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Thermodynamics question

2025 · Shift 1 · Q11
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Thermodynamics question

2025 · Shift 1 · Q11

JEE AdvancedChemistryThermodynamicsNumerical+4 / −1
Considering ideal gas behavior, the expansion work done (in kJ) when 144 g of water is electrolyzed completely under constant pressure at 300 K is ‾\underline{\hspace{2cm}}​. Use: Universal gas constant (R) = 8.3 J K−1 mol−1; Atomic mass (in amu): H = 1, O = 16
Numerical answer
View written solutionFree

Correct answer: -29.95TO-29.8OR29.8TO29.95

Step-by-Step Solution:

  1. Write the balanced chemical equation for the electrolysis of water. The electrolysis of liquid water (H2O(l)H_2O(l)H2​O(l)) produces hydrogen gas (H2(g)H_2(g)H2​(g)) and oxygen gas (O2(g)O_2(g)O2​(g)). The balanced chemical equation for the decomposition of one mole of water is: H2O(l)→H2(g)+12O2(g)H_2O(l) \rightarrow H_2(g) + \frac{1}{2}O_2(g)H2​O(l)→H2​(g)+21​O2​(g)

  2. Calculate the number of moles of water being electrolyzed. First, we determine the molar mass of water (H2OH_2OH2​O). Molar mass of H = 1 g/mol Molar mass of O = 16 g/mol Molar mass of H2O=2×(1)+16=18H_2O = 2 \times (1) + 16 = 18H2​O=2×(1)+16=18 g/mol.

    Given mass of water = 144 g. The number of moles of water (nH2On_{H_2O}nH2​O​) is: nH2O=massmolar mass=144 g18 g/mol=8 moln_{H_2O} = \frac{\text{mass}}{\text{molar mass}} = \frac{144 \text{ g}}{18 \text{ g/mol}} = 8 \text{ mol}nH2​O​=molar massmass​=18 g/mol144 g​=8 mol

  3. Determine the change in the number of moles of gas (Δng\Delta n_gΔng​). The overall reaction for the electrolysis of 8 moles of water is: 8H2O(l)→8H2(g)+4O2(g)8 H_2O(l) \rightarrow 8 H_2(g) + 4 O_2(g)8H2​O(l)→8H2​(g)+4O2​(g) The initial state has 8 moles of liquid water, so the initial number of moles of gas is ng,initial=0n_{g, initial} = 0ng,initial​=0. The final state consists of gaseous products. The total number of moles of gas in the final state is: ng,final=nH2+nO2=8 mol+4 mol=12 moln_{g, final} = n_{H_2} + n_{O_2} = 8 \text{ mol} + 4 \text{ mol} = 12 \text{ mol}ng,final​=nH2​​+nO2​​=8 mol+4 mol=12 mol The change in the number of moles of gas is: Δng=ng,final−ng,initial=12−0=12 mol\Delta n_g = n_{g, final} - n_{g, initial} = 12 - 0 = 12 \text{ mol}Δng​=ng,final​−ng,initial​=12−0=12 mol

  4. Calculate the expansion work done. The work done on the system during a process at constant pressure and temperature is given by the formula: w=−PextΔVw = -P_{ext}\Delta Vw=−Pext​ΔV Assuming the gases behave ideally and the external pressure is equal to the gas pressure, we can use the ideal gas law, PV=nRTPV = nRTPV=nRT. For a change in the number of moles of gas at constant T and P, this becomes: PΔV=(Δng)RTP\Delta V = (\Delta n_g)RTPΔV=(Δng​)RT Substituting this into the work equation, we get: w=−ΔngRTw = -\Delta n_g RTw=−Δng​RT

  5. Substitute the values and compute the work. We are given: Δng=12\Delta n_g = 12Δng​=12 mol R=8.3R = 8.3R=8.3 J K⁻¹ mol⁻¹ T=300T = 300T=300 K

    Now, we calculate the work in Joules: w=−(12 mol)×(8.3 J K⁻¹ mol⁻¹)×(300 K)w = -(12 \text{ mol}) \times (8.3 \text{ J K⁻¹ mol⁻¹}) \times (300 \text{ K})w=−(12 mol)×(8.3 J K⁻¹ mol⁻¹)×(300 K) w=−12×8.3×300 Jw = -12 \times 8.3 \times 300 \text{ J}w=−12×8.3×300 J w=−12×2490 Jw = -12 \times 2490 \text{ J}w=−12×2490 J w=−29880 Jw = -29880 \text{ J}w=−29880 J

  6. Convert the work from Joules (J) to kiloJoules (kJ). To convert Joules to kiloJoules, we divide by 1000: w=−298801000 kJ=−29.88 kJw = \frac{-29880}{1000} \text{ kJ} = -29.88 \text{ kJ}w=1000−29880​ kJ=−29.88 kJ

    The negative sign indicates that work is done by the system on the surroundings (expansion). The question asks for the "expansion work done". This can be interpreted either as the work done on the system (w=−29.88w = -29.88w=−29.88 kJ) or the work done by the system (wby=−w=29.88w_{by} = -w = 29.88wby​=−w=29.88 kJ). The provided answer range accepts both positive and negative values. Our calculated value of 29.88 falls within the range [29.8, 29.95] and -29.88 falls within [-29.95, -29.8].

Final Answer: The expansion work done is -29.88 kJ.

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