

# Chemical Thermodynamics Chapter 4 Chemistry Class 12 Textbook Solution

### 4. Answer the following questions.

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- ii. Obtain the relatioship between ΔH and ΔU for gas phase reactions.

ii. Obtain the relatioship between ΔH and ΔU for gas phase reactions.

Answer:-

**i.** At constant pressure, \( \Delta H \) and \( \Delta U \) are related as:

**ii.** For reactions involving gases, \( \Delta V \) cannot be neglected.

Therefore, \( \Delta H = \Delta U + P\Delta V \)

\[ = \Delta U + P(V_2 - V_1) \quad \text{(2)} \]where \( V_1 \) is the volume of gas-phase reactants and \( V_2 \) is that of the gaseous products.

**iii.** We assume reactant and product behave ideally. Applying an ideal gas equation, \( PV = nRT \). Suppose that \( n_1 \) moles of gaseous reactants produce \( n_2 \) moles of gaseous products. Then,

\[ PV_1 = n_1RT \quad \text{and} \quad PV_2 = n_2RT \quad \text{(3)} \]

**iv.** Substituting equation (3) into equation (2) yields

where \( \Delta n_g \) is the difference between the number of moles of products and those of reactants.

Chemical Thermodynamics Chapter 4 Chemistry Class 12 Textbook Solution