The most exothermic ‘ordinary’ chemical reaction for a given mass of reactants is ; . The theoretica — Thermodynamics and Thermochemistry Chemistry Question
Question
The most exothermic ‘ordinary’ chemical reaction for a given mass of reactants is $2\text{H(g)} \to \text{H}_2\text{(g)}$; $\Delta E = 103\text{ kcal}$. The theoretical decrease in mass on combination of 2.0 moles of hydrogen atoms to form 1.0 mole of hydrogen molecules, assuming that the energy is released only due to decrease in mass of the system, is
Answer: A
💡 Solution & Explanation
The energy released $\Delta E = 103\text{ kcal} = 103 \times 10^3 \times 4.184\text{ J} \approx 4.31 \times 10^5\text{ J}$. Using Einstein's equation $\Delta E = \Delta m \cdot c^2$, the mass decrease $\Delta m = \frac{\Delta E}{c^2} = \frac{4.31 \times 10^5}{(3 \times 10^8)^2} = \frac{4.31 \times 10^5}{9 \times 10^{16}} \approx 4.8 \times 10^{-12}\text{ kg}$.
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